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1 | libpng-manual.txt - A description on how to use and modify libpng | ||
2 | |||
3 | libpng version 1.5.9 - February 18, 2012 | ||
4 | Updated and distributed by Glenn Randers-Pehrson | ||
5 | <glennrp at users.sourceforge.net> | ||
6 | Copyright (c) 1998-2011 Glenn Randers-Pehrson | ||
7 | |||
8 | This document is released under the libpng license. | ||
9 | For conditions of distribution and use, see the disclaimer | ||
10 | and license in png.h | ||
11 | |||
12 | Based on: | ||
13 | |||
14 | libpng versions 0.97, January 1998, through 1.5.9 - February 18, 2012 | ||
15 | Updated and distributed by Glenn Randers-Pehrson | ||
16 | Copyright (c) 1998-2011 Glenn Randers-Pehrson | ||
17 | |||
18 | libpng 1.0 beta 6 version 0.96 May 28, 1997 | ||
19 | Updated and distributed by Andreas Dilger | ||
20 | Copyright (c) 1996, 1997 Andreas Dilger | ||
21 | |||
22 | libpng 1.0 beta 2 - version 0.88 January 26, 1996 | ||
23 | For conditions of distribution and use, see copyright | ||
24 | notice in png.h. Copyright (c) 1995, 1996 Guy Eric | ||
25 | Schalnat, Group 42, Inc. | ||
26 | |||
27 | Updated/rewritten per request in the libpng FAQ | ||
28 | Copyright (c) 1995, 1996 Frank J. T. Wojcik | ||
29 | December 18, 1995 & January 20, 1996 | ||
30 | |||
31 | I. Introduction | ||
32 | |||
33 | This file describes how to use and modify the PNG reference library | ||
34 | (known as libpng) for your own use. There are five sections to this | ||
35 | file: introduction, structures, reading, writing, and modification and | ||
36 | configuration notes for various special platforms. In addition to this | ||
37 | file, example.c is a good starting point for using the library, as | ||
38 | it is heavily commented and should include everything most people | ||
39 | will need. We assume that libpng is already installed; see the | ||
40 | INSTALL file for instructions on how to install libpng. | ||
41 | |||
42 | For examples of libpng usage, see the files "example.c", "pngtest.c", | ||
43 | and the files in the "contrib" directory, all of which are included in | ||
44 | the libpng distribution. | ||
45 | |||
46 | Libpng was written as a companion to the PNG specification, as a way | ||
47 | of reducing the amount of time and effort it takes to support the PNG | ||
48 | file format in application programs. | ||
49 | |||
50 | The PNG specification (second edition), November 2003, is available as | ||
51 | a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2003 (E)) at | ||
52 | <http://www.w3.org/TR/2003/REC-PNG-20031110/ | ||
53 | The W3C and ISO documents have identical technical content. | ||
54 | |||
55 | The PNG-1.2 specification is available at | ||
56 | <http://www.libpng.org/pub/png/documents/>. It is technically equivalent | ||
57 | to the PNG specification (second edition) but has some additional material. | ||
58 | |||
59 | The PNG-1.0 specification is available | ||
60 | as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a | ||
61 | W3C Recommendation <http://www.w3.org/TR/REC.png.html>. | ||
62 | |||
63 | Some additional chunks are described in the special-purpose public chunks | ||
64 | documents at <http://www.libpng.org/pub/png/documents/>. | ||
65 | |||
66 | Other information | ||
67 | about PNG, and the latest version of libpng, can be found at the PNG home | ||
68 | page, <http://www.libpng.org/pub/png/>. | ||
69 | |||
70 | Most users will not have to modify the library significantly; advanced | ||
71 | users may want to modify it more. All attempts were made to make it as | ||
72 | complete as possible, while keeping the code easy to understand. | ||
73 | Currently, this library only supports C. Support for other languages | ||
74 | is being considered. | ||
75 | |||
76 | Libpng has been designed to handle multiple sessions at one time, | ||
77 | to be easily modifiable, to be portable to the vast majority of | ||
78 | machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy | ||
79 | to use. The ultimate goal of libpng is to promote the acceptance of | ||
80 | the PNG file format in whatever way possible. While there is still | ||
81 | work to be done (see the TODO file), libpng should cover the | ||
82 | majority of the needs of its users. | ||
83 | |||
84 | Libpng uses zlib for its compression and decompression of PNG files. | ||
85 | Further information about zlib, and the latest version of zlib, can | ||
86 | be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>. | ||
87 | The zlib compression utility is a general purpose utility that is | ||
88 | useful for more than PNG files, and can be used without libpng. | ||
89 | See the documentation delivered with zlib for more details. | ||
90 | You can usually find the source files for the zlib utility wherever you | ||
91 | find the libpng source files. | ||
92 | |||
93 | Libpng is thread safe, provided the threads are using different | ||
94 | instances of the structures. Each thread should have its own | ||
95 | png_struct and png_info instances, and thus its own image. | ||
96 | Libpng does not protect itself against two threads using the | ||
97 | same instance of a structure. | ||
98 | |||
99 | II. Structures | ||
100 | |||
101 | There are two main structures that are important to libpng, png_struct | ||
102 | and png_info. Both are internal structures that are no longer exposed | ||
103 | in the libpng interface (as of libpng 1.5.0). | ||
104 | |||
105 | The png_info structure is designed to provide information about the | ||
106 | PNG file. At one time, the fields of png_info were intended to be | ||
107 | directly accessible to the user. However, this tended to cause problems | ||
108 | with applications using dynamically loaded libraries, and as a result | ||
109 | a set of interface functions for png_info (the png_get_*() and png_set_*() | ||
110 | functions) was developed, and direct access to the png_info fields was | ||
111 | deprecated.. | ||
112 | |||
113 | The png_struct structure is the object used by the library to decode a | ||
114 | single image. As of 1.5.0 this structure is also not exposed. | ||
115 | |||
116 | Almost all libpng APIs require a pointer to a png_struct as the first argument. | ||
117 | Many (in particular the png_set and png_get APIs) also require a pointer | ||
118 | to png_info as the second argument. Some application visible macros | ||
119 | defined in png.h designed for basic data access (reading and writing | ||
120 | integers in the PNG format) don't take a png_info pointer, but it's almost | ||
121 | always safe to assume that a (png_struct*) has to be passed to call an API | ||
122 | function. | ||
123 | |||
124 | You can have more than one png_info structure associated with an image, | ||
125 | as illustrated in pngtest.c, one for information valid prior to the | ||
126 | IDAT chunks and another (called "end_info" below) for things after them. | ||
127 | |||
128 | The png.h header file is an invaluable reference for programming with libpng. | ||
129 | And while I'm on the topic, make sure you include the libpng header file: | ||
130 | |||
131 | #include <png.h> | ||
132 | |||
133 | and also (as of libpng-1.5.0) the zlib header file, if you need it: | ||
134 | |||
135 | #include <zlib.h> | ||
136 | |||
137 | Types | ||
138 | |||
139 | The png.h header file defines a number of integral types used by the | ||
140 | APIs. Most of these are fairly obvious; for example types corresponding | ||
141 | to integers of particular sizes and types for passing color values. | ||
142 | |||
143 | One exception is how non-integral numbers are handled. For application | ||
144 | convenience most APIs that take such numbers have C (double) arguments, | ||
145 | however internally PNG, and libpng, use 32 bit signed integers and encode | ||
146 | the value by multiplying by 100,000. As of libpng 1.5.0 a convenience | ||
147 | macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point) | ||
148 | which is simply (png_int_32). | ||
149 | |||
150 | All APIs that take (double) arguments also have a matching API that | ||
151 | takes the corresponding fixed point integer arguments. The fixed point | ||
152 | API has the same name as the floating point one with "_fixed" appended. | ||
153 | The actual range of values permitted in the APIs is frequently less than | ||
154 | the full range of (png_fixed_point) (-21474 to +21474). When APIs require | ||
155 | a non-negative argument the type is recorded as png_uint_32 above. Consult | ||
156 | the header file and the text below for more information. | ||
157 | |||
158 | Special care must be take with sCAL chunk handling because the chunk itself | ||
159 | uses non-integral values encoded as strings containing decimal floating point | ||
160 | numbers. See the comments in the header file. | ||
161 | |||
162 | Configuration | ||
163 | |||
164 | The main header file function declarations are frequently protected by C | ||
165 | preprocessing directives of the form: | ||
166 | |||
167 | #ifdef PNG_feature_SUPPORTED | ||
168 | declare-function | ||
169 | #endif | ||
170 | ... | ||
171 | #ifdef PNG_feature_SUPPORTED | ||
172 | use-function | ||
173 | #endif | ||
174 | |||
175 | The library can be built without support for these APIs, although a | ||
176 | standard build will have all implemented APIs. Application programs | ||
177 | should check the feature macros before using an API for maximum | ||
178 | portability. From libpng 1.5.0 the feature macros set during the build | ||
179 | of libpng are recorded in the header file "pnglibconf.h" and this file | ||
180 | is always included by png.h. | ||
181 | |||
182 | If you don't need to change the library configuration from the default, skip to | ||
183 | the next section ("Reading"). | ||
184 | |||
185 | Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all | ||
186 | of the build project files in the 'projects' directory simply copy | ||
187 | scripts/pnglibconf.h.prebuilt to pnglibconf.h. This means that these build | ||
188 | systems do not permit easy auto-configuration of the library - they only | ||
189 | support the default configuration. | ||
190 | |||
191 | The easiest way to make minor changes to the libpng configuration when | ||
192 | auto-configuration is supported is to add definitions to the command line | ||
193 | using (typically) CPPFLAGS. For example: | ||
194 | |||
195 | CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC | ||
196 | |||
197 | will change the internal libpng math implementation for gamma correction and | ||
198 | other arithmetic calculations to fixed point, avoiding the need for fast | ||
199 | floating point support. The result can be seen in the generated pnglibconf.h - | ||
200 | make sure it contains the changed feature macro setting. | ||
201 | |||
202 | If you need to make more extensive configuration changes - more than one or two | ||
203 | feature macro settings - you can either add -DPNG_USER_CONFIG to the build | ||
204 | command line and put a list of feature macro settings in pngusr.h or you can set | ||
205 | DFA_XTRA (a makefile variable) to a file containing the same information in the | ||
206 | form of 'option' settings. | ||
207 | |||
208 | A. Changing pnglibconf.h | ||
209 | |||
210 | A variety of methods exist to build libpng. Not all of these support | ||
211 | reconfiguration of pnglibconf.h. To reconfigure pnglibconf.h it must either be | ||
212 | rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand. | ||
213 | |||
214 | Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to | ||
215 | pnglibconf.h and changing the lines defining the supported features, paying | ||
216 | very close attention to the 'option' information in scripts/pnglibconf.dfa | ||
217 | that describes those features and their requirements. This is easy to get | ||
218 | wrong. | ||
219 | |||
220 | B. Configuration using DFA_XTRA | ||
221 | |||
222 | Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later | ||
223 | variant such as 'nawk' or 'gawk', is available. The configure build will | ||
224 | automatically find an appropriate awk and build pnglibconf.h. | ||
225 | The scripts/pnglibconf.mak file contains a set of make rules for doing the | ||
226 | same thing if configure is not used, and many of the makefiles in the scripts | ||
227 | directory use this approach. | ||
228 | |||
229 | When rebuilding simply write a new file containing changed options and set | ||
230 | DFA_XTRA to the name of this file. This causes the build to append the new file | ||
231 | to the end of scripts/pnglibconf.dfa. The pngusr.dfa file should contain lines | ||
232 | of the following forms: | ||
233 | |||
234 | everything = off | ||
235 | |||
236 | This turns all optional features off. Include it at the start of pngusr.dfa to | ||
237 | make it easier to build a minimal configuration. You will need to turn at least | ||
238 | some features on afterward to enable either reading or writing code, or both. | ||
239 | |||
240 | option feature on | ||
241 | option feature off | ||
242 | |||
243 | Enable or disable a single feature. This will automatically enable other | ||
244 | features required by a feature that is turned on or disable other features that | ||
245 | require a feature which is turned off. Conflicting settings will cause an error | ||
246 | message to be emitted by awk. | ||
247 | |||
248 | setting feature default value | ||
249 | |||
250 | Changes the default value of setting 'feature' to 'value'. There are a small | ||
251 | number of settings listed at the top of pnglibconf.h, they are documented in the | ||
252 | source code. Most of these values have performance implications for the library | ||
253 | but most of them have no visible effect on the API. Some can also be overridden | ||
254 | from the API. | ||
255 | |||
256 | This method of building a customized pnglibconf.h is illustrated in | ||
257 | contrib/pngminim/*. See the "$(PNGCONF):" target in the makefile and | ||
258 | pngusr.dfa in these directories. | ||
259 | |||
260 | C. Configuration using PNG_USR_CONFIG | ||
261 | |||
262 | If -DPNG_USR_CONFIG is added to the CFLAGS when pnglibconf.h is built the file | ||
263 | pngusr.h will automatically be included before the options in | ||
264 | scripts/pnglibconf.dfa are processed. Your pngusr.h file should contain only | ||
265 | macro definitions turning features on or off or setting settings. | ||
266 | |||
267 | Apart from the global setting "everything = off" all the options listed above | ||
268 | can be set using macros in pngusr.h: | ||
269 | |||
270 | #define PNG_feature_SUPPORTED | ||
271 | |||
272 | is equivalent to: | ||
273 | |||
274 | option feature on | ||
275 | |||
276 | #define PNG_NO_feature | ||
277 | |||
278 | is equivalent to: | ||
279 | |||
280 | option feature off | ||
281 | |||
282 | #define PNG_feature value | ||
283 | |||
284 | is equivalent to: | ||
285 | |||
286 | setting feature default value | ||
287 | |||
288 | Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the | ||
289 | pngusr file you supply override the contents of scripts/pnglibconf.dfa | ||
290 | |||
291 | If confusing or incomprehensible behavior results it is possible to | ||
292 | examine the intermediate file pnglibconf.dfn to find the full set of | ||
293 | dependency information for each setting and option. Simply locate the | ||
294 | feature in the file and read the C comments that precede it. | ||
295 | |||
296 | This method is also illustrated in the contrib/pngminim/* makefiles and | ||
297 | pngusr.h. | ||
298 | |||
299 | III. Reading | ||
300 | |||
301 | We'll now walk you through the possible functions to call when reading | ||
302 | in a PNG file sequentially, briefly explaining the syntax and purpose | ||
303 | of each one. See example.c and png.h for more detail. While | ||
304 | progressive reading is covered in the next section, you will still | ||
305 | need some of the functions discussed in this section to read a PNG | ||
306 | file. | ||
307 | |||
308 | Setup | ||
309 | |||
310 | You will want to do the I/O initialization(*) before you get into libpng, | ||
311 | so if it doesn't work, you don't have much to undo. Of course, you | ||
312 | will also want to insure that you are, in fact, dealing with a PNG | ||
313 | file. Libpng provides a simple check to see if a file is a PNG file. | ||
314 | To use it, pass in the first 1 to 8 bytes of the file to the function | ||
315 | png_sig_cmp(), and it will return 0 (false) if the bytes match the | ||
316 | corresponding bytes of the PNG signature, or nonzero (true) otherwise. | ||
317 | Of course, the more bytes you pass in, the greater the accuracy of the | ||
318 | prediction. | ||
319 | |||
320 | If you are intending to keep the file pointer open for use in libpng, | ||
321 | you must ensure you don't read more than 8 bytes from the beginning | ||
322 | of the file, and you also have to make a call to png_set_sig_bytes_read() | ||
323 | with the number of bytes you read from the beginning. Libpng will | ||
324 | then only check the bytes (if any) that your program didn't read. | ||
325 | |||
326 | (*): If you are not using the standard I/O functions, you will need | ||
327 | to replace them with custom functions. See the discussion under | ||
328 | Customizing libpng. | ||
329 | |||
330 | |||
331 | FILE *fp = fopen(file_name, "rb"); | ||
332 | if (!fp) | ||
333 | { | ||
334 | return (ERROR); | ||
335 | } | ||
336 | |||
337 | fread(header, 1, number, fp); | ||
338 | is_png = !png_sig_cmp(header, 0, number); | ||
339 | |||
340 | if (!is_png) | ||
341 | { | ||
342 | return (NOT_PNG); | ||
343 | } | ||
344 | |||
345 | |||
346 | Next, png_struct and png_info need to be allocated and initialized. In | ||
347 | order to ensure that the size of these structures is correct even with a | ||
348 | dynamically linked libpng, there are functions to initialize and | ||
349 | allocate the structures. We also pass the library version, optional | ||
350 | pointers to error handling functions, and a pointer to a data struct for | ||
351 | use by the error functions, if necessary (the pointer and functions can | ||
352 | be NULL if the default error handlers are to be used). See the section | ||
353 | on Changes to Libpng below regarding the old initialization functions. | ||
354 | The structure allocation functions quietly return NULL if they fail to | ||
355 | create the structure, so your application should check for that. | ||
356 | |||
357 | png_structp png_ptr = png_create_read_struct | ||
358 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, | ||
359 | user_error_fn, user_warning_fn); | ||
360 | |||
361 | if (!png_ptr) | ||
362 | return (ERROR); | ||
363 | |||
364 | png_infop info_ptr = png_create_info_struct(png_ptr); | ||
365 | |||
366 | if (!info_ptr) | ||
367 | { | ||
368 | png_destroy_read_struct(&png_ptr, | ||
369 | (png_infopp)NULL, (png_infopp)NULL); | ||
370 | return (ERROR); | ||
371 | } | ||
372 | |||
373 | If you want to use your own memory allocation routines, | ||
374 | use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use | ||
375 | png_create_read_struct_2() instead of png_create_read_struct(): | ||
376 | |||
377 | png_structp png_ptr = png_create_read_struct_2 | ||
378 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, | ||
379 | user_error_fn, user_warning_fn, (png_voidp) | ||
380 | user_mem_ptr, user_malloc_fn, user_free_fn); | ||
381 | |||
382 | The error handling routines passed to png_create_read_struct() | ||
383 | and the memory alloc/free routines passed to png_create_struct_2() | ||
384 | are only necessary if you are not using the libpng supplied error | ||
385 | handling and memory alloc/free functions. | ||
386 | |||
387 | When libpng encounters an error, it expects to longjmp back | ||
388 | to your routine. Therefore, you will need to call setjmp and pass | ||
389 | your png_jmpbuf(png_ptr). If you read the file from different | ||
390 | routines, you will need to update the longjmp buffer every time you enter | ||
391 | a new routine that will call a png_*() function. | ||
392 | |||
393 | See your documentation of setjmp/longjmp for your compiler for more | ||
394 | information on setjmp/longjmp. See the discussion on libpng error | ||
395 | handling in the Customizing Libpng section below for more information | ||
396 | on the libpng error handling. If an error occurs, and libpng longjmp's | ||
397 | back to your setjmp, you will want to call png_destroy_read_struct() to | ||
398 | free any memory. | ||
399 | |||
400 | if (setjmp(png_jmpbuf(png_ptr))) | ||
401 | { | ||
402 | png_destroy_read_struct(&png_ptr, &info_ptr, | ||
403 | &end_info); | ||
404 | fclose(fp); | ||
405 | return (ERROR); | ||
406 | } | ||
407 | |||
408 | Pass (png_infopp)NULL instead of &end_info if you didn't create | ||
409 | an end_info structure. | ||
410 | |||
411 | If you would rather avoid the complexity of setjmp/longjmp issues, | ||
412 | you can compile libpng with PNG_NO_SETJMP, in which case | ||
413 | errors will result in a call to PNG_ABORT() which defaults to abort(). | ||
414 | |||
415 | You can #define PNG_ABORT() to a function that does something | ||
416 | more useful than abort(), as long as your function does not | ||
417 | return. | ||
418 | |||
419 | Now you need to set up the input code. The default for libpng is to | ||
420 | use the C function fread(). If you use this, you will need to pass a | ||
421 | valid FILE * in the function png_init_io(). Be sure that the file is | ||
422 | opened in binary mode. If you wish to handle reading data in another | ||
423 | way, you need not call the png_init_io() function, but you must then | ||
424 | implement the libpng I/O methods discussed in the Customizing Libpng | ||
425 | section below. | ||
426 | |||
427 | png_init_io(png_ptr, fp); | ||
428 | |||
429 | If you had previously opened the file and read any of the signature from | ||
430 | the beginning in order to see if this was a PNG file, you need to let | ||
431 | libpng know that there are some bytes missing from the start of the file. | ||
432 | |||
433 | png_set_sig_bytes(png_ptr, number); | ||
434 | |||
435 | You can change the zlib compression buffer size to be used while | ||
436 | reading compressed data with | ||
437 | |||
438 | png_set_compression_buffer_size(png_ptr, buffer_size); | ||
439 | |||
440 | where the default size is 8192 bytes. Note that the buffer size | ||
441 | is changed immediately and the buffer is reallocated immediately, | ||
442 | instead of setting a flag to be acted upon later. | ||
443 | |||
444 | If you want CRC errors to be handled in a different manner than | ||
445 | the default, use | ||
446 | |||
447 | png_set_crc_action(png_ptr, crit_action, ancil_action); | ||
448 | |||
449 | The values for png_set_crc_action() say how libpng is to handle CRC errors in | ||
450 | ancillary and critical chunks, and whether to use the data contained | ||
451 | therein. Note that it is impossible to "discard" data in a critical | ||
452 | chunk. | ||
453 | |||
454 | Choices for (int) crit_action are | ||
455 | PNG_CRC_DEFAULT 0 error/quit | ||
456 | PNG_CRC_ERROR_QUIT 1 error/quit | ||
457 | PNG_CRC_WARN_USE 3 warn/use data | ||
458 | PNG_CRC_QUIET_USE 4 quiet/use data | ||
459 | PNG_CRC_NO_CHANGE 5 use the current value | ||
460 | |||
461 | Choices for (int) ancil_action are | ||
462 | PNG_CRC_DEFAULT 0 error/quit | ||
463 | PNG_CRC_ERROR_QUIT 1 error/quit | ||
464 | PNG_CRC_WARN_DISCARD 2 warn/discard data | ||
465 | PNG_CRC_WARN_USE 3 warn/use data | ||
466 | PNG_CRC_QUIET_USE 4 quiet/use data | ||
467 | PNG_CRC_NO_CHANGE 5 use the current value | ||
468 | |||
469 | Setting up callback code | ||
470 | |||
471 | You can set up a callback function to handle any unknown chunks in the | ||
472 | input stream. You must supply the function | ||
473 | |||
474 | read_chunk_callback(png_structp png_ptr, | ||
475 | png_unknown_chunkp chunk); | ||
476 | { | ||
477 | /* The unknown chunk structure contains your | ||
478 | chunk data, along with similar data for any other | ||
479 | unknown chunks: */ | ||
480 | |||
481 | png_byte name[5]; | ||
482 | png_byte *data; | ||
483 | png_size_t size; | ||
484 | |||
485 | /* Note that libpng has already taken care of | ||
486 | the CRC handling */ | ||
487 | |||
488 | /* put your code here. Search for your chunk in the | ||
489 | unknown chunk structure, process it, and return one | ||
490 | of the following: */ | ||
491 | |||
492 | return (-n); /* chunk had an error */ | ||
493 | return (0); /* did not recognize */ | ||
494 | return (n); /* success */ | ||
495 | } | ||
496 | |||
497 | (You can give your function another name that you like instead of | ||
498 | "read_chunk_callback") | ||
499 | |||
500 | To inform libpng about your function, use | ||
501 | |||
502 | png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr, | ||
503 | read_chunk_callback); | ||
504 | |||
505 | This names not only the callback function, but also a user pointer that | ||
506 | you can retrieve with | ||
507 | |||
508 | png_get_user_chunk_ptr(png_ptr); | ||
509 | |||
510 | If you call the png_set_read_user_chunk_fn() function, then all unknown | ||
511 | chunks will be saved when read, in case your callback function will need | ||
512 | one or more of them. This behavior can be changed with the | ||
513 | png_set_keep_unknown_chunks() function, described below. | ||
514 | |||
515 | At this point, you can set up a callback function that will be | ||
516 | called after each row has been read, which you can use to control | ||
517 | a progress meter or the like. It's demonstrated in pngtest.c. | ||
518 | You must supply a function | ||
519 | |||
520 | void read_row_callback(png_structp png_ptr, | ||
521 | png_uint_32 row, int pass); | ||
522 | { | ||
523 | /* put your code here */ | ||
524 | } | ||
525 | |||
526 | (You can give it another name that you like instead of "read_row_callback") | ||
527 | |||
528 | To inform libpng about your function, use | ||
529 | |||
530 | png_set_read_status_fn(png_ptr, read_row_callback); | ||
531 | |||
532 | When this function is called the row has already been completely processed and | ||
533 | the 'row' and 'pass' refer to the next row to be handled. For the | ||
534 | non-interlaced case the row that was just handled is simply one less than the | ||
535 | passed in row number, and pass will always be 0. For the interlaced case the | ||
536 | same applies unless the row value is 0, in which case the row just handled was | ||
537 | the last one from one of the preceding passes. Because interlacing may skip a | ||
538 | pass you cannot be sure that the preceding pass is just 'pass-1', if you really | ||
539 | need to know what the last pass is record (row,pass) from the callback and use | ||
540 | the last recorded value each time. | ||
541 | |||
542 | As with the user transform you can find the output row using the | ||
543 | PNG_ROW_FROM_PASS_ROW macro. | ||
544 | |||
545 | Unknown-chunk handling | ||
546 | |||
547 | Now you get to set the way the library processes unknown chunks in the | ||
548 | input PNG stream. Both known and unknown chunks will be read. Normal | ||
549 | behavior is that known chunks will be parsed into information in | ||
550 | various info_ptr members while unknown chunks will be discarded. This | ||
551 | behavior can be wasteful if your application will never use some known | ||
552 | chunk types. To change this, you can call: | ||
553 | |||
554 | png_set_keep_unknown_chunks(png_ptr, keep, | ||
555 | chunk_list, num_chunks); | ||
556 | keep - 0: default unknown chunk handling | ||
557 | 1: ignore; do not keep | ||
558 | 2: keep only if safe-to-copy | ||
559 | 3: keep even if unsafe-to-copy | ||
560 | |||
561 | You can use these definitions: | ||
562 | PNG_HANDLE_CHUNK_AS_DEFAULT 0 | ||
563 | PNG_HANDLE_CHUNK_NEVER 1 | ||
564 | PNG_HANDLE_CHUNK_IF_SAFE 2 | ||
565 | PNG_HANDLE_CHUNK_ALWAYS 3 | ||
566 | |||
567 | chunk_list - list of chunks affected (a byte string, | ||
568 | five bytes per chunk, NULL or '\0' if | ||
569 | num_chunks is 0) | ||
570 | |||
571 | num_chunks - number of chunks affected; if 0, all | ||
572 | unknown chunks are affected. If nonzero, | ||
573 | only the chunks in the list are affected | ||
574 | |||
575 | Unknown chunks declared in this way will be saved as raw data onto a | ||
576 | list of png_unknown_chunk structures. If a chunk that is normally | ||
577 | known to libpng is named in the list, it will be handled as unknown, | ||
578 | according to the "keep" directive. If a chunk is named in successive | ||
579 | instances of png_set_keep_unknown_chunks(), the final instance will | ||
580 | take precedence. The IHDR and IEND chunks should not be named in | ||
581 | chunk_list; if they are, libpng will process them normally anyway. | ||
582 | If you know that your application will never make use of some particular | ||
583 | chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below. | ||
584 | |||
585 | Here is an example of the usage of png_set_keep_unknown_chunks(), | ||
586 | where the private "vpAg" chunk will later be processed by a user chunk | ||
587 | callback function: | ||
588 | |||
589 | png_byte vpAg[5]={118, 112, 65, 103, (png_byte) '\0'}; | ||
590 | |||
591 | #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED) | ||
592 | png_byte unused_chunks[]= | ||
593 | { | ||
594 | 104, 73, 83, 84, (png_byte) '\0', /* hIST */ | ||
595 | 105, 84, 88, 116, (png_byte) '\0', /* iTXt */ | ||
596 | 112, 67, 65, 76, (png_byte) '\0', /* pCAL */ | ||
597 | 115, 67, 65, 76, (png_byte) '\0', /* sCAL */ | ||
598 | 115, 80, 76, 84, (png_byte) '\0', /* sPLT */ | ||
599 | 116, 73, 77, 69, (png_byte) '\0', /* tIME */ | ||
600 | }; | ||
601 | #endif | ||
602 | |||
603 | ... | ||
604 | |||
605 | #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED) | ||
606 | /* ignore all unknown chunks: */ | ||
607 | png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0); | ||
608 | |||
609 | /* except for vpAg: */ | ||
610 | png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1); | ||
611 | |||
612 | /* also ignore unused known chunks: */ | ||
613 | png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks, | ||
614 | (int)sizeof(unused_chunks)/5); | ||
615 | #endif | ||
616 | |||
617 | User limits | ||
618 | |||
619 | The PNG specification allows the width and height of an image to be as | ||
620 | large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns. | ||
621 | Since very few applications really need to process such large images, | ||
622 | we have imposed an arbitrary 1-million limit on rows and columns. | ||
623 | Larger images will be rejected immediately with a png_error() call. If | ||
624 | you wish to change this limit, you can use | ||
625 | |||
626 | png_set_user_limits(png_ptr, width_max, height_max); | ||
627 | |||
628 | to set your own limits, or use width_max = height_max = 0x7fffffffL | ||
629 | to allow all valid dimensions (libpng may reject some very large images | ||
630 | anyway because of potential buffer overflow conditions). | ||
631 | |||
632 | You should put this statement after you create the PNG structure and | ||
633 | before calling png_read_info(), png_read_png(), or png_process_data(). | ||
634 | |||
635 | When writing a PNG datastream, put this statement before calling | ||
636 | png_write_info() or png_write_png(). | ||
637 | |||
638 | If you need to retrieve the limits that are being applied, use | ||
639 | |||
640 | width_max = png_get_user_width_max(png_ptr); | ||
641 | height_max = png_get_user_height_max(png_ptr); | ||
642 | |||
643 | The PNG specification sets no limit on the number of ancillary chunks | ||
644 | allowed in a PNG datastream. You can impose a limit on the total number | ||
645 | of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with | ||
646 | |||
647 | png_set_chunk_cache_max(png_ptr, user_chunk_cache_max); | ||
648 | |||
649 | where 0x7fffffffL means unlimited. You can retrieve this limit with | ||
650 | |||
651 | chunk_cache_max = png_get_chunk_cache_max(png_ptr); | ||
652 | |||
653 | This limit also applies to the number of buffers that can be allocated | ||
654 | by png_decompress_chunk() while decompressing iTXt, zTXt, and iCCP chunks. | ||
655 | |||
656 | You can also set a limit on the amount of memory that a compressed chunk | ||
657 | other than IDAT can occupy, with | ||
658 | |||
659 | png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max); | ||
660 | |||
661 | and you can retrieve the limit with | ||
662 | |||
663 | chunk_malloc_max = png_get_chunk_malloc_max(png_ptr); | ||
664 | |||
665 | Any chunks that would cause either of these limits to be exceeded will | ||
666 | be ignored. | ||
667 | |||
668 | Information about your system | ||
669 | |||
670 | If you intend to display the PNG or to incorporate it in other image data you | ||
671 | need to tell libpng information about your display or drawing surface so that | ||
672 | libpng can convert the values in the image to match the display. | ||
673 | |||
674 | From libpng-1.5.4 this information can be set before reading the PNG file | ||
675 | header. In earlier versions png_set_gamma() existed but behaved incorrectly if | ||
676 | called before the PNG file header had been read and png_set_alpha_mode() did not | ||
677 | exist. | ||
678 | |||
679 | If you need to support versions prior to libpng-1.5.4 test the version number | ||
680 | as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures | ||
681 | described in the appropriate manual page. | ||
682 | |||
683 | You give libpng the encoding expected by your system expressed as a 'gamma' | ||
684 | value. You can also specify a default encoding for the PNG file in | ||
685 | case the required information is missing from the file. By default libpng | ||
686 | assumes that the PNG data matches your system, to keep this default call: | ||
687 | |||
688 | png_set_gamma(png_ptr, screen_gamma, 1/screen_gamma/*file gamma*/); | ||
689 | |||
690 | or you can use the fixed point equivalent: | ||
691 | |||
692 | png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma, PNG_FP_1/screen_gamma); | ||
693 | |||
694 | If you don't know the gamma for your system it is probably 2.2 - a good | ||
695 | approximation to the IEC standard for display systems (sRGB). If images are | ||
696 | too contrasty or washed out you got the value wrong - check your system | ||
697 | documentation! | ||
698 | |||
699 | Many systems permit the system gamma to be changed via a lookup table in the | ||
700 | display driver, a few systems, including older Macs, change the response by | ||
701 | default. As of 1.5.4 three special values are available to handle common | ||
702 | situations: | ||
703 | |||
704 | PNG_DEFAULT_sRGB: Indicates that the system conforms to the IEC 61966-2-1 | ||
705 | standard. This matches almost all systems. | ||
706 | PNG_GAMMA_MAC_18: Indicates that the system is an older (pre Mac OS 10.6) | ||
707 | Apple Macintosh system with the default settings. | ||
708 | PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates that the | ||
709 | system expects data with no gamma encoding. | ||
710 | |||
711 | You would use the linear (unencoded) value if you need to process the pixel | ||
712 | values further because this avoids the need to decode and reencode each | ||
713 | component value whenever arithmetic is performed. A lot of graphics software | ||
714 | uses linear values for this reason, often with higher precision component values | ||
715 | to preserve overall accuracy. | ||
716 | |||
717 | The second thing you may need to tell libpng about is how your system handles | ||
718 | alpha channel information. Some, but not all, PNG files contain an alpha | ||
719 | channel. To display these files correctly you need to compose the data onto a | ||
720 | suitable background, as described in the PNG specification. | ||
721 | |||
722 | Libpng only supports composing onto a single color (using png_set_background; | ||
723 | see below). Otherwise you must do the composition yourself and, in this case, | ||
724 | you may need to call png_set_alpha_mode: | ||
725 | |||
726 | #if PNG_LIBPNG_VER >= 10504 | ||
727 | png_set_alpha_mode(png_ptr, mode, screen_gamma); | ||
728 | #else | ||
729 | png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma); | ||
730 | #endif | ||
731 | |||
732 | The screen_gamma value is the same as the argument to png_set_gamma; however, | ||
733 | how it affects the output depends on the mode. png_set_alpha_mode() sets the | ||
734 | file gamma default to 1/screen_gamma, so normally you don't need to call | ||
735 | png_set_gamma. If you need different defaults call png_set_gamma() before | ||
736 | png_set_alpha_mode() - if you call it after it will override the settings made | ||
737 | by png_set_alpha_mode(). | ||
738 | |||
739 | The mode is as follows: | ||
740 | |||
741 | PNG_ALPHA_PNG: The data is encoded according to the PNG specification. Red, | ||
742 | green and blue, or gray, components are gamma encoded color | ||
743 | values and are not premultiplied by the alpha value. The | ||
744 | alpha value is a linear measure of the contribution of the | ||
745 | pixel to the corresponding final output pixel. | ||
746 | |||
747 | You should normally use this format if you intend to perform | ||
748 | color correction on the color values; most, maybe all, color | ||
749 | correction software has no handling for the alpha channel and, | ||
750 | anyway, the math to handle pre-multiplied component values is | ||
751 | unnecessarily complex. | ||
752 | |||
753 | Before you do any arithmetic on the component values you need | ||
754 | to remove the gamma encoding and multiply out the alpha | ||
755 | channel. See the PNG specification for more detail. It is | ||
756 | important to note that when an image with an alpha channel is | ||
757 | scaled, linear encoded, pre-multiplied component values must | ||
758 | be used! | ||
759 | |||
760 | The remaining modes assume you don't need to do any further color correction or | ||
761 | that if you do, your color correction software knows all about alpha (it | ||
762 | probably doesn't!) | ||
763 | |||
764 | PNG_ALPHA_STANDARD: The data libpng produces | ||
765 | is encoded in the standard way | ||
766 | assumed by most correctly written graphics software. | ||
767 | The gamma encoding will be removed by libpng and the | ||
768 | linear component values will be pre-multiplied by the | ||
769 | alpha channel. | ||
770 | |||
771 | With this format the final image must be re-encoded to | ||
772 | match the display gamma before the image is displayed. | ||
773 | If your system doesn't do that, yet still seems to | ||
774 | perform arithmetic on the pixels without decoding them, | ||
775 | it is broken - check out the modes below. | ||
776 | |||
777 | With PNG_ALPHA_STANDARD libpng always produces linear | ||
778 | component values, whatever screen_gamma you supply. The | ||
779 | screen_gamma value is, however, used as a default for | ||
780 | the file gamma if the PNG file has no gamma information. | ||
781 | |||
782 | If you call png_set_gamma() after png_set_alpha_mode() you | ||
783 | will override the linear encoding. Instead the | ||
784 | pre-multiplied pixel values will be gamma encoded but | ||
785 | the alpha channel will still be linear. This may | ||
786 | actually match the requirements of some broken software, | ||
787 | but it is unlikely. | ||
788 | |||
789 | While linear 8-bit data is often used it has | ||
790 | insufficient precision for any image with a reasonable | ||
791 | dynamic range. To avoid problems, and if your software | ||
792 | supports it, use png_set_expand_16() to force all | ||
793 | components to 16 bits. | ||
794 | |||
795 | PNG_ALPHA_OPTIMIZED: This mode is the same | ||
796 | as PNG_ALPHA_STANDARD except that | ||
797 | completely opaque pixels are gamma encoded according to | ||
798 | the screen_gamma value. Pixels with alpha less than 1.0 | ||
799 | will still have linear components. | ||
800 | |||
801 | Use this format if you have control over your | ||
802 | compositing software and do don't do other arithmetic | ||
803 | (such as scaling) on the data you get from libpng. Your | ||
804 | compositing software can simply copy opaque pixels to | ||
805 | the output but still has linear values for the | ||
806 | non-opaque pixels. | ||
807 | |||
808 | In normal compositing, where the alpha channel encodes | ||
809 | partial pixel coverage (as opposed to broad area | ||
810 | translucency), the inaccuracies of the 8-bit | ||
811 | representation of non-opaque pixels are irrelevant. | ||
812 | |||
813 | You can also try this format if your software is broken; | ||
814 | it might look better. | ||
815 | |||
816 | PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; | ||
817 | however, all component values, | ||
818 | including the alpha channel are gamma encoded. This is | ||
819 | an appropriate format to try if your software, or more | ||
820 | likely hardware, is totally broken, i.e., if it performs | ||
821 | linear arithmetic directly on gamma encoded values. | ||
822 | |||
823 | In most cases of broken software or hardware the bug in the final display | ||
824 | manifests as a subtle halo around composited parts of the image. You may not | ||
825 | even perceive this as a halo; the composited part of the image may simply appear | ||
826 | separate from the background, as though it had been cut out of paper and pasted | ||
827 | on afterward. | ||
828 | |||
829 | If you don't have to deal with bugs in software or hardware, or if you can fix | ||
830 | them, there are three recommended ways of using png_set_alpha_mode(): | ||
831 | |||
832 | png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG, | ||
833 | screen_gamma); | ||
834 | |||
835 | You can do color correction on the result (libpng does not currently | ||
836 | support color correction internally). When you handle the alpha channel | ||
837 | you need to undo the gamma encoding and multiply out the alpha. | ||
838 | |||
839 | png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD, | ||
840 | screen_gamma); | ||
841 | png_set_expand_16(png_ptr); | ||
842 | |||
843 | If you are using the high level interface, don't call png_set_expand_16(); | ||
844 | instead pass PNG_TRANSFORM_EXPAND_16 to the interface. | ||
845 | |||
846 | With this mode you can't do color correction, but you can do arithmetic, | ||
847 | including composition and scaling, on the data without further processing. | ||
848 | |||
849 | png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED, | ||
850 | screen_gamma); | ||
851 | |||
852 | You can avoid the expansion to 16-bit components with this mode, but you | ||
853 | lose the ability to scale the image or perform other linear arithmetic. | ||
854 | All you can do is compose the result onto a matching output. Since this | ||
855 | mode is libpng-specific you also need to write your own composition | ||
856 | software. | ||
857 | |||
858 | If you don't need, or can't handle, the alpha channel you can call | ||
859 | png_set_background() to remove it by compositing against a fixed color. Don't | ||
860 | call png_set_strip_alpha() to do this - it will leave spurious pixel values in | ||
861 | transparent parts of this image. | ||
862 | |||
863 | png_set_background(png_ptr, &background_color, | ||
864 | PNG_BACKGROUND_GAMMA_SCREEN, 0, 1); | ||
865 | |||
866 | The background_color is an RGB or grayscale value according to the data format | ||
867 | libpng will produce for you. Because you don't yet know the format of the PNG | ||
868 | file, if you call png_set_background at this point you must arrange for the | ||
869 | format produced by libpng to always have 8-bit or 16-bit components and then | ||
870 | store the color as an 8-bit or 16-bit color as appropriate. The color contains | ||
871 | separate gray and RGB component values, so you can let libpng produce gray or | ||
872 | RGB output according to the input format, but low bit depth grayscale images | ||
873 | must always be converted to at least 8-bit format. (Even though low bit depth | ||
874 | grayscale images can't have an alpha channel they can have a transparent | ||
875 | color!) | ||
876 | |||
877 | You set the transforms you need later, either as flags to the high level | ||
878 | interface or libpng API calls for the low level interface. For reference the | ||
879 | settings and API calls required are: | ||
880 | |||
881 | 8-bit values: | ||
882 | PNG_TRANSFORM_SCALE_16 | PNG_EXPAND | ||
883 | png_set_expand(png_ptr); png_set_scale_16(png_ptr); | ||
884 | |||
885 | If you must get exactly the same inaccurate results | ||
886 | produced by default in versions prior to libpng-1.5.4, | ||
887 | use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr) | ||
888 | instead. | ||
889 | |||
890 | 16-bit values: | ||
891 | PNG_TRANSFORM_EXPAND_16 | ||
892 | png_set_expand_16(png_ptr); | ||
893 | |||
894 | In either case palette image data will be expanded to RGB. If you just want | ||
895 | color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr) | ||
896 | to the list. | ||
897 | |||
898 | Calling png_set_background before the PNG file header is read will not work | ||
899 | prior to libpng-1.5.4. Because the failure may result in unexpected warnings or | ||
900 | errors it is therefore much safer to call png_set_background after the head has | ||
901 | been read. Unfortunately this means that prior to libpng-1.5.4 it cannot be | ||
902 | used with the high level interface. | ||
903 | |||
904 | The high-level read interface | ||
905 | |||
906 | At this point there are two ways to proceed; through the high-level | ||
907 | read interface, or through a sequence of low-level read operations. | ||
908 | You can use the high-level interface if (a) you are willing to read | ||
909 | the entire image into memory, and (b) the input transformations | ||
910 | you want to do are limited to the following set: | ||
911 | |||
912 | PNG_TRANSFORM_IDENTITY No transformation | ||
913 | PNG_TRANSFORM_SCALE_16 Strip 16-bit samples to | ||
914 | 8-bit accurately | ||
915 | PNG_TRANSFORM_STRIP_16 Chop 16-bit samples to | ||
916 | 8-bit less accurately | ||
917 | PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel | ||
918 | PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit | ||
919 | samples to bytes | ||
920 | PNG_TRANSFORM_PACKSWAP Change order of packed | ||
921 | pixels to LSB first | ||
922 | PNG_TRANSFORM_EXPAND Perform set_expand() | ||
923 | PNG_TRANSFORM_INVERT_MONO Invert monochrome images | ||
924 | PNG_TRANSFORM_SHIFT Normalize pixels to the | ||
925 | sBIT depth | ||
926 | PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA | ||
927 | to BGRA | ||
928 | PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA | ||
929 | to AG | ||
930 | PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity | ||
931 | to transparency | ||
932 | PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples | ||
933 | PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples | ||
934 | to RGB (or GA to RGBA) | ||
935 | PNG_TRANSFORM_EXPAND_16 Expand samples to 16 bits | ||
936 | |||
937 | (This excludes setting a background color, doing gamma transformation, | ||
938 | quantizing, and setting filler.) If this is the case, simply do this: | ||
939 | |||
940 | png_read_png(png_ptr, info_ptr, png_transforms, NULL) | ||
941 | |||
942 | where png_transforms is an integer containing the bitwise OR of some | ||
943 | set of transformation flags. This call is equivalent to png_read_info(), | ||
944 | followed the set of transformations indicated by the transform mask, | ||
945 | then png_read_image(), and finally png_read_end(). | ||
946 | |||
947 | (The final parameter of this call is not yet used. Someday it might point | ||
948 | to transformation parameters required by some future input transform.) | ||
949 | |||
950 | You must use png_transforms and not call any png_set_transform() functions | ||
951 | when you use png_read_png(). | ||
952 | |||
953 | After you have called png_read_png(), you can retrieve the image data | ||
954 | with | ||
955 | |||
956 | row_pointers = png_get_rows(png_ptr, info_ptr); | ||
957 | |||
958 | where row_pointers is an array of pointers to the pixel data for each row: | ||
959 | |||
960 | png_bytep row_pointers[height]; | ||
961 | |||
962 | If you know your image size and pixel size ahead of time, you can allocate | ||
963 | row_pointers prior to calling png_read_png() with | ||
964 | |||
965 | if (height > PNG_UINT_32_MAX/png_sizeof(png_byte)) | ||
966 | png_error (png_ptr, | ||
967 | "Image is too tall to process in memory"); | ||
968 | |||
969 | if (width > PNG_UINT_32_MAX/pixel_size) | ||
970 | png_error (png_ptr, | ||
971 | "Image is too wide to process in memory"); | ||
972 | |||
973 | row_pointers = png_malloc(png_ptr, | ||
974 | height*png_sizeof(png_bytep)); | ||
975 | |||
976 | for (int i=0; i<height, i++) | ||
977 | row_pointers[i]=NULL; /* security precaution */ | ||
978 | |||
979 | for (int i=0; i<height, i++) | ||
980 | row_pointers[i]=png_malloc(png_ptr, | ||
981 | width*pixel_size); | ||
982 | |||
983 | png_set_rows(png_ptr, info_ptr, &row_pointers); | ||
984 | |||
985 | Alternatively you could allocate your image in one big block and define | ||
986 | row_pointers[i] to point into the proper places in your block. | ||
987 | |||
988 | If you use png_set_rows(), the application is responsible for freeing | ||
989 | row_pointers (and row_pointers[i], if they were separately allocated). | ||
990 | |||
991 | If you don't allocate row_pointers ahead of time, png_read_png() will | ||
992 | do it, and it'll be free'ed by libpng when you call png_destroy_*(). | ||
993 | |||
994 | The low-level read interface | ||
995 | |||
996 | If you are going the low-level route, you are now ready to read all | ||
997 | the file information up to the actual image data. You do this with a | ||
998 | call to png_read_info(). | ||
999 | |||
1000 | png_read_info(png_ptr, info_ptr); | ||
1001 | |||
1002 | This will process all chunks up to but not including the image data. | ||
1003 | |||
1004 | This also copies some of the data from the PNG file into the decode structure | ||
1005 | for use in later transformations. Important information copied in is: | ||
1006 | |||
1007 | 1) The PNG file gamma from the gAMA chunk. This overwrites the default value | ||
1008 | provided by an earlier call to png_set_gamma or png_set_alpha_mode. | ||
1009 | |||
1010 | 2) Prior to libpng-1.5.4 the background color from a bKGd chunk. This | ||
1011 | damages the information provided by an earlier call to png_set_background | ||
1012 | resulting in unexpected behavior. Libpng-1.5.4 no longer does this. | ||
1013 | |||
1014 | 3) The number of significant bits in each component value. Libpng uses this to | ||
1015 | optimize gamma handling by reducing the internal lookup table sizes. | ||
1016 | |||
1017 | 4) The transparent color information from a tRNS chunk. This can be modified by | ||
1018 | a later call to png_set_tRNS. | ||
1019 | |||
1020 | Querying the info structure | ||
1021 | |||
1022 | Functions are used to get the information from the info_ptr once it | ||
1023 | has been read. Note that these fields may not be completely filled | ||
1024 | in until png_read_end() has read the chunk data following the image. | ||
1025 | |||
1026 | png_get_IHDR(png_ptr, info_ptr, &width, &height, | ||
1027 | &bit_depth, &color_type, &interlace_type, | ||
1028 | &compression_type, &filter_method); | ||
1029 | |||
1030 | width - holds the width of the image | ||
1031 | in pixels (up to 2^31). | ||
1032 | |||
1033 | height - holds the height of the image | ||
1034 | in pixels (up to 2^31). | ||
1035 | |||
1036 | bit_depth - holds the bit depth of one of the | ||
1037 | image channels. (valid values are | ||
1038 | 1, 2, 4, 8, 16 and depend also on | ||
1039 | the color_type. See also | ||
1040 | significant bits (sBIT) below). | ||
1041 | |||
1042 | color_type - describes which color/alpha channels | ||
1043 | are present. | ||
1044 | PNG_COLOR_TYPE_GRAY | ||
1045 | (bit depths 1, 2, 4, 8, 16) | ||
1046 | PNG_COLOR_TYPE_GRAY_ALPHA | ||
1047 | (bit depths 8, 16) | ||
1048 | PNG_COLOR_TYPE_PALETTE | ||
1049 | (bit depths 1, 2, 4, 8) | ||
1050 | PNG_COLOR_TYPE_RGB | ||
1051 | (bit_depths 8, 16) | ||
1052 | PNG_COLOR_TYPE_RGB_ALPHA | ||
1053 | (bit_depths 8, 16) | ||
1054 | |||
1055 | PNG_COLOR_MASK_PALETTE | ||
1056 | PNG_COLOR_MASK_COLOR | ||
1057 | PNG_COLOR_MASK_ALPHA | ||
1058 | |||
1059 | interlace_type - (PNG_INTERLACE_NONE or | ||
1060 | PNG_INTERLACE_ADAM7) | ||
1061 | |||
1062 | compression_type - (must be PNG_COMPRESSION_TYPE_BASE | ||
1063 | for PNG 1.0) | ||
1064 | |||
1065 | filter_method - (must be PNG_FILTER_TYPE_BASE | ||
1066 | for PNG 1.0, and can also be | ||
1067 | PNG_INTRAPIXEL_DIFFERENCING if | ||
1068 | the PNG datastream is embedded in | ||
1069 | a MNG-1.0 datastream) | ||
1070 | |||
1071 | Any or all of interlace_type, compression_type, or | ||
1072 | filter_method can be NULL if you are | ||
1073 | not interested in their values. | ||
1074 | |||
1075 | Note that png_get_IHDR() returns 32-bit data into | ||
1076 | the application's width and height variables. | ||
1077 | This is an unsafe situation if these are 16-bit | ||
1078 | variables. In such situations, the | ||
1079 | png_get_image_width() and png_get_image_height() | ||
1080 | functions described below are safer. | ||
1081 | |||
1082 | width = png_get_image_width(png_ptr, | ||
1083 | info_ptr); | ||
1084 | |||
1085 | height = png_get_image_height(png_ptr, | ||
1086 | info_ptr); | ||
1087 | |||
1088 | bit_depth = png_get_bit_depth(png_ptr, | ||
1089 | info_ptr); | ||
1090 | |||
1091 | color_type = png_get_color_type(png_ptr, | ||
1092 | info_ptr); | ||
1093 | |||
1094 | interlace_type = png_get_interlace_type(png_ptr, | ||
1095 | info_ptr); | ||
1096 | |||
1097 | compression_type = png_get_compression_type(png_ptr, | ||
1098 | info_ptr); | ||
1099 | |||
1100 | filter_method = png_get_filter_type(png_ptr, | ||
1101 | info_ptr); | ||
1102 | |||
1103 | channels = png_get_channels(png_ptr, info_ptr); | ||
1104 | |||
1105 | channels - number of channels of info for the | ||
1106 | color type (valid values are 1 (GRAY, | ||
1107 | PALETTE), 2 (GRAY_ALPHA), 3 (RGB), | ||
1108 | 4 (RGB_ALPHA or RGB + filler byte)) | ||
1109 | |||
1110 | rowbytes = png_get_rowbytes(png_ptr, info_ptr); | ||
1111 | |||
1112 | rowbytes - number of bytes needed to hold a row | ||
1113 | |||
1114 | signature = png_get_signature(png_ptr, info_ptr); | ||
1115 | |||
1116 | signature - holds the signature read from the | ||
1117 | file (if any). The data is kept in | ||
1118 | the same offset it would be if the | ||
1119 | whole signature were read (i.e. if an | ||
1120 | application had already read in 4 | ||
1121 | bytes of signature before starting | ||
1122 | libpng, the remaining 4 bytes would | ||
1123 | be in signature[4] through signature[7] | ||
1124 | (see png_set_sig_bytes())). | ||
1125 | |||
1126 | These are also important, but their validity depends on whether the chunk | ||
1127 | has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and | ||
1128 | png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the | ||
1129 | data has been read, or zero if it is missing. The parameters to the | ||
1130 | png_get_<chunk> are set directly if they are simple data types, or a | ||
1131 | pointer into the info_ptr is returned for any complex types. | ||
1132 | |||
1133 | png_get_PLTE(png_ptr, info_ptr, &palette, | ||
1134 | &num_palette); | ||
1135 | |||
1136 | palette - the palette for the file | ||
1137 | (array of png_color) | ||
1138 | |||
1139 | num_palette - number of entries in the palette | ||
1140 | |||
1141 | png_get_gAMA(png_ptr, info_ptr, &file_gamma); | ||
1142 | png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma); | ||
1143 | |||
1144 | file_gamma - the gamma at which the file is | ||
1145 | written (PNG_INFO_gAMA) | ||
1146 | |||
1147 | int_file_gamma - 100,000 times the gamma at which the | ||
1148 | file is written | ||
1149 | |||
1150 | png_get_cHRM(png_ptr, info_ptr, &white_x, &white_y, &red_x, &red_y, | ||
1151 | &green_x, &green_y, &blue_x, &blue_y) | ||
1152 | png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z, &green_X, | ||
1153 | &green_Y, &green_Z, &blue_X, &blue_Y, &blue_Z) | ||
1154 | png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x, &int_white_y, | ||
1155 | &int_red_x, &int_red_y, &int_green_x, &int_green_y, | ||
1156 | &int_blue_x, &int_blue_y) | ||
1157 | png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y, | ||
1158 | &int_red_Z, &int_green_X, &int_green_Y, &int_green_Z, | ||
1159 | &int_blue_X, &int_blue_Y, &int_blue_Z) | ||
1160 | |||
1161 | {white,red,green,blue}_{x,y} | ||
1162 | A color space encoding specified using the chromaticities | ||
1163 | of the end points and the white point. (PNG_INFO_cHRM) | ||
1164 | |||
1165 | {red,green,blue}_{X,Y,Z} | ||
1166 | A color space encoding specified using the encoding end | ||
1167 | points - the CIE tristimulus specification of the intended | ||
1168 | color of the red, green and blue channels in the PNG RGB | ||
1169 | data. The white point is simply the sum of the three end | ||
1170 | points. (PNG_INFO_cHRM) | ||
1171 | |||
1172 | png_get_sRGB(png_ptr, info_ptr, &srgb_intent); | ||
1173 | |||
1174 | file_srgb_intent - the rendering intent (PNG_INFO_sRGB) | ||
1175 | The presence of the sRGB chunk | ||
1176 | means that the pixel data is in the | ||
1177 | sRGB color space. This chunk also | ||
1178 | implies specific values of gAMA and | ||
1179 | cHRM. | ||
1180 | |||
1181 | png_get_iCCP(png_ptr, info_ptr, &name, | ||
1182 | &compression_type, &profile, &proflen); | ||
1183 | |||
1184 | name - The profile name. | ||
1185 | |||
1186 | compression_type - The compression type; always | ||
1187 | PNG_COMPRESSION_TYPE_BASE for PNG 1.0. | ||
1188 | You may give NULL to this argument to | ||
1189 | ignore it. | ||
1190 | |||
1191 | profile - International Color Consortium color | ||
1192 | profile data. May contain NULs. | ||
1193 | |||
1194 | proflen - length of profile data in bytes. | ||
1195 | |||
1196 | png_get_sBIT(png_ptr, info_ptr, &sig_bit); | ||
1197 | |||
1198 | sig_bit - the number of significant bits for | ||
1199 | (PNG_INFO_sBIT) each of the gray, | ||
1200 | red, green, and blue channels, | ||
1201 | whichever are appropriate for the | ||
1202 | given color type (png_color_16) | ||
1203 | |||
1204 | png_get_tRNS(png_ptr, info_ptr, &trans_alpha, | ||
1205 | &num_trans, &trans_color); | ||
1206 | |||
1207 | trans_alpha - array of alpha (transparency) | ||
1208 | entries for palette (PNG_INFO_tRNS) | ||
1209 | |||
1210 | num_trans - number of transparent entries | ||
1211 | (PNG_INFO_tRNS) | ||
1212 | |||
1213 | trans_color - graylevel or color sample values of | ||
1214 | the single transparent color for | ||
1215 | non-paletted images (PNG_INFO_tRNS) | ||
1216 | |||
1217 | png_get_hIST(png_ptr, info_ptr, &hist); | ||
1218 | (PNG_INFO_hIST) | ||
1219 | |||
1220 | hist - histogram of palette (array of | ||
1221 | png_uint_16) | ||
1222 | |||
1223 | png_get_tIME(png_ptr, info_ptr, &mod_time); | ||
1224 | |||
1225 | mod_time - time image was last modified | ||
1226 | (PNG_VALID_tIME) | ||
1227 | |||
1228 | png_get_bKGD(png_ptr, info_ptr, &background); | ||
1229 | |||
1230 | background - background color (of type | ||
1231 | png_color_16p) (PNG_VALID_bKGD) | ||
1232 | valid 16-bit red, green and blue | ||
1233 | values, regardless of color_type | ||
1234 | |||
1235 | num_comments = png_get_text(png_ptr, info_ptr, | ||
1236 | &text_ptr, &num_text); | ||
1237 | |||
1238 | num_comments - number of comments | ||
1239 | |||
1240 | text_ptr - array of png_text holding image | ||
1241 | comments | ||
1242 | |||
1243 | text_ptr[i].compression - type of compression used | ||
1244 | on "text" PNG_TEXT_COMPRESSION_NONE | ||
1245 | PNG_TEXT_COMPRESSION_zTXt | ||
1246 | PNG_ITXT_COMPRESSION_NONE | ||
1247 | PNG_ITXT_COMPRESSION_zTXt | ||
1248 | |||
1249 | text_ptr[i].key - keyword for comment. Must contain | ||
1250 | 1-79 characters. | ||
1251 | |||
1252 | text_ptr[i].text - text comments for current | ||
1253 | keyword. Can be empty. | ||
1254 | |||
1255 | text_ptr[i].text_length - length of text string, | ||
1256 | after decompression, 0 for iTXt | ||
1257 | |||
1258 | text_ptr[i].itxt_length - length of itxt string, | ||
1259 | after decompression, 0 for tEXt/zTXt | ||
1260 | |||
1261 | text_ptr[i].lang - language of comment (empty | ||
1262 | string for unknown). | ||
1263 | |||
1264 | text_ptr[i].lang_key - keyword in UTF-8 | ||
1265 | (empty string for unknown). | ||
1266 | |||
1267 | Note that the itxt_length, lang, and lang_key | ||
1268 | members of the text_ptr structure only exist when the | ||
1269 | library is built with iTXt chunk support. Prior to | ||
1270 | libpng-1.4.0 the library was built by default without | ||
1271 | iTXt support. Also note that when iTXt is supported, | ||
1272 | they contain NULL pointers when the "compression" | ||
1273 | field contains PNG_TEXT_COMPRESSION_NONE or | ||
1274 | PNG_TEXT_COMPRESSION_zTXt. | ||
1275 | |||
1276 | num_text - number of comments (same as | ||
1277 | num_comments; you can put NULL here | ||
1278 | to avoid the duplication) | ||
1279 | |||
1280 | Note while png_set_text() will accept text, language, | ||
1281 | and translated keywords that can be NULL pointers, the | ||
1282 | structure returned by png_get_text will always contain | ||
1283 | regular zero-terminated C strings. They might be | ||
1284 | empty strings but they will never be NULL pointers. | ||
1285 | |||
1286 | num_spalettes = png_get_sPLT(png_ptr, info_ptr, | ||
1287 | &palette_ptr); | ||
1288 | |||
1289 | num_spalettes - number of sPLT chunks read. | ||
1290 | |||
1291 | palette_ptr - array of palette structures holding | ||
1292 | contents of one or more sPLT chunks | ||
1293 | read. | ||
1294 | |||
1295 | png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y, | ||
1296 | &unit_type); | ||
1297 | |||
1298 | offset_x - positive offset from the left edge | ||
1299 | of the screen (can be negative) | ||
1300 | |||
1301 | offset_y - positive offset from the top edge | ||
1302 | of the screen (can be negative) | ||
1303 | |||
1304 | unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER | ||
1305 | |||
1306 | png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, | ||
1307 | &unit_type); | ||
1308 | |||
1309 | res_x - pixels/unit physical resolution in | ||
1310 | x direction | ||
1311 | |||
1312 | res_y - pixels/unit physical resolution in | ||
1313 | x direction | ||
1314 | |||
1315 | unit_type - PNG_RESOLUTION_UNKNOWN, | ||
1316 | PNG_RESOLUTION_METER | ||
1317 | |||
1318 | png_get_sCAL(png_ptr, info_ptr, &unit, &width, | ||
1319 | &height) | ||
1320 | |||
1321 | unit - physical scale units (an integer) | ||
1322 | |||
1323 | width - width of a pixel in physical scale units | ||
1324 | |||
1325 | height - height of a pixel in physical scale units | ||
1326 | (width and height are doubles) | ||
1327 | |||
1328 | png_get_sCAL_s(png_ptr, info_ptr, &unit, &width, | ||
1329 | &height) | ||
1330 | |||
1331 | unit - physical scale units (an integer) | ||
1332 | |||
1333 | width - width of a pixel in physical scale units | ||
1334 | (expressed as a string) | ||
1335 | |||
1336 | height - height of a pixel in physical scale units | ||
1337 | (width and height are strings like "2.54") | ||
1338 | |||
1339 | num_unknown_chunks = png_get_unknown_chunks(png_ptr, | ||
1340 | info_ptr, &unknowns) | ||
1341 | |||
1342 | unknowns - array of png_unknown_chunk | ||
1343 | structures holding unknown chunks | ||
1344 | |||
1345 | unknowns[i].name - name of unknown chunk | ||
1346 | |||
1347 | unknowns[i].data - data of unknown chunk | ||
1348 | |||
1349 | unknowns[i].size - size of unknown chunk's data | ||
1350 | |||
1351 | unknowns[i].location - position of chunk in file | ||
1352 | |||
1353 | The value of "i" corresponds to the order in which the | ||
1354 | chunks were read from the PNG file or inserted with the | ||
1355 | png_set_unknown_chunks() function. | ||
1356 | |||
1357 | The value of "location" is a bitwise "or" of | ||
1358 | |||
1359 | PNG_HAVE_IHDR (0x01) | ||
1360 | PNG_HAVE_PLTE (0x02) | ||
1361 | PNG_AFTER_IDAT (0x08) | ||
1362 | |||
1363 | The data from the pHYs chunk can be retrieved in several convenient | ||
1364 | forms: | ||
1365 | |||
1366 | res_x = png_get_x_pixels_per_meter(png_ptr, | ||
1367 | info_ptr) | ||
1368 | |||
1369 | res_y = png_get_y_pixels_per_meter(png_ptr, | ||
1370 | info_ptr) | ||
1371 | |||
1372 | res_x_and_y = png_get_pixels_per_meter(png_ptr, | ||
1373 | info_ptr) | ||
1374 | |||
1375 | res_x = png_get_x_pixels_per_inch(png_ptr, | ||
1376 | info_ptr) | ||
1377 | |||
1378 | res_y = png_get_y_pixels_per_inch(png_ptr, | ||
1379 | info_ptr) | ||
1380 | |||
1381 | res_x_and_y = png_get_pixels_per_inch(png_ptr, | ||
1382 | info_ptr) | ||
1383 | |||
1384 | aspect_ratio = png_get_pixel_aspect_ratio(png_ptr, | ||
1385 | info_ptr) | ||
1386 | |||
1387 | Each of these returns 0 [signifying "unknown"] if | ||
1388 | the data is not present or if res_x is 0; | ||
1389 | res_x_and_y is 0 if res_x != res_y | ||
1390 | |||
1391 | Note that because of the way the resolutions are | ||
1392 | stored internally, the inch conversions won't | ||
1393 | come out to exactly even number. For example, | ||
1394 | 72 dpi is stored as 0.28346 pixels/meter, and | ||
1395 | when this is retrieved it is 71.9988 dpi, so | ||
1396 | be sure to round the returned value appropriately | ||
1397 | if you want to display a reasonable-looking result. | ||
1398 | |||
1399 | The data from the oFFs chunk can be retrieved in several convenient | ||
1400 | forms: | ||
1401 | |||
1402 | x_offset = png_get_x_offset_microns(png_ptr, info_ptr); | ||
1403 | |||
1404 | y_offset = png_get_y_offset_microns(png_ptr, info_ptr); | ||
1405 | |||
1406 | x_offset = png_get_x_offset_inches(png_ptr, info_ptr); | ||
1407 | |||
1408 | y_offset = png_get_y_offset_inches(png_ptr, info_ptr); | ||
1409 | |||
1410 | Each of these returns 0 [signifying "unknown" if both | ||
1411 | x and y are 0] if the data is not present or if the | ||
1412 | chunk is present but the unit is the pixel. The | ||
1413 | remark about inexact inch conversions applies here | ||
1414 | as well, because a value in inches can't always be | ||
1415 | converted to microns and back without some loss | ||
1416 | of precision. | ||
1417 | |||
1418 | For more information, see the | ||
1419 | PNG specification for chunk contents. Be careful with trusting | ||
1420 | rowbytes, as some of the transformations could increase the space | ||
1421 | needed to hold a row (expand, filler, gray_to_rgb, etc.). | ||
1422 | See png_read_update_info(), below. | ||
1423 | |||
1424 | A quick word about text_ptr and num_text. PNG stores comments in | ||
1425 | keyword/text pairs, one pair per chunk, with no limit on the number | ||
1426 | of text chunks, and a 2^31 byte limit on their size. While there are | ||
1427 | suggested keywords, there is no requirement to restrict the use to these | ||
1428 | strings. It is strongly suggested that keywords and text be sensible | ||
1429 | to humans (that's the point), so don't use abbreviations. Non-printing | ||
1430 | symbols are not allowed. See the PNG specification for more details. | ||
1431 | There is also no requirement to have text after the keyword. | ||
1432 | |||
1433 | Keywords should be limited to 79 Latin-1 characters without leading or | ||
1434 | trailing spaces, but non-consecutive spaces are allowed within the | ||
1435 | keyword. It is possible to have the same keyword any number of times. | ||
1436 | The text_ptr is an array of png_text structures, each holding a | ||
1437 | pointer to a language string, a pointer to a keyword and a pointer to | ||
1438 | a text string. The text string, language code, and translated | ||
1439 | keyword may be empty or NULL pointers. The keyword/text | ||
1440 | pairs are put into the array in the order that they are received. | ||
1441 | However, some or all of the text chunks may be after the image, so, to | ||
1442 | make sure you have read all the text chunks, don't mess with these | ||
1443 | until after you read the stuff after the image. This will be | ||
1444 | mentioned again below in the discussion that goes with png_read_end(). | ||
1445 | |||
1446 | Input transformations | ||
1447 | |||
1448 | After you've read the header information, you can set up the library | ||
1449 | to handle any special transformations of the image data. The various | ||
1450 | ways to transform the data will be described in the order that they | ||
1451 | should occur. This is important, as some of these change the color | ||
1452 | type and/or bit depth of the data, and some others only work on | ||
1453 | certain color types and bit depths. | ||
1454 | |||
1455 | Transformations you request are ignored if they don't have any meaning for a | ||
1456 | particular input data format. However some transformations can have an effect | ||
1457 | as a result of a previous transformation. If you specify a contradictory set of | ||
1458 | transformations, for example both adding and removing the alpha channel, you | ||
1459 | cannot predict the final result. | ||
1460 | |||
1461 | The color used for the transparency values should be supplied in the same | ||
1462 | format/depth as the current image data. It is stored in the same format/depth | ||
1463 | as the image data in a tRNS chunk, so this is what libpng expects for this data. | ||
1464 | |||
1465 | The color used for the background value depends on the need_expand argument as | ||
1466 | described below. | ||
1467 | |||
1468 | Data will be decoded into the supplied row buffers packed into bytes | ||
1469 | unless the library has been told to transform it into another format. | ||
1470 | For example, 4 bit/pixel paletted or grayscale data will be returned | ||
1471 | 2 pixels/byte with the leftmost pixel in the high-order bits of the | ||
1472 | byte, unless png_set_packing() is called. 8-bit RGB data will be stored | ||
1473 | in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha() | ||
1474 | is called to insert filler bytes, either before or after each RGB triplet. | ||
1475 | 16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant | ||
1476 | byte of the color value first, unless png_set_scale_16() is called to | ||
1477 | transform it to regular RGB RGB triplets, or png_set_filler() or | ||
1478 | png_set_add alpha() is called to insert filler bytes, either before or | ||
1479 | after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can | ||
1480 | be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(), | ||
1481 | or png_set_scale_16(). | ||
1482 | |||
1483 | The following code transforms grayscale images of less than 8 to 8 bits, | ||
1484 | changes paletted images to RGB, and adds a full alpha channel if there is | ||
1485 | transparency information in a tRNS chunk. This is most useful on | ||
1486 | grayscale images with bit depths of 2 or 4 or if there is a multiple-image | ||
1487 | viewing application that wishes to treat all images in the same way. | ||
1488 | |||
1489 | if (color_type == PNG_COLOR_TYPE_PALETTE) | ||
1490 | png_set_palette_to_rgb(png_ptr); | ||
1491 | |||
1492 | if (png_get_valid(png_ptr, info_ptr, | ||
1493 | PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr); | ||
1494 | |||
1495 | if (color_type == PNG_COLOR_TYPE_GRAY && | ||
1496 | bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr); | ||
1497 | |||
1498 | The first two functions are actually aliases for png_set_expand(), added | ||
1499 | in libpng version 1.0.4, with the function names expanded to improve code | ||
1500 | readability. In some future version they may actually do different | ||
1501 | things. | ||
1502 | |||
1503 | As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was | ||
1504 | added. It expands the sample depth without changing tRNS to alpha. | ||
1505 | |||
1506 | As of libpng version 1.5.2, png_set_expand_16() was added. It behaves as | ||
1507 | png_set_expand(); however, the resultant channels have 16 bits rather than 8. | ||
1508 | Use this when the output color or gray channels are made linear to avoid fairly | ||
1509 | severe accuracy loss. | ||
1510 | |||
1511 | if (bit_depth < 16) | ||
1512 | png_set_expand_16(png_ptr); | ||
1513 | |||
1514 | PNG can have files with 16 bits per channel. If you only can handle | ||
1515 | 8 bits per channel, this will strip the pixels down to 8-bit. | ||
1516 | |||
1517 | if (bit_depth == 16) | ||
1518 | #if PNG_LIBPNG_VER >= 10504 | ||
1519 | png_set_scale_16(png_ptr); | ||
1520 | #else | ||
1521 | png_set_strip_16(png_ptr); | ||
1522 | #endif | ||
1523 | |||
1524 | (The more accurate "png_set_scale_16()" API became available in libpng version | ||
1525 | 1.5.4). | ||
1526 | |||
1527 | If you need to process the alpha channel on the image separately from the image | ||
1528 | data (for example if you convert it to a bitmap mask) it is possible to have | ||
1529 | libpng strip the channel leaving just RGB or gray data: | ||
1530 | |||
1531 | if (color_type & PNG_COLOR_MASK_ALPHA) | ||
1532 | png_set_strip_alpha(png_ptr); | ||
1533 | |||
1534 | If you strip the alpha channel you need to find some other way of dealing with | ||
1535 | the information. If, instead, you want to convert the image to an opaque | ||
1536 | version with no alpha channel use png_set_background; see below. | ||
1537 | |||
1538 | As of libpng version 1.5.2, almost all useful expansions are supported, the | ||
1539 | major ommissions are conversion of grayscale to indexed images (which can be | ||
1540 | done trivially in the application) and conversion of indexed to grayscale (which | ||
1541 | can be done by a trivial manipulation of the palette.) | ||
1542 | |||
1543 | In the following table, the 01 means grayscale with depth<8, 31 means | ||
1544 | indexed with depth<8, other numerals represent the color type, "T" means | ||
1545 | the tRNS chunk is present, A means an alpha channel is present, and O | ||
1546 | means tRNS or alpha is present but all pixels in the image are opaque. | ||
1547 | |||
1548 | FROM 01 31 0 0T 0O 2 2T 2O 3 3T 3O 4A 4O 6A 6O | ||
1549 | TO | ||
1550 | 01 - [G] - - - - - - - - - - - - - | ||
1551 | 31 [Q] Q [Q] [Q] [Q] Q Q Q Q Q Q [Q] [Q] Q Q | ||
1552 | 0 1 G + . . G G G G G G B B GB GB | ||
1553 | 0T lt Gt t + . Gt G G Gt G G Bt Bt GBt GBt | ||
1554 | 0O lt Gt t . + Gt Gt G Gt Gt G Bt Bt GBt GBt | ||
1555 | 2 C P C C C + . . C - - CB CB B B | ||
1556 | 2T Ct - Ct C C t + t - - - CBt CBt Bt Bt | ||
1557 | 2O Ct - Ct C C t t + - - - CBt CBt Bt Bt | ||
1558 | 3 [Q] p [Q] [Q] [Q] Q Q Q + . . [Q] [Q] Q Q | ||
1559 | 3T [Qt] p [Qt][Q] [Q] Qt Qt Qt t + t [Qt][Qt] Qt Qt | ||
1560 | 3O [Qt] p [Qt][Q] [Q] Qt Qt Qt t t + [Qt][Qt] Qt Qt | ||
1561 | 4A lA G A T T GA GT GT GA GT GT + BA G GBA | ||
1562 | 4O lA GBA A T T GA GT GT GA GT GT BA + GBA G | ||
1563 | 6A CA PA CA C C A T tT PA P P C CBA + BA | ||
1564 | 6O CA PBA CA C C A tT T PA P P CBA C BA + | ||
1565 | |||
1566 | Within the matrix, | ||
1567 | "+" identifies entries where 'from' and 'to' are the same. | ||
1568 | "-" means the transformation is not supported. | ||
1569 | "." means nothing is necessary (a tRNS chunk can just be ignored). | ||
1570 | "t" means the transformation is obtained by png_set_tRNS. | ||
1571 | "A" means the transformation is obtained by png_set_add_alpha(). | ||
1572 | "X" means the transformation is obtained by png_set_expand(). | ||
1573 | "1" means the transformation is obtained by | ||
1574 | png_set_expand_gray_1_2_4_to_8() (and by png_set_expand() if there | ||
1575 | is no transparency in the original or the final format). | ||
1576 | "C" means the transformation is obtained by png_set_gray_to_rgb(). | ||
1577 | "G" means the transformation is obtained by png_set_rgb_to_gray(). | ||
1578 | "P" means the transformation is obtained by | ||
1579 | png_set_expand_palette_to_rgb(). | ||
1580 | "p" means the transformation is obtained by png_set_packing(). | ||
1581 | "Q" means the transformation is obtained by png_set_quantize(). | ||
1582 | "T" means the transformation is obtained by png_set_tRNS_to_alpha(). | ||
1583 | "B" means the transformation is obtained by png_set_background(), or | ||
1584 | png_strip_alpha(). | ||
1585 | |||
1586 | When an entry has multiple transforms listed all are required to cause the | ||
1587 | right overall transformation. When two transforms are separated by a comma | ||
1588 | either will do the job. When transforms are enclosed in [] the transform should | ||
1589 | do the job but this is currently unimplemented - a different format will result | ||
1590 | if the suggested transformations are used. | ||
1591 | |||
1592 | In PNG files, the alpha channel in an image | ||
1593 | is the level of opacity. If you need the alpha channel in an image to | ||
1594 | be the level of transparency instead of opacity, you can invert the | ||
1595 | alpha channel (or the tRNS chunk data) after it's read, so that 0 is | ||
1596 | fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit | ||
1597 | images) is fully transparent, with | ||
1598 | |||
1599 | png_set_invert_alpha(png_ptr); | ||
1600 | |||
1601 | PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as | ||
1602 | they can, resulting in, for example, 8 pixels per byte for 1 bit | ||
1603 | files. This code expands to 1 pixel per byte without changing the | ||
1604 | values of the pixels: | ||
1605 | |||
1606 | if (bit_depth < 8) | ||
1607 | png_set_packing(png_ptr); | ||
1608 | |||
1609 | PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels | ||
1610 | stored in a PNG image have been "scaled" or "shifted" up to the next | ||
1611 | higher possible bit depth (e.g. from 5 bits/sample in the range [0,31] | ||
1612 | to 8 bits/sample in the range [0, 255]). However, it is also possible | ||
1613 | to convert the PNG pixel data back to the original bit depth of the | ||
1614 | image. This call reduces the pixels back down to the original bit depth: | ||
1615 | |||
1616 | png_color_8p sig_bit; | ||
1617 | |||
1618 | if (png_get_sBIT(png_ptr, info_ptr, &sig_bit)) | ||
1619 | png_set_shift(png_ptr, sig_bit); | ||
1620 | |||
1621 | PNG files store 3-color pixels in red, green, blue order. This code | ||
1622 | changes the storage of the pixels to blue, green, red: | ||
1623 | |||
1624 | if (color_type == PNG_COLOR_TYPE_RGB || | ||
1625 | color_type == PNG_COLOR_TYPE_RGB_ALPHA) | ||
1626 | png_set_bgr(png_ptr); | ||
1627 | |||
1628 | PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them | ||
1629 | into 4 or 8 bytes for windowing systems that need them in this format: | ||
1630 | |||
1631 | if (color_type == PNG_COLOR_TYPE_RGB) | ||
1632 | png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE); | ||
1633 | |||
1634 | where "filler" is the 8 or 16-bit number to fill with, and the location is | ||
1635 | either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether | ||
1636 | you want the filler before the RGB or after. This transformation | ||
1637 | does not affect images that already have full alpha channels. To add an | ||
1638 | opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which | ||
1639 | will generate RGBA pixels. | ||
1640 | |||
1641 | Note that png_set_filler() does not change the color type. If you want | ||
1642 | to do that, you can add a true alpha channel with | ||
1643 | |||
1644 | if (color_type == PNG_COLOR_TYPE_RGB || | ||
1645 | color_type == PNG_COLOR_TYPE_GRAY) | ||
1646 | png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER); | ||
1647 | |||
1648 | where "filler" contains the alpha value to assign to each pixel. | ||
1649 | This function was added in libpng-1.2.7. | ||
1650 | |||
1651 | If you are reading an image with an alpha channel, and you need the | ||
1652 | data as ARGB instead of the normal PNG format RGBA: | ||
1653 | |||
1654 | if (color_type == PNG_COLOR_TYPE_RGB_ALPHA) | ||
1655 | png_set_swap_alpha(png_ptr); | ||
1656 | |||
1657 | For some uses, you may want a grayscale image to be represented as | ||
1658 | RGB. This code will do that conversion: | ||
1659 | |||
1660 | if (color_type == PNG_COLOR_TYPE_GRAY || | ||
1661 | color_type == PNG_COLOR_TYPE_GRAY_ALPHA) | ||
1662 | png_set_gray_to_rgb(png_ptr); | ||
1663 | |||
1664 | Conversely, you can convert an RGB or RGBA image to grayscale or grayscale | ||
1665 | with alpha. | ||
1666 | |||
1667 | if (color_type == PNG_COLOR_TYPE_RGB || | ||
1668 | color_type == PNG_COLOR_TYPE_RGB_ALPHA) | ||
1669 | png_set_rgb_to_gray(png_ptr, error_action, double red_weight, | ||
1670 | double green_weight); | ||
1671 | |||
1672 | error_action = 1: silently do the conversion | ||
1673 | |||
1674 | error_action = 2: issue a warning if the original | ||
1675 | image has any pixel where | ||
1676 | red != green or red != blue | ||
1677 | |||
1678 | error_action = 3: issue an error and abort the | ||
1679 | conversion if the original | ||
1680 | image has any pixel where | ||
1681 | red != green or red != blue | ||
1682 | |||
1683 | red_weight: weight of red component | ||
1684 | |||
1685 | green_weight: weight of green component | ||
1686 | If either weight is negative, default | ||
1687 | weights are used. | ||
1688 | |||
1689 | In the corresponding fixed point API the red_weight and green_weight values are | ||
1690 | simply scaled by 100,000: | ||
1691 | |||
1692 | png_set_rgb_to_gray(png_ptr, error_action, png_fixed_point red_weight, | ||
1693 | png_fixed_point green_weight); | ||
1694 | |||
1695 | If you have set error_action = 1 or 2, you can | ||
1696 | later check whether the image really was gray, after processing | ||
1697 | the image rows, with the png_get_rgb_to_gray_status(png_ptr) function. | ||
1698 | It will return a png_byte that is zero if the image was gray or | ||
1699 | 1 if there were any non-gray pixels. Background and sBIT data | ||
1700 | will be silently converted to grayscale, using the green channel | ||
1701 | data for sBIT, regardless of the error_action setting. | ||
1702 | |||
1703 | The default values come from the PNG file cHRM chunk if present; otherwise, the | ||
1704 | defaults correspond to the ITU-R recommendation 709, and also the sRGB color | ||
1705 | space, as recommended in the Charles Poynton's Colour FAQ, | ||
1706 | <http://www.poynton.com/>, in section 9: | ||
1707 | |||
1708 | <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9> | ||
1709 | |||
1710 | Y = 0.2126 * R + 0.7152 * G + 0.0722 * B | ||
1711 | |||
1712 | Previous versions of this document, 1998 through 2002, recommended a slightly | ||
1713 | different formula: | ||
1714 | |||
1715 | Y = 0.212671 * R + 0.715160 * G + 0.072169 * B | ||
1716 | |||
1717 | Libpng uses an integer approximation: | ||
1718 | |||
1719 | Y = (6968 * R + 23434 * G + 2366 * B)/32768 | ||
1720 | |||
1721 | The calculation is done in a linear colorspace, if the image gamma | ||
1722 | can be determined. | ||
1723 | |||
1724 | The png_set_background() function has been described already; it tells libpng to | ||
1725 | composite images with alpha or simple transparency against the supplied | ||
1726 | background color. For compatibility with versions of libpng earlier than | ||
1727 | libpng-1.5.4 it is recommended that you call the function after reading the file | ||
1728 | header, even if you don't want to use the color in a bKGD chunk, if one exists. | ||
1729 | |||
1730 | If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid), | ||
1731 | you may use this color, or supply another color more suitable for | ||
1732 | the current display (e.g., the background color from a web page). You | ||
1733 | need to tell libpng how the color is represented, both the format of the | ||
1734 | component values in the color (the number of bits) and the gamma encoding of the | ||
1735 | color. The function takes two arguments, background_gamma_mode and need_expand | ||
1736 | to convey this information, however only two combinations are likely to be | ||
1737 | useful: | ||
1738 | |||
1739 | png_color_16 my_background; | ||
1740 | png_color_16p image_background; | ||
1741 | |||
1742 | if (png_get_bKGD(png_ptr, info_ptr, &image_background)) | ||
1743 | png_set_background(png_ptr, image_background, | ||
1744 | PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1); | ||
1745 | else | ||
1746 | png_set_background(png_ptr, &my_background, | ||
1747 | PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1); | ||
1748 | |||
1749 | The second call was described above - my_background is in the format of the | ||
1750 | final, display, output produced by libpng. Because you now know the format of | ||
1751 | the PNG it is possible to avoid the need to choose either 8-bit or 16-bit | ||
1752 | output and to retain palette images (the palette colors will be modified | ||
1753 | appropriately and the tRNS chunk removed.) However, if you are doing this, | ||
1754 | take great care not to ask for transformations without checking first that | ||
1755 | they apply! | ||
1756 | |||
1757 | In the first call the background color has the original bit depth and color type | ||
1758 | of the PNG file. So, for palette images the color is supplied as a palette | ||
1759 | index and for low bit greyscale images the color is a reduced bit value in | ||
1760 | image_background->gray. | ||
1761 | |||
1762 | If you didn't call png_set_gamma() before reading the file header, for example | ||
1763 | if you need your code to remain compatible with older versions of libpng prior | ||
1764 | to libpng-1.5.4, this is the place to call it. | ||
1765 | |||
1766 | Do not call it if you called png_set_alpha_mode(); doing so will damage the | ||
1767 | settings put in place by png_set_alpha_mode(). (If png_set_alpha_mode() is | ||
1768 | supported then you can certainly do png_set_gamma() before reading the PNG | ||
1769 | header.) | ||
1770 | |||
1771 | This API unconditionally sets the screen and file gamma values, so it will | ||
1772 | override the value in the PNG file unless it is called before the PNG file | ||
1773 | reading starts. For this reason you must always call it with the PNG file | ||
1774 | value when you call it in this position: | ||
1775 | |||
1776 | if (png_get_gAMA(png_ptr, info_ptr, &file_gamma)) | ||
1777 | png_set_gamma(png_ptr, screen_gamma, file_gamma); | ||
1778 | |||
1779 | else | ||
1780 | png_set_gamma(png_ptr, screen_gamma, 0.45455); | ||
1781 | |||
1782 | If you need to reduce an RGB file to a paletted file, or if a paletted | ||
1783 | file has more entries then will fit on your screen, png_set_quantize() | ||
1784 | will do that. Note that this is a simple match quantization that merely | ||
1785 | finds the closest color available. This should work fairly well with | ||
1786 | optimized palettes, but fairly badly with linear color cubes. If you | ||
1787 | pass a palette that is larger than maximum_colors, the file will | ||
1788 | reduce the number of colors in the palette so it will fit into | ||
1789 | maximum_colors. If there is a histogram, libpng will use it to make | ||
1790 | more intelligent choices when reducing the palette. If there is no | ||
1791 | histogram, it may not do as good a job. | ||
1792 | |||
1793 | if (color_type & PNG_COLOR_MASK_COLOR) | ||
1794 | { | ||
1795 | if (png_get_valid(png_ptr, info_ptr, | ||
1796 | PNG_INFO_PLTE)) | ||
1797 | { | ||
1798 | png_uint_16p histogram = NULL; | ||
1799 | |||
1800 | png_get_hIST(png_ptr, info_ptr, | ||
1801 | &histogram); | ||
1802 | png_set_quantize(png_ptr, palette, num_palette, | ||
1803 | max_screen_colors, histogram, 1); | ||
1804 | } | ||
1805 | |||
1806 | else | ||
1807 | { | ||
1808 | png_color std_color_cube[MAX_SCREEN_COLORS] = | ||
1809 | { ... colors ... }; | ||
1810 | |||
1811 | png_set_quantize(png_ptr, std_color_cube, | ||
1812 | MAX_SCREEN_COLORS, MAX_SCREEN_COLORS, | ||
1813 | NULL,0); | ||
1814 | } | ||
1815 | } | ||
1816 | |||
1817 | PNG files describe monochrome as black being zero and white being one. | ||
1818 | The following code will reverse this (make black be one and white be | ||
1819 | zero): | ||
1820 | |||
1821 | if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY) | ||
1822 | png_set_invert_mono(png_ptr); | ||
1823 | |||
1824 | This function can also be used to invert grayscale and gray-alpha images: | ||
1825 | |||
1826 | if (color_type == PNG_COLOR_TYPE_GRAY || | ||
1827 | color_type == PNG_COLOR_TYPE_GRAY_ALPHA) | ||
1828 | png_set_invert_mono(png_ptr); | ||
1829 | |||
1830 | PNG files store 16-bit pixels in network byte order (big-endian, | ||
1831 | ie. most significant bits first). This code changes the storage to the | ||
1832 | other way (little-endian, i.e. least significant bits first, the | ||
1833 | way PCs store them): | ||
1834 | |||
1835 | if (bit_depth == 16) | ||
1836 | png_set_swap(png_ptr); | ||
1837 | |||
1838 | If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you | ||
1839 | need to change the order the pixels are packed into bytes, you can use: | ||
1840 | |||
1841 | if (bit_depth < 8) | ||
1842 | png_set_packswap(png_ptr); | ||
1843 | |||
1844 | Finally, you can write your own transformation function if none of | ||
1845 | the existing ones meets your needs. This is done by setting a callback | ||
1846 | with | ||
1847 | |||
1848 | png_set_read_user_transform_fn(png_ptr, | ||
1849 | read_transform_fn); | ||
1850 | |||
1851 | You must supply the function | ||
1852 | |||
1853 | void read_transform_fn(png_structp png_ptr, png_row_infop | ||
1854 | row_info, png_bytep data) | ||
1855 | |||
1856 | See pngtest.c for a working example. Your function will be called | ||
1857 | after all of the other transformations have been processed. Take care with | ||
1858 | interlaced images if you do the interlace yourself - the width of the row is the | ||
1859 | width in 'row_info', not the overall image width. | ||
1860 | |||
1861 | If supported, libpng provides two information routines that you can use to find | ||
1862 | where you are in processing the image: | ||
1863 | |||
1864 | png_get_current_pass_number(png_structp png_ptr); | ||
1865 | png_get_current_row_number(png_structp png_ptr); | ||
1866 | |||
1867 | Don't try using these outside a transform callback - firstly they are only | ||
1868 | supported if user transforms are supported, secondly they may well return | ||
1869 | unexpected results unless the row is actually being processed at the moment they | ||
1870 | are called. | ||
1871 | |||
1872 | With interlaced | ||
1873 | images the value returned is the row in the input sub-image image. Use | ||
1874 | PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to | ||
1875 | find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass). | ||
1876 | |||
1877 | The discussion of interlace handling above contains more information on how to | ||
1878 | use these values. | ||
1879 | |||
1880 | You can also set up a pointer to a user structure for use by your | ||
1881 | callback function, and you can inform libpng that your transform | ||
1882 | function will change the number of channels or bit depth with the | ||
1883 | function | ||
1884 | |||
1885 | png_set_user_transform_info(png_ptr, user_ptr, | ||
1886 | user_depth, user_channels); | ||
1887 | |||
1888 | The user's application, not libpng, is responsible for allocating and | ||
1889 | freeing any memory required for the user structure. | ||
1890 | |||
1891 | You can retrieve the pointer via the function | ||
1892 | png_get_user_transform_ptr(). For example: | ||
1893 | |||
1894 | voidp read_user_transform_ptr = | ||
1895 | png_get_user_transform_ptr(png_ptr); | ||
1896 | |||
1897 | The last thing to handle is interlacing; this is covered in detail below, | ||
1898 | but you must call the function here if you want libpng to handle expansion | ||
1899 | of the interlaced image. | ||
1900 | |||
1901 | number_of_passes = png_set_interlace_handling(png_ptr); | ||
1902 | |||
1903 | After setting the transformations, libpng can update your png_info | ||
1904 | structure to reflect any transformations you've requested with this | ||
1905 | call. | ||
1906 | |||
1907 | png_read_update_info(png_ptr, info_ptr); | ||
1908 | |||
1909 | This is most useful to update the info structure's rowbytes | ||
1910 | field so you can use it to allocate your image memory. This function | ||
1911 | will also update your palette with the correct screen_gamma and | ||
1912 | background if these have been given with the calls above. You may | ||
1913 | only call png_read_update_info() once with a particular info_ptr. | ||
1914 | |||
1915 | After you call png_read_update_info(), you can allocate any | ||
1916 | memory you need to hold the image. The row data is simply | ||
1917 | raw byte data for all forms of images. As the actual allocation | ||
1918 | varies among applications, no example will be given. If you | ||
1919 | are allocating one large chunk, you will need to build an | ||
1920 | array of pointers to each row, as it will be needed for some | ||
1921 | of the functions below. | ||
1922 | |||
1923 | Remember: Before you call png_read_update_info(), the png_get_*() | ||
1924 | functions return the values corresponding to the original PNG image. | ||
1925 | After you call png_read_update_info the values refer to the image | ||
1926 | that libpng will output. Consequently you must call all the png_set_ | ||
1927 | functions before you call png_read_update_info(). This is particularly | ||
1928 | important for png_set_interlace_handling() - if you are going to call | ||
1929 | png_read_update_info() you must call png_set_interlace_handling() before | ||
1930 | it unless you want to receive interlaced output. | ||
1931 | |||
1932 | Reading image data | ||
1933 | |||
1934 | After you've allocated memory, you can read the image data. | ||
1935 | The simplest way to do this is in one function call. If you are | ||
1936 | allocating enough memory to hold the whole image, you can just | ||
1937 | call png_read_image() and libpng will read in all the image data | ||
1938 | and put it in the memory area supplied. You will need to pass in | ||
1939 | an array of pointers to each row. | ||
1940 | |||
1941 | This function automatically handles interlacing, so you don't | ||
1942 | need to call png_set_interlace_handling() (unless you call | ||
1943 | png_read_update_info()) or call this function multiple times, or any | ||
1944 | of that other stuff necessary with png_read_rows(). | ||
1945 | |||
1946 | png_read_image(png_ptr, row_pointers); | ||
1947 | |||
1948 | where row_pointers is: | ||
1949 | |||
1950 | png_bytep row_pointers[height]; | ||
1951 | |||
1952 | You can point to void or char or whatever you use for pixels. | ||
1953 | |||
1954 | If you don't want to read in the whole image at once, you can | ||
1955 | use png_read_rows() instead. If there is no interlacing (check | ||
1956 | interlace_type == PNG_INTERLACE_NONE), this is simple: | ||
1957 | |||
1958 | png_read_rows(png_ptr, row_pointers, NULL, | ||
1959 | number_of_rows); | ||
1960 | |||
1961 | where row_pointers is the same as in the png_read_image() call. | ||
1962 | |||
1963 | If you are doing this just one row at a time, you can do this with | ||
1964 | a single row_pointer instead of an array of row_pointers: | ||
1965 | |||
1966 | png_bytep row_pointer = row; | ||
1967 | png_read_row(png_ptr, row_pointer, NULL); | ||
1968 | |||
1969 | If the file is interlaced (interlace_type != 0 in the IHDR chunk), things | ||
1970 | get somewhat harder. The only current (PNG Specification version 1.2) | ||
1971 | interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7); | ||
1972 | a somewhat complicated 2D interlace scheme, known as Adam7, that | ||
1973 | breaks down an image into seven smaller images of varying size, based | ||
1974 | on an 8x8 grid. This number is defined (from libpng 1.5) as | ||
1975 | PNG_INTERLACE_ADAM7_PASSES in png.h | ||
1976 | |||
1977 | libpng can fill out those images or it can give them to you "as is". | ||
1978 | It is almost always better to have libpng handle the interlacing for you. | ||
1979 | If you want the images filled out, there are two ways to do that. The one | ||
1980 | mentioned in the PNG specification is to expand each pixel to cover | ||
1981 | those pixels that have not been read yet (the "rectangle" method). | ||
1982 | This results in a blocky image for the first pass, which gradually | ||
1983 | smooths out as more pixels are read. The other method is the "sparkle" | ||
1984 | method, where pixels are drawn only in their final locations, with the | ||
1985 | rest of the image remaining whatever colors they were initialized to | ||
1986 | before the start of the read. The first method usually looks better, | ||
1987 | but tends to be slower, as there are more pixels to put in the rows. | ||
1988 | |||
1989 | If, as is likely, you want libpng to expand the images, call this before | ||
1990 | calling png_start_read_image() or png_read_update_info(): | ||
1991 | |||
1992 | if (interlace_type == PNG_INTERLACE_ADAM7) | ||
1993 | number_of_passes | ||
1994 | = png_set_interlace_handling(png_ptr); | ||
1995 | |||
1996 | This will return the number of passes needed. Currently, this is seven, | ||
1997 | but may change if another interlace type is added. This function can be | ||
1998 | called even if the file is not interlaced, where it will return one pass. | ||
1999 | You then need to read the whole image 'number_of_passes' times. Each time | ||
2000 | will distribute the pixels from the current pass to the correct place in | ||
2001 | the output image, so you need to supply the same rows to png_read_rows in | ||
2002 | each pass. | ||
2003 | |||
2004 | If you are not going to display the image after each pass, but are | ||
2005 | going to wait until the entire image is read in, use the sparkle | ||
2006 | effect. This effect is faster and the end result of either method | ||
2007 | is exactly the same. If you are planning on displaying the image | ||
2008 | after each pass, the "rectangle" effect is generally considered the | ||
2009 | better looking one. | ||
2010 | |||
2011 | If you only want the "sparkle" effect, just call png_read_rows() as | ||
2012 | normal, with the third parameter NULL. Make sure you make pass over | ||
2013 | the image number_of_passes times, and you don't change the data in the | ||
2014 | rows between calls. You can change the locations of the data, just | ||
2015 | not the data. Each pass only writes the pixels appropriate for that | ||
2016 | pass, and assumes the data from previous passes is still valid. | ||
2017 | |||
2018 | png_read_rows(png_ptr, row_pointers, NULL, | ||
2019 | number_of_rows); | ||
2020 | |||
2021 | If you only want the first effect (the rectangles), do the same as | ||
2022 | before except pass the row buffer in the third parameter, and leave | ||
2023 | the second parameter NULL. | ||
2024 | |||
2025 | png_read_rows(png_ptr, NULL, row_pointers, | ||
2026 | number_of_rows); | ||
2027 | |||
2028 | If you don't want libpng to handle the interlacing details, just call | ||
2029 | png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images. | ||
2030 | Each of the images is a valid image by itself, however you will almost | ||
2031 | certainly need to distribute the pixels from each sub-image to the | ||
2032 | correct place. This is where everything gets very tricky. | ||
2033 | |||
2034 | If you want to retrieve the separate images you must pass the correct | ||
2035 | number of rows to each successive call of png_read_rows(). The calculation | ||
2036 | gets pretty complicated for small images, where some sub-images may | ||
2037 | not even exist because either their width or height ends up zero. | ||
2038 | libpng provides two macros to help you in 1.5 and later versions: | ||
2039 | |||
2040 | png_uint_32 width = PNG_PASS_COLS(image_width, pass_number); | ||
2041 | png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number); | ||
2042 | |||
2043 | Respectively these tell you the width and height of the sub-image | ||
2044 | corresponding to the numbered pass. 'pass' is in in the range 0 to 6 - | ||
2045 | this can be confusing because the specification refers to the same passes | ||
2046 | as 1 to 7! Be careful, you must check both the width and height before | ||
2047 | calling png_read_rows() and not call it for that pass if either is zero. | ||
2048 | |||
2049 | You can, of course, read each sub-image row by row. If you want to | ||
2050 | produce optimal code to make a pixel-by-pixel transformation of an | ||
2051 | interlaced image this is the best approach; read each row of each pass, | ||
2052 | transform it, and write it out to a new interlaced image. | ||
2053 | |||
2054 | If you want to de-interlace the image yourself libpng provides further | ||
2055 | macros to help that tell you where to place the pixels in the output image. | ||
2056 | Because the interlacing scheme is rectangular - sub-image pixels are always | ||
2057 | arranged on a rectangular grid - all you need to know for each pass is the | ||
2058 | starting column and row in the output image of the first pixel plus the | ||
2059 | spacing between each pixel. As of libpng 1.5 there are four macros to | ||
2060 | retrieve this information: | ||
2061 | |||
2062 | png_uint_32 x = PNG_PASS_START_COL(pass); | ||
2063 | png_uint_32 y = PNG_PASS_START_ROW(pass); | ||
2064 | png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass); | ||
2065 | png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass); | ||
2066 | |||
2067 | These allow you to write the obvious loop: | ||
2068 | |||
2069 | png_uint_32 input_y = 0; | ||
2070 | png_uint_32 output_y = PNG_PASS_START_ROW(pass); | ||
2071 | |||
2072 | while (output_y < output_image_height) | ||
2073 | { | ||
2074 | png_uint_32 input_x = 0; | ||
2075 | png_uint_32 output_x = PNG_PASS_START_COL(pass); | ||
2076 | |||
2077 | while (output_x < output_image_width) | ||
2078 | { | ||
2079 | image[output_y][output_x] = | ||
2080 | subimage[pass][input_y][input_x++]; | ||
2081 | |||
2082 | output_x += xStep; | ||
2083 | } | ||
2084 | |||
2085 | ++input_y; | ||
2086 | output_y += yStep; | ||
2087 | } | ||
2088 | |||
2089 | Notice that the steps between successive output rows and columns are | ||
2090 | returned as shifts. This is possible because the pixels in the subimages | ||
2091 | are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original | ||
2092 | image. In practice you may need to directly calculate the output coordinate | ||
2093 | given an input coordinate. libpng provides two further macros for this | ||
2094 | purpose: | ||
2095 | |||
2096 | png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass); | ||
2097 | png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass); | ||
2098 | |||
2099 | Finally a pair of macros are provided to tell you if a particular image | ||
2100 | row or column appears in a given pass: | ||
2101 | |||
2102 | int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass); | ||
2103 | int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass); | ||
2104 | |||
2105 | Bear in mind that you will probably also need to check the width and height | ||
2106 | of the pass in addition to the above to be sure the pass even exists! | ||
2107 | |||
2108 | With any luck you are convinced by now that you don't want to do your own | ||
2109 | interlace handling. In reality normally the only good reason for doing this | ||
2110 | is if you are processing PNG files on a pixel-by-pixel basis and don't want | ||
2111 | to load the whole file into memory when it is interlaced. | ||
2112 | |||
2113 | libpng includes a test program, pngvalid, that illustrates reading and | ||
2114 | writing of interlaced images. If you can't get interlacing to work in your | ||
2115 | code and don't want to leave it to libpng (the recommended approach), see | ||
2116 | how pngvalid.c does it. | ||
2117 | |||
2118 | Finishing a sequential read | ||
2119 | |||
2120 | After you are finished reading the image through the | ||
2121 | low-level interface, you can finish reading the file. If you are | ||
2122 | interested in comments or time, which may be stored either before or | ||
2123 | after the image data, you should pass the separate png_info struct if | ||
2124 | you want to keep the comments from before and after the image | ||
2125 | separate. | ||
2126 | |||
2127 | png_infop end_info = png_create_info_struct(png_ptr); | ||
2128 | |||
2129 | if (!end_info) | ||
2130 | { | ||
2131 | png_destroy_read_struct(&png_ptr, &info_ptr, | ||
2132 | (png_infopp)NULL); | ||
2133 | return (ERROR); | ||
2134 | } | ||
2135 | |||
2136 | png_read_end(png_ptr, end_info); | ||
2137 | |||
2138 | If you are not interested, you should still call png_read_end() | ||
2139 | but you can pass NULL, avoiding the need to create an end_info structure. | ||
2140 | |||
2141 | png_read_end(png_ptr, (png_infop)NULL); | ||
2142 | |||
2143 | If you don't call png_read_end(), then your file pointer will be | ||
2144 | left pointing to the first chunk after the last IDAT, which is probably | ||
2145 | not what you want if you expect to read something beyond the end of | ||
2146 | the PNG datastream. | ||
2147 | |||
2148 | When you are done, you can free all memory allocated by libpng like this: | ||
2149 | |||
2150 | png_destroy_read_struct(&png_ptr, &info_ptr, | ||
2151 | &end_info); | ||
2152 | |||
2153 | or, if you didn't create an end_info structure, | ||
2154 | |||
2155 | png_destroy_read_struct(&png_ptr, &info_ptr, | ||
2156 | (png_infopp)NULL); | ||
2157 | |||
2158 | It is also possible to individually free the info_ptr members that | ||
2159 | point to libpng-allocated storage with the following function: | ||
2160 | |||
2161 | png_free_data(png_ptr, info_ptr, mask, seq) | ||
2162 | |||
2163 | mask - identifies data to be freed, a mask | ||
2164 | containing the bitwise OR of one or | ||
2165 | more of | ||
2166 | PNG_FREE_PLTE, PNG_FREE_TRNS, | ||
2167 | PNG_FREE_HIST, PNG_FREE_ICCP, | ||
2168 | PNG_FREE_PCAL, PNG_FREE_ROWS, | ||
2169 | PNG_FREE_SCAL, PNG_FREE_SPLT, | ||
2170 | PNG_FREE_TEXT, PNG_FREE_UNKN, | ||
2171 | or simply PNG_FREE_ALL | ||
2172 | |||
2173 | seq - sequence number of item to be freed | ||
2174 | (-1 for all items) | ||
2175 | |||
2176 | This function may be safely called when the relevant storage has | ||
2177 | already been freed, or has not yet been allocated, or was allocated | ||
2178 | by the user and not by libpng, and will in those cases do nothing. | ||
2179 | The "seq" parameter is ignored if only one item of the selected data | ||
2180 | type, such as PLTE, is allowed. If "seq" is not -1, and multiple items | ||
2181 | are allowed for the data type identified in the mask, such as text or | ||
2182 | sPLT, only the n'th item in the structure is freed, where n is "seq". | ||
2183 | |||
2184 | The default behavior is only to free data that was allocated internally | ||
2185 | by libpng. This can be changed, so that libpng will not free the data, | ||
2186 | or so that it will free data that was allocated by the user with png_malloc() | ||
2187 | or png_zalloc() and passed in via a png_set_*() function, with | ||
2188 | |||
2189 | png_data_freer(png_ptr, info_ptr, freer, mask) | ||
2190 | |||
2191 | freer - one of | ||
2192 | PNG_DESTROY_WILL_FREE_DATA | ||
2193 | PNG_SET_WILL_FREE_DATA | ||
2194 | PNG_USER_WILL_FREE_DATA | ||
2195 | |||
2196 | mask - which data elements are affected | ||
2197 | same choices as in png_free_data() | ||
2198 | |||
2199 | This function only affects data that has already been allocated. | ||
2200 | You can call this function after reading the PNG data but before calling | ||
2201 | any png_set_*() functions, to control whether the user or the png_set_*() | ||
2202 | function is responsible for freeing any existing data that might be present, | ||
2203 | and again after the png_set_*() functions to control whether the user | ||
2204 | or png_destroy_*() is supposed to free the data. When the user assumes | ||
2205 | responsibility for libpng-allocated data, the application must use | ||
2206 | png_free() to free it, and when the user transfers responsibility to libpng | ||
2207 | for data that the user has allocated, the user must have used png_malloc() | ||
2208 | or png_zalloc() to allocate it. | ||
2209 | |||
2210 | If you allocated your row_pointers in a single block, as suggested above in | ||
2211 | the description of the high level read interface, you must not transfer | ||
2212 | responsibility for freeing it to the png_set_rows or png_read_destroy function, | ||
2213 | because they would also try to free the individual row_pointers[i]. | ||
2214 | |||
2215 | If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword | ||
2216 | separately, do not transfer responsibility for freeing text_ptr to libpng, | ||
2217 | because when libpng fills a png_text structure it combines these members with | ||
2218 | the key member, and png_free_data() will free only text_ptr.key. Similarly, | ||
2219 | if you transfer responsibility for free'ing text_ptr from libpng to your | ||
2220 | application, your application must not separately free those members. | ||
2221 | |||
2222 | The png_free_data() function will turn off the "valid" flag for anything | ||
2223 | it frees. If you need to turn the flag off for a chunk that was freed by | ||
2224 | your application instead of by libpng, you can use | ||
2225 | |||
2226 | png_set_invalid(png_ptr, info_ptr, mask); | ||
2227 | |||
2228 | mask - identifies the chunks to be made invalid, | ||
2229 | containing the bitwise OR of one or | ||
2230 | more of | ||
2231 | PNG_INFO_gAMA, PNG_INFO_sBIT, | ||
2232 | PNG_INFO_cHRM, PNG_INFO_PLTE, | ||
2233 | PNG_INFO_tRNS, PNG_INFO_bKGD, | ||
2234 | PNG_INFO_hIST, PNG_INFO_pHYs, | ||
2235 | PNG_INFO_oFFs, PNG_INFO_tIME, | ||
2236 | PNG_INFO_pCAL, PNG_INFO_sRGB, | ||
2237 | PNG_INFO_iCCP, PNG_INFO_sPLT, | ||
2238 | PNG_INFO_sCAL, PNG_INFO_IDAT | ||
2239 | |||
2240 | For a more compact example of reading a PNG image, see the file example.c. | ||
2241 | |||
2242 | Reading PNG files progressively | ||
2243 | |||
2244 | The progressive reader is slightly different then the non-progressive | ||
2245 | reader. Instead of calling png_read_info(), png_read_rows(), and | ||
2246 | png_read_end(), you make one call to png_process_data(), which calls | ||
2247 | callbacks when it has the info, a row, or the end of the image. You | ||
2248 | set up these callbacks with png_set_progressive_read_fn(). You don't | ||
2249 | have to worry about the input/output functions of libpng, as you are | ||
2250 | giving the library the data directly in png_process_data(). I will | ||
2251 | assume that you have read the section on reading PNG files above, | ||
2252 | so I will only highlight the differences (although I will show | ||
2253 | all of the code). | ||
2254 | |||
2255 | png_structp png_ptr; | ||
2256 | png_infop info_ptr; | ||
2257 | |||
2258 | /* An example code fragment of how you would | ||
2259 | initialize the progressive reader in your | ||
2260 | application. */ | ||
2261 | int | ||
2262 | initialize_png_reader() | ||
2263 | { | ||
2264 | png_ptr = png_create_read_struct | ||
2265 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, | ||
2266 | user_error_fn, user_warning_fn); | ||
2267 | |||
2268 | if (!png_ptr) | ||
2269 | return (ERROR); | ||
2270 | |||
2271 | info_ptr = png_create_info_struct(png_ptr); | ||
2272 | |||
2273 | if (!info_ptr) | ||
2274 | { | ||
2275 | png_destroy_read_struct(&png_ptr, | ||
2276 | (png_infopp)NULL, (png_infopp)NULL); | ||
2277 | return (ERROR); | ||
2278 | } | ||
2279 | |||
2280 | if (setjmp(png_jmpbuf(png_ptr))) | ||
2281 | { | ||
2282 | png_destroy_read_struct(&png_ptr, &info_ptr, | ||
2283 | (png_infopp)NULL); | ||
2284 | return (ERROR); | ||
2285 | } | ||
2286 | |||
2287 | /* This one's new. You can provide functions | ||
2288 | to be called when the header info is valid, | ||
2289 | when each row is completed, and when the image | ||
2290 | is finished. If you aren't using all functions, | ||
2291 | you can specify NULL parameters. Even when all | ||
2292 | three functions are NULL, you need to call | ||
2293 | png_set_progressive_read_fn(). You can use | ||
2294 | any struct as the user_ptr (cast to a void pointer | ||
2295 | for the function call), and retrieve the pointer | ||
2296 | from inside the callbacks using the function | ||
2297 | |||
2298 | png_get_progressive_ptr(png_ptr); | ||
2299 | |||
2300 | which will return a void pointer, which you have | ||
2301 | to cast appropriately. | ||
2302 | */ | ||
2303 | png_set_progressive_read_fn(png_ptr, (void *)user_ptr, | ||
2304 | info_callback, row_callback, end_callback); | ||
2305 | |||
2306 | return 0; | ||
2307 | } | ||
2308 | |||
2309 | /* A code fragment that you call as you receive blocks | ||
2310 | of data */ | ||
2311 | int | ||
2312 | process_data(png_bytep buffer, png_uint_32 length) | ||
2313 | { | ||
2314 | if (setjmp(png_jmpbuf(png_ptr))) | ||
2315 | { | ||
2316 | png_destroy_read_struct(&png_ptr, &info_ptr, | ||
2317 | (png_infopp)NULL); | ||
2318 | return (ERROR); | ||
2319 | } | ||
2320 | |||
2321 | /* This one's new also. Simply give it a chunk | ||
2322 | of data from the file stream (in order, of | ||
2323 | course). On machines with segmented memory | ||
2324 | models machines, don't give it any more than | ||
2325 | 64K. The library seems to run fine with sizes | ||
2326 | of 4K. Although you can give it much less if | ||
2327 | necessary (I assume you can give it chunks of | ||
2328 | 1 byte, I haven't tried less then 256 bytes | ||
2329 | yet). When this function returns, you may | ||
2330 | want to display any rows that were generated | ||
2331 | in the row callback if you don't already do | ||
2332 | so there. | ||
2333 | */ | ||
2334 | png_process_data(png_ptr, info_ptr, buffer, length); | ||
2335 | |||
2336 | /* At this point you can call png_process_data_skip if | ||
2337 | you want to handle data the library will skip yourself; | ||
2338 | it simply returns the number of bytes to skip (and stops | ||
2339 | libpng skipping that number of bytes on the next | ||
2340 | png_process_data call). | ||
2341 | return 0; | ||
2342 | } | ||
2343 | |||
2344 | /* This function is called (as set by | ||
2345 | png_set_progressive_read_fn() above) when enough data | ||
2346 | has been supplied so all of the header has been | ||
2347 | read. | ||
2348 | */ | ||
2349 | void | ||
2350 | info_callback(png_structp png_ptr, png_infop info) | ||
2351 | { | ||
2352 | /* Do any setup here, including setting any of | ||
2353 | the transformations mentioned in the Reading | ||
2354 | PNG files section. For now, you _must_ call | ||
2355 | either png_start_read_image() or | ||
2356 | png_read_update_info() after all the | ||
2357 | transformations are set (even if you don't set | ||
2358 | any). You may start getting rows before | ||
2359 | png_process_data() returns, so this is your | ||
2360 | last chance to prepare for that. | ||
2361 | |||
2362 | This is where you turn on interlace handling, | ||
2363 | assuming you don't want to do it yourself. | ||
2364 | |||
2365 | If you need to you can stop the processing of | ||
2366 | your original input data at this point by calling | ||
2367 | png_process_data_pause. This returns the number | ||
2368 | of unprocessed bytes from the last png_process_data | ||
2369 | call - it is up to you to ensure that the next call | ||
2370 | sees these bytes again. If you don't want to bother | ||
2371 | with this you can get libpng to cache the unread | ||
2372 | bytes by setting the 'save' parameter (see png.h) but | ||
2373 | then libpng will have to copy the data internally. | ||
2374 | */ | ||
2375 | } | ||
2376 | |||
2377 | /* This function is called when each row of image | ||
2378 | data is complete */ | ||
2379 | void | ||
2380 | row_callback(png_structp png_ptr, png_bytep new_row, | ||
2381 | png_uint_32 row_num, int pass) | ||
2382 | { | ||
2383 | /* If the image is interlaced, and you turned | ||
2384 | on the interlace handler, this function will | ||
2385 | be called for every row in every pass. Some | ||
2386 | of these rows will not be changed from the | ||
2387 | previous pass. When the row is not changed, | ||
2388 | the new_row variable will be NULL. The rows | ||
2389 | and passes are called in order, so you don't | ||
2390 | really need the row_num and pass, but I'm | ||
2391 | supplying them because it may make your life | ||
2392 | easier. | ||
2393 | |||
2394 | If you did not turn on interlace handling then | ||
2395 | the callback is called for each row of each | ||
2396 | sub-image when the image is interlaced. In this | ||
2397 | case 'row_num' is the row in the sub-image, not | ||
2398 | the row in the output image as it is in all other | ||
2399 | cases. | ||
2400 | |||
2401 | For the non-NULL rows of interlaced images when | ||
2402 | you have switched on libpng interlace handling, | ||
2403 | you must call png_progressive_combine_row() | ||
2404 | passing in the row and the old row. You can | ||
2405 | call this function for NULL rows (it will just | ||
2406 | return) and for non-interlaced images (it just | ||
2407 | does the memcpy for you) if it will make the | ||
2408 | code easier. Thus, you can just do this for | ||
2409 | all cases if you switch on interlace handling; | ||
2410 | */ | ||
2411 | |||
2412 | png_progressive_combine_row(png_ptr, old_row, | ||
2413 | new_row); | ||
2414 | |||
2415 | /* where old_row is what was displayed for | ||
2416 | previously for the row. Note that the first | ||
2417 | pass (pass == 0, really) will completely cover | ||
2418 | the old row, so the rows do not have to be | ||
2419 | initialized. After the first pass (and only | ||
2420 | for interlaced images), you will have to pass | ||
2421 | the current row, and the function will combine | ||
2422 | the old row and the new row. | ||
2423 | |||
2424 | You can also call png_process_data_pause in this | ||
2425 | callback - see above. | ||
2426 | */ | ||
2427 | } | ||
2428 | |||
2429 | void | ||
2430 | end_callback(png_structp png_ptr, png_infop info) | ||
2431 | { | ||
2432 | /* This function is called after the whole image | ||
2433 | has been read, including any chunks after the | ||
2434 | image (up to and including the IEND). You | ||
2435 | will usually have the same info chunk as you | ||
2436 | had in the header, although some data may have | ||
2437 | been added to the comments and time fields. | ||
2438 | |||
2439 | Most people won't do much here, perhaps setting | ||
2440 | a flag that marks the image as finished. | ||
2441 | */ | ||
2442 | } | ||
2443 | |||
2444 | |||
2445 | |||
2446 | IV. Writing | ||
2447 | |||
2448 | Much of this is very similar to reading. However, everything of | ||
2449 | importance is repeated here, so you won't have to constantly look | ||
2450 | back up in the reading section to understand writing. | ||
2451 | |||
2452 | Setup | ||
2453 | |||
2454 | You will want to do the I/O initialization before you get into libpng, | ||
2455 | so if it doesn't work, you don't have anything to undo. If you are not | ||
2456 | using the standard I/O functions, you will need to replace them with | ||
2457 | custom writing functions. See the discussion under Customizing libpng. | ||
2458 | |||
2459 | FILE *fp = fopen(file_name, "wb"); | ||
2460 | |||
2461 | if (!fp) | ||
2462 | return (ERROR); | ||
2463 | |||
2464 | Next, png_struct and png_info need to be allocated and initialized. | ||
2465 | As these can be both relatively large, you may not want to store these | ||
2466 | on the stack, unless you have stack space to spare. Of course, you | ||
2467 | will want to check if they return NULL. If you are also reading, | ||
2468 | you won't want to name your read structure and your write structure | ||
2469 | both "png_ptr"; you can call them anything you like, such as | ||
2470 | "read_ptr" and "write_ptr". Look at pngtest.c, for example. | ||
2471 | |||
2472 | png_structp png_ptr = png_create_write_struct | ||
2473 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, | ||
2474 | user_error_fn, user_warning_fn); | ||
2475 | |||
2476 | if (!png_ptr) | ||
2477 | return (ERROR); | ||
2478 | |||
2479 | png_infop info_ptr = png_create_info_struct(png_ptr); | ||
2480 | if (!info_ptr) | ||
2481 | { | ||
2482 | png_destroy_write_struct(&png_ptr, | ||
2483 | (png_infopp)NULL); | ||
2484 | return (ERROR); | ||
2485 | } | ||
2486 | |||
2487 | If you want to use your own memory allocation routines, | ||
2488 | define PNG_USER_MEM_SUPPORTED and use | ||
2489 | png_create_write_struct_2() instead of png_create_write_struct(): | ||
2490 | |||
2491 | png_structp png_ptr = png_create_write_struct_2 | ||
2492 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, | ||
2493 | user_error_fn, user_warning_fn, (png_voidp) | ||
2494 | user_mem_ptr, user_malloc_fn, user_free_fn); | ||
2495 | |||
2496 | After you have these structures, you will need to set up the | ||
2497 | error handling. When libpng encounters an error, it expects to | ||
2498 | longjmp() back to your routine. Therefore, you will need to call | ||
2499 | setjmp() and pass the png_jmpbuf(png_ptr). If you | ||
2500 | write the file from different routines, you will need to update | ||
2501 | the png_jmpbuf(png_ptr) every time you enter a new routine that will | ||
2502 | call a png_*() function. See your documentation of setjmp/longjmp | ||
2503 | for your compiler for more information on setjmp/longjmp. See | ||
2504 | the discussion on libpng error handling in the Customizing Libpng | ||
2505 | section below for more information on the libpng error handling. | ||
2506 | |||
2507 | if (setjmp(png_jmpbuf(png_ptr))) | ||
2508 | { | ||
2509 | png_destroy_write_struct(&png_ptr, &info_ptr); | ||
2510 | fclose(fp); | ||
2511 | return (ERROR); | ||
2512 | } | ||
2513 | ... | ||
2514 | return; | ||
2515 | |||
2516 | If you would rather avoid the complexity of setjmp/longjmp issues, | ||
2517 | you can compile libpng with PNG_NO_SETJMP, in which case | ||
2518 | errors will result in a call to PNG_ABORT() which defaults to abort(). | ||
2519 | |||
2520 | You can #define PNG_ABORT() to a function that does something | ||
2521 | more useful than abort(), as long as your function does not | ||
2522 | return. | ||
2523 | |||
2524 | Now you need to set up the output code. The default for libpng is to | ||
2525 | use the C function fwrite(). If you use this, you will need to pass a | ||
2526 | valid FILE * in the function png_init_io(). Be sure that the file is | ||
2527 | opened in binary mode. Again, if you wish to handle writing data in | ||
2528 | another way, see the discussion on libpng I/O handling in the Customizing | ||
2529 | Libpng section below. | ||
2530 | |||
2531 | png_init_io(png_ptr, fp); | ||
2532 | |||
2533 | If you are embedding your PNG into a datastream such as MNG, and don't | ||
2534 | want libpng to write the 8-byte signature, or if you have already | ||
2535 | written the signature in your application, use | ||
2536 | |||
2537 | png_set_sig_bytes(png_ptr, 8); | ||
2538 | |||
2539 | to inform libpng that it should not write a signature. | ||
2540 | |||
2541 | Write callbacks | ||
2542 | |||
2543 | At this point, you can set up a callback function that will be | ||
2544 | called after each row has been written, which you can use to control | ||
2545 | a progress meter or the like. It's demonstrated in pngtest.c. | ||
2546 | You must supply a function | ||
2547 | |||
2548 | void write_row_callback(png_structp png_ptr, png_uint_32 row, | ||
2549 | int pass); | ||
2550 | { | ||
2551 | /* put your code here */ | ||
2552 | } | ||
2553 | |||
2554 | (You can give it another name that you like instead of "write_row_callback") | ||
2555 | |||
2556 | To inform libpng about your function, use | ||
2557 | |||
2558 | png_set_write_status_fn(png_ptr, write_row_callback); | ||
2559 | |||
2560 | When this function is called the row has already been completely processed and | ||
2561 | it has also been written out. The 'row' and 'pass' refer to the next row to be | ||
2562 | handled. For the | ||
2563 | non-interlaced case the row that was just handled is simply one less than the | ||
2564 | passed in row number, and pass will always be 0. For the interlaced case the | ||
2565 | same applies unless the row value is 0, in which case the row just handled was | ||
2566 | the last one from one of the preceding passes. Because interlacing may skip a | ||
2567 | pass you cannot be sure that the preceding pass is just 'pass-1', if you really | ||
2568 | need to know what the last pass is record (row,pass) from the callback and use | ||
2569 | the last recorded value each time. | ||
2570 | |||
2571 | As with the user transform you can find the output row using the | ||
2572 | PNG_ROW_FROM_PASS_ROW macro. | ||
2573 | |||
2574 | You now have the option of modifying how the compression library will | ||
2575 | run. The following functions are mainly for testing, but may be useful | ||
2576 | in some cases, like if you need to write PNG files extremely fast and | ||
2577 | are willing to give up some compression, or if you want to get the | ||
2578 | maximum possible compression at the expense of slower writing. If you | ||
2579 | have no special needs in this area, let the library do what it wants by | ||
2580 | not calling this function at all, as it has been tuned to deliver a good | ||
2581 | speed/compression ratio. The second parameter to png_set_filter() is | ||
2582 | the filter method, for which the only valid values are 0 (as of the | ||
2583 | July 1999 PNG specification, version 1.2) or 64 (if you are writing | ||
2584 | a PNG datastream that is to be embedded in a MNG datastream). The third | ||
2585 | parameter is a flag that indicates which filter type(s) are to be tested | ||
2586 | for each scanline. See the PNG specification for details on the specific | ||
2587 | filter types. | ||
2588 | |||
2589 | |||
2590 | /* turn on or off filtering, and/or choose | ||
2591 | specific filters. You can use either a single | ||
2592 | PNG_FILTER_VALUE_NAME or the bitwise OR of one | ||
2593 | or more PNG_FILTER_NAME masks. | ||
2594 | */ | ||
2595 | png_set_filter(png_ptr, 0, | ||
2596 | PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE | | ||
2597 | PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB | | ||
2598 | PNG_FILTER_UP | PNG_FILTER_VALUE_UP | | ||
2599 | PNG_FILTER_AVG | PNG_FILTER_VALUE_AVG | | ||
2600 | PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH| | ||
2601 | PNG_ALL_FILTERS); | ||
2602 | |||
2603 | If an application wants to start and stop using particular filters during | ||
2604 | compression, it should start out with all of the filters (to ensure that | ||
2605 | the previous row of pixels will be stored in case it's needed later), | ||
2606 | and then add and remove them after the start of compression. | ||
2607 | |||
2608 | If you are writing a PNG datastream that is to be embedded in a MNG | ||
2609 | datastream, the second parameter can be either 0 or 64. | ||
2610 | |||
2611 | The png_set_compression_*() functions interface to the zlib compression | ||
2612 | library, and should mostly be ignored unless you really know what you are | ||
2613 | doing. The only generally useful call is png_set_compression_level() | ||
2614 | which changes how much time zlib spends on trying to compress the image | ||
2615 | data. See the Compression Library (zlib.h and algorithm.txt, distributed | ||
2616 | with zlib) for details on the compression levels. | ||
2617 | |||
2618 | #include zlib.h | ||
2619 | |||
2620 | /* Set the zlib compression level */ | ||
2621 | png_set_compression_level(png_ptr, | ||
2622 | Z_BEST_COMPRESSION); | ||
2623 | |||
2624 | /* Set other zlib parameters for compressing IDAT */ | ||
2625 | png_set_compression_mem_level(png_ptr, 8); | ||
2626 | png_set_compression_strategy(png_ptr, | ||
2627 | Z_DEFAULT_STRATEGY); | ||
2628 | png_set_compression_window_bits(png_ptr, 15); | ||
2629 | png_set_compression_method(png_ptr, 8); | ||
2630 | png_set_compression_buffer_size(png_ptr, 8192) | ||
2631 | |||
2632 | /* Set zlib parameters for text compression | ||
2633 | * If you don't call these, the parameters | ||
2634 | * fall back on those defined for IDAT chunks | ||
2635 | */ | ||
2636 | png_set_text_compression_mem_level(png_ptr, 8); | ||
2637 | png_set_text_compression_strategy(png_ptr, | ||
2638 | Z_DEFAULT_STRATEGY); | ||
2639 | png_set_text_compression_window_bits(png_ptr, 15); | ||
2640 | png_set_text_compression_method(png_ptr, 8); | ||
2641 | |||
2642 | Setting the contents of info for output | ||
2643 | |||
2644 | You now need to fill in the png_info structure with all the data you | ||
2645 | wish to write before the actual image. Note that the only thing you | ||
2646 | are allowed to write after the image is the text chunks and the time | ||
2647 | chunk (as of PNG Specification 1.2, anyway). See png_write_end() and | ||
2648 | the latest PNG specification for more information on that. If you | ||
2649 | wish to write them before the image, fill them in now, and flag that | ||
2650 | data as being valid. If you want to wait until after the data, don't | ||
2651 | fill them until png_write_end(). For all the fields in png_info and | ||
2652 | their data types, see png.h. For explanations of what the fields | ||
2653 | contain, see the PNG specification. | ||
2654 | |||
2655 | Some of the more important parts of the png_info are: | ||
2656 | |||
2657 | png_set_IHDR(png_ptr, info_ptr, width, height, | ||
2658 | bit_depth, color_type, interlace_type, | ||
2659 | compression_type, filter_method) | ||
2660 | |||
2661 | width - holds the width of the image | ||
2662 | in pixels (up to 2^31). | ||
2663 | |||
2664 | height - holds the height of the image | ||
2665 | in pixels (up to 2^31). | ||
2666 | |||
2667 | bit_depth - holds the bit depth of one of the | ||
2668 | image channels. | ||
2669 | (valid values are 1, 2, 4, 8, 16 | ||
2670 | and depend also on the | ||
2671 | color_type. See also significant | ||
2672 | bits (sBIT) below). | ||
2673 | |||
2674 | color_type - describes which color/alpha | ||
2675 | channels are present. | ||
2676 | PNG_COLOR_TYPE_GRAY | ||
2677 | (bit depths 1, 2, 4, 8, 16) | ||
2678 | PNG_COLOR_TYPE_GRAY_ALPHA | ||
2679 | (bit depths 8, 16) | ||
2680 | PNG_COLOR_TYPE_PALETTE | ||
2681 | (bit depths 1, 2, 4, 8) | ||
2682 | PNG_COLOR_TYPE_RGB | ||
2683 | (bit_depths 8, 16) | ||
2684 | PNG_COLOR_TYPE_RGB_ALPHA | ||
2685 | (bit_depths 8, 16) | ||
2686 | |||
2687 | PNG_COLOR_MASK_PALETTE | ||
2688 | PNG_COLOR_MASK_COLOR | ||
2689 | PNG_COLOR_MASK_ALPHA | ||
2690 | |||
2691 | interlace_type - PNG_INTERLACE_NONE or | ||
2692 | PNG_INTERLACE_ADAM7 | ||
2693 | |||
2694 | compression_type - (must be | ||
2695 | PNG_COMPRESSION_TYPE_DEFAULT) | ||
2696 | |||
2697 | filter_method - (must be PNG_FILTER_TYPE_DEFAULT | ||
2698 | or, if you are writing a PNG to | ||
2699 | be embedded in a MNG datastream, | ||
2700 | can also be | ||
2701 | PNG_INTRAPIXEL_DIFFERENCING) | ||
2702 | |||
2703 | If you call png_set_IHDR(), the call must appear before any of the | ||
2704 | other png_set_*() functions, because they might require access to some of | ||
2705 | the IHDR settings. The remaining png_set_*() functions can be called | ||
2706 | in any order. | ||
2707 | |||
2708 | If you wish, you can reset the compression_type, interlace_type, or | ||
2709 | filter_method later by calling png_set_IHDR() again; if you do this, the | ||
2710 | width, height, bit_depth, and color_type must be the same in each call. | ||
2711 | |||
2712 | png_set_PLTE(png_ptr, info_ptr, palette, | ||
2713 | num_palette); | ||
2714 | |||
2715 | palette - the palette for the file | ||
2716 | (array of png_color) | ||
2717 | num_palette - number of entries in the palette | ||
2718 | |||
2719 | png_set_gAMA(png_ptr, info_ptr, file_gamma); | ||
2720 | png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma); | ||
2721 | |||
2722 | file_gamma - the gamma at which the image was | ||
2723 | created (PNG_INFO_gAMA) | ||
2724 | |||
2725 | int_file_gamma - 100,000 times the gamma at which | ||
2726 | the image was created | ||
2727 | |||
2728 | png_set_cHRM(png_ptr, info_ptr, white_x, white_y, red_x, red_y, | ||
2729 | green_x, green_y, blue_x, blue_y) | ||
2730 | png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X, | ||
2731 | green_Y, green_Z, blue_X, blue_Y, blue_Z) | ||
2732 | png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y, | ||
2733 | int_red_x, int_red_y, int_green_x, int_green_y, | ||
2734 | int_blue_x, int_blue_y) | ||
2735 | png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y, | ||
2736 | int_red_Z, int_green_X, int_green_Y, int_green_Z, | ||
2737 | int_blue_X, int_blue_Y, int_blue_Z) | ||
2738 | |||
2739 | {white,red,green,blue}_{x,y} | ||
2740 | A color space encoding specified using the chromaticities | ||
2741 | of the end points and the white point. | ||
2742 | |||
2743 | {red,green,blue}_{X,Y,Z} | ||
2744 | A color space encoding specified using the encoding end | ||
2745 | points - the CIE tristimulus specification of the intended | ||
2746 | color of the red, green and blue channels in the PNG RGB | ||
2747 | data. The white point is simply the sum of the three end | ||
2748 | points. | ||
2749 | |||
2750 | png_set_sRGB(png_ptr, info_ptr, srgb_intent); | ||
2751 | |||
2752 | srgb_intent - the rendering intent | ||
2753 | (PNG_INFO_sRGB) The presence of | ||
2754 | the sRGB chunk means that the pixel | ||
2755 | data is in the sRGB color space. | ||
2756 | This chunk also implies specific | ||
2757 | values of gAMA and cHRM. Rendering | ||
2758 | intent is the CSS-1 property that | ||
2759 | has been defined by the International | ||
2760 | Color Consortium | ||
2761 | (http://www.color.org). | ||
2762 | It can be one of | ||
2763 | PNG_sRGB_INTENT_SATURATION, | ||
2764 | PNG_sRGB_INTENT_PERCEPTUAL, | ||
2765 | PNG_sRGB_INTENT_ABSOLUTE, or | ||
2766 | PNG_sRGB_INTENT_RELATIVE. | ||
2767 | |||
2768 | |||
2769 | png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr, | ||
2770 | srgb_intent); | ||
2771 | |||
2772 | srgb_intent - the rendering intent | ||
2773 | (PNG_INFO_sRGB) The presence of the | ||
2774 | sRGB chunk means that the pixel | ||
2775 | data is in the sRGB color space. | ||
2776 | This function also causes gAMA and | ||
2777 | cHRM chunks with the specific values | ||
2778 | that are consistent with sRGB to be | ||
2779 | written. | ||
2780 | |||
2781 | png_set_iCCP(png_ptr, info_ptr, name, compression_type, | ||
2782 | profile, proflen); | ||
2783 | |||
2784 | name - The profile name. | ||
2785 | |||
2786 | compression_type - The compression type; always | ||
2787 | PNG_COMPRESSION_TYPE_BASE for PNG 1.0. | ||
2788 | You may give NULL to this argument to | ||
2789 | ignore it. | ||
2790 | |||
2791 | profile - International Color Consortium color | ||
2792 | profile data. May contain NULs. | ||
2793 | |||
2794 | proflen - length of profile data in bytes. | ||
2795 | |||
2796 | png_set_sBIT(png_ptr, info_ptr, sig_bit); | ||
2797 | |||
2798 | sig_bit - the number of significant bits for | ||
2799 | (PNG_INFO_sBIT) each of the gray, red, | ||
2800 | green, and blue channels, whichever are | ||
2801 | appropriate for the given color type | ||
2802 | (png_color_16) | ||
2803 | |||
2804 | png_set_tRNS(png_ptr, info_ptr, trans_alpha, | ||
2805 | num_trans, trans_color); | ||
2806 | |||
2807 | trans_alpha - array of alpha (transparency) | ||
2808 | entries for palette (PNG_INFO_tRNS) | ||
2809 | |||
2810 | num_trans - number of transparent entries | ||
2811 | (PNG_INFO_tRNS) | ||
2812 | |||
2813 | trans_color - graylevel or color sample values | ||
2814 | (in order red, green, blue) of the | ||
2815 | single transparent color for | ||
2816 | non-paletted images (PNG_INFO_tRNS) | ||
2817 | |||
2818 | png_set_hIST(png_ptr, info_ptr, hist); | ||
2819 | |||
2820 | hist - histogram of palette (array of | ||
2821 | png_uint_16) (PNG_INFO_hIST) | ||
2822 | |||
2823 | png_set_tIME(png_ptr, info_ptr, mod_time); | ||
2824 | |||
2825 | mod_time - time image was last modified | ||
2826 | (PNG_VALID_tIME) | ||
2827 | |||
2828 | png_set_bKGD(png_ptr, info_ptr, background); | ||
2829 | |||
2830 | background - background color (of type | ||
2831 | png_color_16p) (PNG_VALID_bKGD) | ||
2832 | |||
2833 | png_set_text(png_ptr, info_ptr, text_ptr, num_text); | ||
2834 | |||
2835 | text_ptr - array of png_text holding image | ||
2836 | comments | ||
2837 | |||
2838 | text_ptr[i].compression - type of compression used | ||
2839 | on "text" PNG_TEXT_COMPRESSION_NONE | ||
2840 | PNG_TEXT_COMPRESSION_zTXt | ||
2841 | PNG_ITXT_COMPRESSION_NONE | ||
2842 | PNG_ITXT_COMPRESSION_zTXt | ||
2843 | text_ptr[i].key - keyword for comment. Must contain | ||
2844 | 1-79 characters. | ||
2845 | text_ptr[i].text - text comments for current | ||
2846 | keyword. Can be NULL or empty. | ||
2847 | text_ptr[i].text_length - length of text string, | ||
2848 | after decompression, 0 for iTXt | ||
2849 | text_ptr[i].itxt_length - length of itxt string, | ||
2850 | after decompression, 0 for tEXt/zTXt | ||
2851 | text_ptr[i].lang - language of comment (NULL or | ||
2852 | empty for unknown). | ||
2853 | text_ptr[i].translated_keyword - keyword in UTF-8 (NULL | ||
2854 | or empty for unknown). | ||
2855 | |||
2856 | Note that the itxt_length, lang, and lang_key | ||
2857 | members of the text_ptr structure only exist when the | ||
2858 | library is built with iTXt chunk support. Prior to | ||
2859 | libpng-1.4.0 the library was built by default without | ||
2860 | iTXt support. Also note that when iTXt is supported, | ||
2861 | they contain NULL pointers when the "compression" | ||
2862 | field contains PNG_TEXT_COMPRESSION_NONE or | ||
2863 | PNG_TEXT_COMPRESSION_zTXt. | ||
2864 | |||
2865 | num_text - number of comments | ||
2866 | |||
2867 | png_set_sPLT(png_ptr, info_ptr, &palette_ptr, | ||
2868 | num_spalettes); | ||
2869 | |||
2870 | palette_ptr - array of png_sPLT_struct structures | ||
2871 | to be added to the list of palettes | ||
2872 | in the info structure. | ||
2873 | num_spalettes - number of palette structures to be | ||
2874 | added. | ||
2875 | |||
2876 | png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y, | ||
2877 | unit_type); | ||
2878 | |||
2879 | offset_x - positive offset from the left | ||
2880 | edge of the screen | ||
2881 | |||
2882 | offset_y - positive offset from the top | ||
2883 | edge of the screen | ||
2884 | |||
2885 | unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER | ||
2886 | |||
2887 | png_set_pHYs(png_ptr, info_ptr, res_x, res_y, | ||
2888 | unit_type); | ||
2889 | |||
2890 | res_x - pixels/unit physical resolution | ||
2891 | in x direction | ||
2892 | |||
2893 | res_y - pixels/unit physical resolution | ||
2894 | in y direction | ||
2895 | |||
2896 | unit_type - PNG_RESOLUTION_UNKNOWN, | ||
2897 | PNG_RESOLUTION_METER | ||
2898 | |||
2899 | png_set_sCAL(png_ptr, info_ptr, unit, width, height) | ||
2900 | |||
2901 | unit - physical scale units (an integer) | ||
2902 | |||
2903 | width - width of a pixel in physical scale units | ||
2904 | |||
2905 | height - height of a pixel in physical scale units | ||
2906 | (width and height are doubles) | ||
2907 | |||
2908 | png_set_sCAL_s(png_ptr, info_ptr, unit, width, height) | ||
2909 | |||
2910 | unit - physical scale units (an integer) | ||
2911 | |||
2912 | width - width of a pixel in physical scale units | ||
2913 | expressed as a string | ||
2914 | |||
2915 | height - height of a pixel in physical scale units | ||
2916 | (width and height are strings like "2.54") | ||
2917 | |||
2918 | png_set_unknown_chunks(png_ptr, info_ptr, &unknowns, | ||
2919 | num_unknowns) | ||
2920 | |||
2921 | unknowns - array of png_unknown_chunk | ||
2922 | structures holding unknown chunks | ||
2923 | unknowns[i].name - name of unknown chunk | ||
2924 | unknowns[i].data - data of unknown chunk | ||
2925 | unknowns[i].size - size of unknown chunk's data | ||
2926 | unknowns[i].location - position to write chunk in file | ||
2927 | 0: do not write chunk | ||
2928 | PNG_HAVE_IHDR: before PLTE | ||
2929 | PNG_HAVE_PLTE: before IDAT | ||
2930 | PNG_AFTER_IDAT: after IDAT | ||
2931 | |||
2932 | The "location" member is set automatically according to | ||
2933 | what part of the output file has already been written. | ||
2934 | You can change its value after calling png_set_unknown_chunks() | ||
2935 | as demonstrated in pngtest.c. Within each of the "locations", | ||
2936 | the chunks are sequenced according to their position in the | ||
2937 | structure (that is, the value of "i", which is the order in which | ||
2938 | the chunk was either read from the input file or defined with | ||
2939 | png_set_unknown_chunks). | ||
2940 | |||
2941 | A quick word about text and num_text. text is an array of png_text | ||
2942 | structures. num_text is the number of valid structures in the array. | ||
2943 | Each png_text structure holds a language code, a keyword, a text value, | ||
2944 | and a compression type. | ||
2945 | |||
2946 | The compression types have the same valid numbers as the compression | ||
2947 | types of the image data. Currently, the only valid number is zero. | ||
2948 | However, you can store text either compressed or uncompressed, unlike | ||
2949 | images, which always have to be compressed. So if you don't want the | ||
2950 | text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE. | ||
2951 | Because tEXt and zTXt chunks don't have a language field, if you | ||
2952 | specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt | ||
2953 | any language code or translated keyword will not be written out. | ||
2954 | |||
2955 | Until text gets around a few hundred bytes, it is not worth compressing it. | ||
2956 | After the text has been written out to the file, the compression type | ||
2957 | is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR, | ||
2958 | so that it isn't written out again at the end (in case you are calling | ||
2959 | png_write_end() with the same struct). | ||
2960 | |||
2961 | The keywords that are given in the PNG Specification are: | ||
2962 | |||
2963 | Title Short (one line) title or | ||
2964 | caption for image | ||
2965 | |||
2966 | Author Name of image's creator | ||
2967 | |||
2968 | Description Description of image (possibly long) | ||
2969 | |||
2970 | Copyright Copyright notice | ||
2971 | |||
2972 | Creation Time Time of original image creation | ||
2973 | (usually RFC 1123 format, see below) | ||
2974 | |||
2975 | Software Software used to create the image | ||
2976 | |||
2977 | Disclaimer Legal disclaimer | ||
2978 | |||
2979 | Warning Warning of nature of content | ||
2980 | |||
2981 | Source Device used to create the image | ||
2982 | |||
2983 | Comment Miscellaneous comment; conversion | ||
2984 | from other image format | ||
2985 | |||
2986 | The keyword-text pairs work like this. Keywords should be short | ||
2987 | simple descriptions of what the comment is about. Some typical | ||
2988 | keywords are found in the PNG specification, as is some recommendations | ||
2989 | on keywords. You can repeat keywords in a file. You can even write | ||
2990 | some text before the image and some after. For example, you may want | ||
2991 | to put a description of the image before the image, but leave the | ||
2992 | disclaimer until after, so viewers working over modem connections | ||
2993 | don't have to wait for the disclaimer to go over the modem before | ||
2994 | they start seeing the image. Finally, keywords should be full | ||
2995 | words, not abbreviations. Keywords and text are in the ISO 8859-1 | ||
2996 | (Latin-1) character set (a superset of regular ASCII) and can not | ||
2997 | contain NUL characters, and should not contain control or other | ||
2998 | unprintable characters. To make the comments widely readable, stick | ||
2999 | with basic ASCII, and avoid machine specific character set extensions | ||
3000 | like the IBM-PC character set. The keyword must be present, but | ||
3001 | you can leave off the text string on non-compressed pairs. | ||
3002 | Compressed pairs must have a text string, as only the text string | ||
3003 | is compressed anyway, so the compression would be meaningless. | ||
3004 | |||
3005 | PNG supports modification time via the png_time structure. Two | ||
3006 | conversion routines are provided, png_convert_from_time_t() for | ||
3007 | time_t and png_convert_from_struct_tm() for struct tm. The | ||
3008 | time_t routine uses gmtime(). You don't have to use either of | ||
3009 | these, but if you wish to fill in the png_time structure directly, | ||
3010 | you should provide the time in universal time (GMT) if possible | ||
3011 | instead of your local time. Note that the year number is the full | ||
3012 | year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and | ||
3013 | that months start with 1. | ||
3014 | |||
3015 | If you want to store the time of the original image creation, you should | ||
3016 | use a plain tEXt chunk with the "Creation Time" keyword. This is | ||
3017 | necessary because the "creation time" of a PNG image is somewhat vague, | ||
3018 | depending on whether you mean the PNG file, the time the image was | ||
3019 | created in a non-PNG format, a still photo from which the image was | ||
3020 | scanned, or possibly the subject matter itself. In order to facilitate | ||
3021 | machine-readable dates, it is recommended that the "Creation Time" | ||
3022 | tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"), | ||
3023 | although this isn't a requirement. Unlike the tIME chunk, the | ||
3024 | "Creation Time" tEXt chunk is not expected to be automatically changed | ||
3025 | by the software. To facilitate the use of RFC 1123 dates, a function | ||
3026 | png_convert_to_rfc1123(png_timep) is provided to convert from PNG | ||
3027 | time to an RFC 1123 format string. | ||
3028 | |||
3029 | Writing unknown chunks | ||
3030 | |||
3031 | You can use the png_set_unknown_chunks function to queue up chunks | ||
3032 | for writing. You give it a chunk name, raw data, and a size; that's | ||
3033 | all there is to it. The chunks will be written by the next following | ||
3034 | png_write_info_before_PLTE, png_write_info, or png_write_end function. | ||
3035 | Any chunks previously read into the info structure's unknown-chunk | ||
3036 | list will also be written out in a sequence that satisfies the PNG | ||
3037 | specification's ordering rules. | ||
3038 | |||
3039 | The high-level write interface | ||
3040 | |||
3041 | At this point there are two ways to proceed; through the high-level | ||
3042 | write interface, or through a sequence of low-level write operations. | ||
3043 | You can use the high-level interface if your image data is present | ||
3044 | in the info structure. All defined output | ||
3045 | transformations are permitted, enabled by the following masks. | ||
3046 | |||
3047 | PNG_TRANSFORM_IDENTITY No transformation | ||
3048 | PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples | ||
3049 | PNG_TRANSFORM_PACKSWAP Change order of packed | ||
3050 | pixels to LSB first | ||
3051 | PNG_TRANSFORM_INVERT_MONO Invert monochrome images | ||
3052 | PNG_TRANSFORM_SHIFT Normalize pixels to the | ||
3053 | sBIT depth | ||
3054 | PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA | ||
3055 | to BGRA | ||
3056 | PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA | ||
3057 | to AG | ||
3058 | PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity | ||
3059 | to transparency | ||
3060 | PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples | ||
3061 | PNG_TRANSFORM_STRIP_FILLER Strip out filler | ||
3062 | bytes (deprecated). | ||
3063 | PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading | ||
3064 | filler bytes | ||
3065 | PNG_TRANSFORM_STRIP_FILLER_AFTER Strip out trailing | ||
3066 | filler bytes | ||
3067 | |||
3068 | If you have valid image data in the info structure (you can use | ||
3069 | png_set_rows() to put image data in the info structure), simply do this: | ||
3070 | |||
3071 | png_write_png(png_ptr, info_ptr, png_transforms, NULL) | ||
3072 | |||
3073 | where png_transforms is an integer containing the bitwise OR of some set of | ||
3074 | transformation flags. This call is equivalent to png_write_info(), | ||
3075 | followed the set of transformations indicated by the transform mask, | ||
3076 | then png_write_image(), and finally png_write_end(). | ||
3077 | |||
3078 | (The final parameter of this call is not yet used. Someday it might point | ||
3079 | to transformation parameters required by some future output transform.) | ||
3080 | |||
3081 | You must use png_transforms and not call any png_set_transform() functions | ||
3082 | when you use png_write_png(). | ||
3083 | |||
3084 | The low-level write interface | ||
3085 | |||
3086 | If you are going the low-level route instead, you are now ready to | ||
3087 | write all the file information up to the actual image data. You do | ||
3088 | this with a call to png_write_info(). | ||
3089 | |||
3090 | png_write_info(png_ptr, info_ptr); | ||
3091 | |||
3092 | Note that there is one transformation you may need to do before | ||
3093 | png_write_info(). In PNG files, the alpha channel in an image is the | ||
3094 | level of opacity. If your data is supplied as a level of transparency, | ||
3095 | you can invert the alpha channel before you write it, so that 0 is | ||
3096 | fully transparent and 255 (in 8-bit or paletted images) or 65535 | ||
3097 | (in 16-bit images) is fully opaque, with | ||
3098 | |||
3099 | png_set_invert_alpha(png_ptr); | ||
3100 | |||
3101 | This must appear before png_write_info() instead of later with the | ||
3102 | other transformations because in the case of paletted images the tRNS | ||
3103 | chunk data has to be inverted before the tRNS chunk is written. If | ||
3104 | your image is not a paletted image, the tRNS data (which in such cases | ||
3105 | represents a single color to be rendered as transparent) won't need to | ||
3106 | be changed, and you can safely do this transformation after your | ||
3107 | png_write_info() call. | ||
3108 | |||
3109 | If you need to write a private chunk that you want to appear before | ||
3110 | the PLTE chunk when PLTE is present, you can write the PNG info in | ||
3111 | two steps, and insert code to write your own chunk between them: | ||
3112 | |||
3113 | png_write_info_before_PLTE(png_ptr, info_ptr); | ||
3114 | png_set_unknown_chunks(png_ptr, info_ptr, ...); | ||
3115 | png_write_info(png_ptr, info_ptr); | ||
3116 | |||
3117 | After you've written the file information, you can set up the library | ||
3118 | to handle any special transformations of the image data. The various | ||
3119 | ways to transform the data will be described in the order that they | ||
3120 | should occur. This is important, as some of these change the color | ||
3121 | type and/or bit depth of the data, and some others only work on | ||
3122 | certain color types and bit depths. Even though each transformation | ||
3123 | checks to see if it has data that it can do something with, you should | ||
3124 | make sure to only enable a transformation if it will be valid for the | ||
3125 | data. For example, don't swap red and blue on grayscale data. | ||
3126 | |||
3127 | PNG files store RGB pixels packed into 3 or 6 bytes. This code tells | ||
3128 | the library to strip input data that has 4 or 8 bytes per pixel down | ||
3129 | to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2 | ||
3130 | bytes per pixel). | ||
3131 | |||
3132 | png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE); | ||
3133 | |||
3134 | where the 0 is unused, and the location is either PNG_FILLER_BEFORE or | ||
3135 | PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel | ||
3136 | is stored XRGB or RGBX. | ||
3137 | |||
3138 | PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as | ||
3139 | they can, resulting in, for example, 8 pixels per byte for 1 bit files. | ||
3140 | If the data is supplied at 1 pixel per byte, use this code, which will | ||
3141 | correctly pack the pixels into a single byte: | ||
3142 | |||
3143 | png_set_packing(png_ptr); | ||
3144 | |||
3145 | PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your | ||
3146 | data is of another bit depth, you can write an sBIT chunk into the | ||
3147 | file so that decoders can recover the original data if desired. | ||
3148 | |||
3149 | /* Set the true bit depth of the image data */ | ||
3150 | if (color_type & PNG_COLOR_MASK_COLOR) | ||
3151 | { | ||
3152 | sig_bit.red = true_bit_depth; | ||
3153 | sig_bit.green = true_bit_depth; | ||
3154 | sig_bit.blue = true_bit_depth; | ||
3155 | } | ||
3156 | |||
3157 | else | ||
3158 | { | ||
3159 | sig_bit.gray = true_bit_depth; | ||
3160 | } | ||
3161 | |||
3162 | if (color_type & PNG_COLOR_MASK_ALPHA) | ||
3163 | { | ||
3164 | sig_bit.alpha = true_bit_depth; | ||
3165 | } | ||
3166 | |||
3167 | png_set_sBIT(png_ptr, info_ptr, &sig_bit); | ||
3168 | |||
3169 | If the data is stored in the row buffer in a bit depth other than | ||
3170 | one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG), | ||
3171 | this will scale the values to appear to be the correct bit depth as | ||
3172 | is required by PNG. | ||
3173 | |||
3174 | png_set_shift(png_ptr, &sig_bit); | ||
3175 | |||
3176 | PNG files store 16-bit pixels in network byte order (big-endian, | ||
3177 | ie. most significant bits first). This code would be used if they are | ||
3178 | supplied the other way (little-endian, i.e. least significant bits | ||
3179 | first, the way PCs store them): | ||
3180 | |||
3181 | if (bit_depth > 8) | ||
3182 | png_set_swap(png_ptr); | ||
3183 | |||
3184 | If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you | ||
3185 | need to change the order the pixels are packed into bytes, you can use: | ||
3186 | |||
3187 | if (bit_depth < 8) | ||
3188 | png_set_packswap(png_ptr); | ||
3189 | |||
3190 | PNG files store 3 color pixels in red, green, blue order. This code | ||
3191 | would be used if they are supplied as blue, green, red: | ||
3192 | |||
3193 | png_set_bgr(png_ptr); | ||
3194 | |||
3195 | PNG files describe monochrome as black being zero and white being | ||
3196 | one. This code would be used if the pixels are supplied with this reversed | ||
3197 | (black being one and white being zero): | ||
3198 | |||
3199 | png_set_invert_mono(png_ptr); | ||
3200 | |||
3201 | Finally, you can write your own transformation function if none of | ||
3202 | the existing ones meets your needs. This is done by setting a callback | ||
3203 | with | ||
3204 | |||
3205 | png_set_write_user_transform_fn(png_ptr, | ||
3206 | write_transform_fn); | ||
3207 | |||
3208 | You must supply the function | ||
3209 | |||
3210 | void write_transform_fn(png_structp png_ptr, png_row_infop | ||
3211 | row_info, png_bytep data) | ||
3212 | |||
3213 | See pngtest.c for a working example. Your function will be called | ||
3214 | before any of the other transformations are processed. If supported | ||
3215 | libpng also supplies an information routine that may be called from | ||
3216 | your callback: | ||
3217 | |||
3218 | png_get_current_row_number(png_ptr); | ||
3219 | png_get_current_pass_number(png_ptr); | ||
3220 | |||
3221 | This returns the current row passed to the transform. With interlaced | ||
3222 | images the value returned is the row in the input sub-image image. Use | ||
3223 | PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to | ||
3224 | find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass). | ||
3225 | |||
3226 | The discussion of interlace handling above contains more information on how to | ||
3227 | use these values. | ||
3228 | |||
3229 | You can also set up a pointer to a user structure for use by your | ||
3230 | callback function. | ||
3231 | |||
3232 | png_set_user_transform_info(png_ptr, user_ptr, 0, 0); | ||
3233 | |||
3234 | The user_channels and user_depth parameters of this function are ignored | ||
3235 | when writing; you can set them to zero as shown. | ||
3236 | |||
3237 | You can retrieve the pointer via the function png_get_user_transform_ptr(). | ||
3238 | For example: | ||
3239 | |||
3240 | voidp write_user_transform_ptr = | ||
3241 | png_get_user_transform_ptr(png_ptr); | ||
3242 | |||
3243 | It is possible to have libpng flush any pending output, either manually, | ||
3244 | or automatically after a certain number of lines have been written. To | ||
3245 | flush the output stream a single time call: | ||
3246 | |||
3247 | png_write_flush(png_ptr); | ||
3248 | |||
3249 | and to have libpng flush the output stream periodically after a certain | ||
3250 | number of scanlines have been written, call: | ||
3251 | |||
3252 | png_set_flush(png_ptr, nrows); | ||
3253 | |||
3254 | Note that the distance between rows is from the last time png_write_flush() | ||
3255 | was called, or the first row of the image if it has never been called. | ||
3256 | So if you write 50 lines, and then png_set_flush 25, it will flush the | ||
3257 | output on the next scanline, and every 25 lines thereafter, unless | ||
3258 | png_write_flush() is called before 25 more lines have been written. | ||
3259 | If nrows is too small (less than about 10 lines for a 640 pixel wide | ||
3260 | RGB image) the image compression may decrease noticeably (although this | ||
3261 | may be acceptable for real-time applications). Infrequent flushing will | ||
3262 | only degrade the compression performance by a few percent over images | ||
3263 | that do not use flushing. | ||
3264 | |||
3265 | Writing the image data | ||
3266 | |||
3267 | That's it for the transformations. Now you can write the image data. | ||
3268 | The simplest way to do this is in one function call. If you have the | ||
3269 | whole image in memory, you can just call png_write_image() and libpng | ||
3270 | will write the image. You will need to pass in an array of pointers to | ||
3271 | each row. This function automatically handles interlacing, so you don't | ||
3272 | need to call png_set_interlace_handling() or call this function multiple | ||
3273 | times, or any of that other stuff necessary with png_write_rows(). | ||
3274 | |||
3275 | png_write_image(png_ptr, row_pointers); | ||
3276 | |||
3277 | where row_pointers is: | ||
3278 | |||
3279 | png_byte *row_pointers[height]; | ||
3280 | |||
3281 | You can point to void or char or whatever you use for pixels. | ||
3282 | |||
3283 | If you don't want to write the whole image at once, you can | ||
3284 | use png_write_rows() instead. If the file is not interlaced, | ||
3285 | this is simple: | ||
3286 | |||
3287 | png_write_rows(png_ptr, row_pointers, | ||
3288 | number_of_rows); | ||
3289 | |||
3290 | row_pointers is the same as in the png_write_image() call. | ||
3291 | |||
3292 | If you are just writing one row at a time, you can do this with | ||
3293 | a single row_pointer instead of an array of row_pointers: | ||
3294 | |||
3295 | png_bytep row_pointer = row; | ||
3296 | |||
3297 | png_write_row(png_ptr, row_pointer); | ||
3298 | |||
3299 | When the file is interlaced, things can get a good deal more complicated. | ||
3300 | The only currently (as of the PNG Specification version 1.2, dated July | ||
3301 | 1999) defined interlacing scheme for PNG files is the "Adam7" interlace | ||
3302 | scheme, that breaks down an image into seven smaller images of varying | ||
3303 | size. libpng will build these images for you, or you can do them | ||
3304 | yourself. If you want to build them yourself, see the PNG specification | ||
3305 | for details of which pixels to write when. | ||
3306 | |||
3307 | If you don't want libpng to handle the interlacing details, just | ||
3308 | use png_set_interlace_handling() and call png_write_rows() the | ||
3309 | correct number of times to write all the sub-images | ||
3310 | (png_set_interlace_handling() returns the number of sub-images.) | ||
3311 | |||
3312 | If you want libpng to build the sub-images, call this before you start | ||
3313 | writing any rows: | ||
3314 | |||
3315 | number_of_passes = png_set_interlace_handling(png_ptr); | ||
3316 | |||
3317 | This will return the number of passes needed. Currently, this is seven, | ||
3318 | but may change if another interlace type is added. | ||
3319 | |||
3320 | Then write the complete image number_of_passes times. | ||
3321 | |||
3322 | png_write_rows(png_ptr, row_pointers, number_of_rows); | ||
3323 | |||
3324 | Think carefully before you write an interlaced image. Typically code that | ||
3325 | reads such images reads all the image data into memory, uncompressed, before | ||
3326 | doing any processing. Only code that can display an image on the fly can | ||
3327 | take advantage of the interlacing and even then the image has to be exactly | ||
3328 | the correct size for the output device, because scaling an image requires | ||
3329 | adjacent pixels and these are not available until all the passes have been | ||
3330 | read. | ||
3331 | |||
3332 | If you do write an interlaced image you will hardly ever need to handle | ||
3333 | the interlacing yourself. Call png_set_interlace_handling() and use the | ||
3334 | approach described above. | ||
3335 | |||
3336 | The only time it is conceivable that you will really need to write an | ||
3337 | interlaced image pass-by-pass is when you have read one pass by pass and | ||
3338 | made some pixel-by-pixel transformation to it, as described in the read | ||
3339 | code above. In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros | ||
3340 | to determine the size of each sub-image in turn and simply write the rows | ||
3341 | you obtained from the read code. | ||
3342 | |||
3343 | Finishing a sequential write | ||
3344 | |||
3345 | After you are finished writing the image, you should finish writing | ||
3346 | the file. If you are interested in writing comments or time, you should | ||
3347 | pass an appropriately filled png_info pointer. If you are not interested, | ||
3348 | you can pass NULL. | ||
3349 | |||
3350 | png_write_end(png_ptr, info_ptr); | ||
3351 | |||
3352 | When you are done, you can free all memory used by libpng like this: | ||
3353 | |||
3354 | png_destroy_write_struct(&png_ptr, &info_ptr); | ||
3355 | |||
3356 | It is also possible to individually free the info_ptr members that | ||
3357 | point to libpng-allocated storage with the following function: | ||
3358 | |||
3359 | png_free_data(png_ptr, info_ptr, mask, seq) | ||
3360 | |||
3361 | mask - identifies data to be freed, a mask | ||
3362 | containing the bitwise OR of one or | ||
3363 | more of | ||
3364 | PNG_FREE_PLTE, PNG_FREE_TRNS, | ||
3365 | PNG_FREE_HIST, PNG_FREE_ICCP, | ||
3366 | PNG_FREE_PCAL, PNG_FREE_ROWS, | ||
3367 | PNG_FREE_SCAL, PNG_FREE_SPLT, | ||
3368 | PNG_FREE_TEXT, PNG_FREE_UNKN, | ||
3369 | or simply PNG_FREE_ALL | ||
3370 | |||
3371 | seq - sequence number of item to be freed | ||
3372 | (-1 for all items) | ||
3373 | |||
3374 | This function may be safely called when the relevant storage has | ||
3375 | already been freed, or has not yet been allocated, or was allocated | ||
3376 | by the user and not by libpng, and will in those cases do nothing. | ||
3377 | The "seq" parameter is ignored if only one item of the selected data | ||
3378 | type, such as PLTE, is allowed. If "seq" is not -1, and multiple items | ||
3379 | are allowed for the data type identified in the mask, such as text or | ||
3380 | sPLT, only the n'th item in the structure is freed, where n is "seq". | ||
3381 | |||
3382 | If you allocated data such as a palette that you passed in to libpng | ||
3383 | with png_set_*, you must not free it until just before the call to | ||
3384 | png_destroy_write_struct(). | ||
3385 | |||
3386 | The default behavior is only to free data that was allocated internally | ||
3387 | by libpng. This can be changed, so that libpng will not free the data, | ||
3388 | or so that it will free data that was allocated by the user with png_malloc() | ||
3389 | or png_zalloc() and passed in via a png_set_*() function, with | ||
3390 | |||
3391 | png_data_freer(png_ptr, info_ptr, freer, mask) | ||
3392 | |||
3393 | freer - one of | ||
3394 | PNG_DESTROY_WILL_FREE_DATA | ||
3395 | PNG_SET_WILL_FREE_DATA | ||
3396 | PNG_USER_WILL_FREE_DATA | ||
3397 | |||
3398 | mask - which data elements are affected | ||
3399 | same choices as in png_free_data() | ||
3400 | |||
3401 | For example, to transfer responsibility for some data from a read structure | ||
3402 | to a write structure, you could use | ||
3403 | |||
3404 | png_data_freer(read_ptr, read_info_ptr, | ||
3405 | PNG_USER_WILL_FREE_DATA, | ||
3406 | PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST) | ||
3407 | |||
3408 | png_data_freer(write_ptr, write_info_ptr, | ||
3409 | PNG_DESTROY_WILL_FREE_DATA, | ||
3410 | PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST) | ||
3411 | |||
3412 | thereby briefly reassigning responsibility for freeing to the user but | ||
3413 | immediately afterwards reassigning it once more to the write_destroy | ||
3414 | function. Having done this, it would then be safe to destroy the read | ||
3415 | structure and continue to use the PLTE, tRNS, and hIST data in the write | ||
3416 | structure. | ||
3417 | |||
3418 | This function only affects data that has already been allocated. | ||
3419 | You can call this function before calling after the png_set_*() functions | ||
3420 | to control whether the user or png_destroy_*() is supposed to free the data. | ||
3421 | When the user assumes responsibility for libpng-allocated data, the | ||
3422 | application must use | ||
3423 | png_free() to free it, and when the user transfers responsibility to libpng | ||
3424 | for data that the user has allocated, the user must have used png_malloc() | ||
3425 | or png_zalloc() to allocate it. | ||
3426 | |||
3427 | If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword | ||
3428 | separately, do not transfer responsibility for freeing text_ptr to libpng, | ||
3429 | because when libpng fills a png_text structure it combines these members with | ||
3430 | the key member, and png_free_data() will free only text_ptr.key. Similarly, | ||
3431 | if you transfer responsibility for free'ing text_ptr from libpng to your | ||
3432 | application, your application must not separately free those members. | ||
3433 | For a more compact example of writing a PNG image, see the file example.c. | ||
3434 | |||
3435 | V. Modifying/Customizing libpng: | ||
3436 | |||
3437 | There are two issues here. The first is changing how libpng does | ||
3438 | standard things like memory allocation, input/output, and error handling. | ||
3439 | The second deals with more complicated things like adding new chunks, | ||
3440 | adding new transformations, and generally changing how libpng works. | ||
3441 | Both of those are compile-time issues; that is, they are generally | ||
3442 | determined at the time the code is written, and there is rarely a need | ||
3443 | to provide the user with a means of changing them. | ||
3444 | |||
3445 | Memory allocation, input/output, and error handling | ||
3446 | |||
3447 | All of the memory allocation, input/output, and error handling in libpng | ||
3448 | goes through callbacks that are user-settable. The default routines are | ||
3449 | in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change | ||
3450 | these functions, call the appropriate png_set_*_fn() function. | ||
3451 | |||
3452 | Memory allocation is done through the functions png_malloc(), png_calloc(), | ||
3453 | and png_free(). These currently just call the standard C functions. | ||
3454 | png_calloc() calls png_malloc() and then clears the newly | ||
3455 | allocated memory to zero. There is limited support for certain systems | ||
3456 | with segmented memory architectures and the types of pointers declared by | ||
3457 | png.h match this; you will have to use appropriate pointers in your | ||
3458 | application. Since it is | ||
3459 | unlikely that the method of handling memory allocation on a platform | ||
3460 | will change between applications, these functions must be modified in | ||
3461 | the library at compile time. If you prefer to use a different method | ||
3462 | of allocating and freeing data, you can use png_create_read_struct_2() or | ||
3463 | png_create_write_struct_2() to register your own functions as described | ||
3464 | above. These functions also provide a void pointer that can be retrieved | ||
3465 | via | ||
3466 | |||
3467 | mem_ptr=png_get_mem_ptr(png_ptr); | ||
3468 | |||
3469 | Your replacement memory functions must have prototypes as follows: | ||
3470 | |||
3471 | png_voidp malloc_fn(png_structp png_ptr, | ||
3472 | png_alloc_size_t size); | ||
3473 | |||
3474 | void free_fn(png_structp png_ptr, png_voidp ptr); | ||
3475 | |||
3476 | Your malloc_fn() must return NULL in case of failure. The png_malloc() | ||
3477 | function will normally call png_error() if it receives a NULL from the | ||
3478 | system memory allocator or from your replacement malloc_fn(). | ||
3479 | |||
3480 | Your free_fn() will never be called with a NULL ptr, since libpng's | ||
3481 | png_free() checks for NULL before calling free_fn(). | ||
3482 | |||
3483 | Input/Output in libpng is done through png_read() and png_write(), | ||
3484 | which currently just call fread() and fwrite(). The FILE * is stored in | ||
3485 | png_struct and is initialized via png_init_io(). If you wish to change | ||
3486 | the method of I/O, the library supplies callbacks that you can set | ||
3487 | through the function png_set_read_fn() and png_set_write_fn() at run | ||
3488 | time, instead of calling the png_init_io() function. These functions | ||
3489 | also provide a void pointer that can be retrieved via the function | ||
3490 | png_get_io_ptr(). For example: | ||
3491 | |||
3492 | png_set_read_fn(png_structp read_ptr, | ||
3493 | voidp read_io_ptr, png_rw_ptr read_data_fn) | ||
3494 | |||
3495 | png_set_write_fn(png_structp write_ptr, | ||
3496 | voidp write_io_ptr, png_rw_ptr write_data_fn, | ||
3497 | png_flush_ptr output_flush_fn); | ||
3498 | |||
3499 | voidp read_io_ptr = png_get_io_ptr(read_ptr); | ||
3500 | voidp write_io_ptr = png_get_io_ptr(write_ptr); | ||
3501 | |||
3502 | The replacement I/O functions must have prototypes as follows: | ||
3503 | |||
3504 | void user_read_data(png_structp png_ptr, | ||
3505 | png_bytep data, png_size_t length); | ||
3506 | |||
3507 | void user_write_data(png_structp png_ptr, | ||
3508 | png_bytep data, png_size_t length); | ||
3509 | |||
3510 | void user_flush_data(png_structp png_ptr); | ||
3511 | |||
3512 | The user_read_data() function is responsible for detecting and | ||
3513 | handling end-of-data errors. | ||
3514 | |||
3515 | Supplying NULL for the read, write, or flush functions sets them back | ||
3516 | to using the default C stream functions, which expect the io_ptr to | ||
3517 | point to a standard *FILE structure. It is probably a mistake | ||
3518 | to use NULL for one of write_data_fn and output_flush_fn but not both | ||
3519 | of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined. | ||
3520 | It is an error to read from a write stream, and vice versa. | ||
3521 | |||
3522 | Error handling in libpng is done through png_error() and png_warning(). | ||
3523 | Errors handled through png_error() are fatal, meaning that png_error() | ||
3524 | should never return to its caller. Currently, this is handled via | ||
3525 | setjmp() and longjmp() (unless you have compiled libpng with | ||
3526 | PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()), | ||
3527 | but you could change this to do things like exit() if you should wish, | ||
3528 | as long as your function does not return. | ||
3529 | |||
3530 | On non-fatal errors, png_warning() is called | ||
3531 | to print a warning message, and then control returns to the calling code. | ||
3532 | By default png_error() and png_warning() print a message on stderr via | ||
3533 | fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined | ||
3534 | (because you don't want the messages) or PNG_NO_STDIO defined (because | ||
3535 | fprintf() isn't available). If you wish to change the behavior of the error | ||
3536 | functions, you will need to set up your own message callbacks. These | ||
3537 | functions are normally supplied at the time that the png_struct is created. | ||
3538 | It is also possible to redirect errors and warnings to your own replacement | ||
3539 | functions after png_create_*_struct() has been called by calling: | ||
3540 | |||
3541 | png_set_error_fn(png_structp png_ptr, | ||
3542 | png_voidp error_ptr, png_error_ptr error_fn, | ||
3543 | png_error_ptr warning_fn); | ||
3544 | |||
3545 | png_voidp error_ptr = png_get_error_ptr(png_ptr); | ||
3546 | |||
3547 | If NULL is supplied for either error_fn or warning_fn, then the libpng | ||
3548 | default function will be used, calling fprintf() and/or longjmp() if a | ||
3549 | problem is encountered. The replacement error functions should have | ||
3550 | parameters as follows: | ||
3551 | |||
3552 | void user_error_fn(png_structp png_ptr, | ||
3553 | png_const_charp error_msg); | ||
3554 | |||
3555 | void user_warning_fn(png_structp png_ptr, | ||
3556 | png_const_charp warning_msg); | ||
3557 | |||
3558 | The motivation behind using setjmp() and longjmp() is the C++ throw and | ||
3559 | catch exception handling methods. This makes the code much easier to write, | ||
3560 | as there is no need to check every return code of every function call. | ||
3561 | However, there are some uncertainties about the status of local variables | ||
3562 | after a longjmp, so the user may want to be careful about doing anything | ||
3563 | after setjmp returns non-zero besides returning itself. Consult your | ||
3564 | compiler documentation for more details. For an alternative approach, you | ||
3565 | may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net), | ||
3566 | which is illustrated in pngvalid.c and in contrib/visupng. | ||
3567 | |||
3568 | Custom chunks | ||
3569 | |||
3570 | If you need to read or write custom chunks, you may need to get deeper | ||
3571 | into the libpng code. The library now has mechanisms for storing | ||
3572 | and writing chunks of unknown type; you can even declare callbacks | ||
3573 | for custom chunks. However, this may not be good enough if the | ||
3574 | library code itself needs to know about interactions between your | ||
3575 | chunk and existing `intrinsic' chunks. | ||
3576 | |||
3577 | If you need to write a new intrinsic chunk, first read the PNG | ||
3578 | specification. Acquire a first level of understanding of how it works. | ||
3579 | Pay particular attention to the sections that describe chunk names, | ||
3580 | and look at how other chunks were designed, so you can do things | ||
3581 | similarly. Second, check out the sections of libpng that read and | ||
3582 | write chunks. Try to find a chunk that is similar to yours and use | ||
3583 | it as a template. More details can be found in the comments inside | ||
3584 | the code. It is best to handle private or unknown chunks in a generic method, | ||
3585 | via callback functions, instead of by modifying libpng functions. This | ||
3586 | is illustrated in pngtest.c, which uses a callback function to handle a | ||
3587 | private "vpAg" chunk and the new "sTER" chunk, which are both unknown to | ||
3588 | libpng. | ||
3589 | |||
3590 | If you wish to write your own transformation for the data, look through | ||
3591 | the part of the code that does the transformations, and check out some of | ||
3592 | the simpler ones to get an idea of how they work. Try to find a similar | ||
3593 | transformation to the one you want to add and copy off of it. More details | ||
3594 | can be found in the comments inside the code itself. | ||
3595 | |||
3596 | Configuring for 16-bit platforms | ||
3597 | |||
3598 | You will want to look into zconf.h to tell zlib (and thus libpng) that | ||
3599 | it cannot allocate more then 64K at a time. Even if you can, the memory | ||
3600 | won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K. | ||
3601 | |||
3602 | Configuring for DOS | ||
3603 | |||
3604 | For DOS users who only have access to the lower 640K, you will | ||
3605 | have to limit zlib's memory usage via a png_set_compression_mem_level() | ||
3606 | call. See zlib.h or zconf.h in the zlib library for more information. | ||
3607 | |||
3608 | Configuring for Medium Model | ||
3609 | |||
3610 | Libpng's support for medium model has been tested on most of the popular | ||
3611 | compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets | ||
3612 | defined, and FAR gets defined to far in pngconf.h, and you should be | ||
3613 | all set. Everything in the library (except for zlib's structure) is | ||
3614 | expecting far data. You must use the typedefs with the p or pp on | ||
3615 | the end for pointers (or at least look at them and be careful). Make | ||
3616 | note that the rows of data are defined as png_bytepp, which is | ||
3617 | an "unsigned char far * far *". | ||
3618 | |||
3619 | Configuring for gui/windowing platforms: | ||
3620 | |||
3621 | You will need to write new error and warning functions that use the GUI | ||
3622 | interface, as described previously, and set them to be the error and | ||
3623 | warning functions at the time that png_create_*_struct() is called, | ||
3624 | in order to have them available during the structure initialization. | ||
3625 | They can be changed later via png_set_error_fn(). On some compilers, | ||
3626 | you may also have to change the memory allocators (png_malloc, etc.). | ||
3627 | |||
3628 | Configuring for compiler xxx: | ||
3629 | |||
3630 | All includes for libpng are in pngconf.h. If you need to add, change | ||
3631 | or delete an include, this is the place to do it. | ||
3632 | The includes that are not needed outside libpng are placed in pngpriv.h, | ||
3633 | which is only used by the routines inside libpng itself. | ||
3634 | The files in libpng proper only include pngpriv.h and png.h, which | ||
3635 | in turn includes pngconf.h and, as of libpng-1.5.0, pnglibconf.h. | ||
3636 | As of libpng-1.5.0, pngpriv.h also includes three other private header | ||
3637 | files, pngstruct.h, pnginfo.h, and pngdebug.h, which contain material | ||
3638 | that previously appeared in the public headers. | ||
3639 | |||
3640 | Configuring zlib: | ||
3641 | |||
3642 | There are special functions to configure the compression. Perhaps the | ||
3643 | most useful one changes the compression level, which currently uses | ||
3644 | input compression values in the range 0 - 9. The library normally | ||
3645 | uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests | ||
3646 | have shown that for a large majority of images, compression values in | ||
3647 | the range 3-6 compress nearly as well as higher levels, and do so much | ||
3648 | faster. For online applications it may be desirable to have maximum speed | ||
3649 | (Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also | ||
3650 | specify no compression (Z_NO_COMPRESSION = 0), but this would create | ||
3651 | files larger than just storing the raw bitmap. You can specify the | ||
3652 | compression level by calling: | ||
3653 | |||
3654 | #include zlib.h | ||
3655 | png_set_compression_level(png_ptr, level); | ||
3656 | |||
3657 | Another useful one is to reduce the memory level used by the library. | ||
3658 | The memory level defaults to 8, but it can be lowered if you are | ||
3659 | short on memory (running DOS, for example, where you only have 640K). | ||
3660 | Note that the memory level does have an effect on compression; among | ||
3661 | other things, lower levels will result in sections of incompressible | ||
3662 | data being emitted in smaller stored blocks, with a correspondingly | ||
3663 | larger relative overhead of up to 15% in the worst case. | ||
3664 | |||
3665 | #include zlib.h | ||
3666 | png_set_compression_mem_level(png_ptr, level); | ||
3667 | |||
3668 | The other functions are for configuring zlib. They are not recommended | ||
3669 | for normal use and may result in writing an invalid PNG file. See | ||
3670 | zlib.h for more information on what these mean. | ||
3671 | |||
3672 | #include zlib.h | ||
3673 | png_set_compression_strategy(png_ptr, | ||
3674 | strategy); | ||
3675 | |||
3676 | png_set_compression_window_bits(png_ptr, | ||
3677 | window_bits); | ||
3678 | |||
3679 | png_set_compression_method(png_ptr, method); | ||
3680 | |||
3681 | png_set_compression_buffer_size(png_ptr, size); | ||
3682 | |||
3683 | As of libpng version 1.5.4, additional APIs became | ||
3684 | available to set these separately for non-IDAT | ||
3685 | compressed chunks such as zTXt, iTXt, and iCCP: | ||
3686 | |||
3687 | #include zlib.h | ||
3688 | #if PNG_LIBPNG_VER <= 10504 | ||
3689 | png_set_text_compression_level(png_ptr, level); | ||
3690 | |||
3691 | png_set_text_compression_mem_level(png_ptr, level); | ||
3692 | |||
3693 | png_set_text_compression_strategy(png_ptr, | ||
3694 | strategy); | ||
3695 | |||
3696 | png_set_text_compression_window_bits(png_ptr, | ||
3697 | window_bits); | ||
3698 | |||
3699 | png_set_text_compression_method(png_ptr, method); | ||
3700 | #endif | ||
3701 | |||
3702 | Controlling row filtering | ||
3703 | |||
3704 | If you want to control whether libpng uses filtering or not, which | ||
3705 | filters are used, and how it goes about picking row filters, you | ||
3706 | can call one of these functions. The selection and configuration | ||
3707 | of row filters can have a significant impact on the size and | ||
3708 | encoding speed and a somewhat lesser impact on the decoding speed | ||
3709 | of an image. Filtering is enabled by default for RGB and grayscale | ||
3710 | images (with and without alpha), but not for paletted images nor | ||
3711 | for any images with bit depths less than 8 bits/pixel. | ||
3712 | |||
3713 | The 'method' parameter sets the main filtering method, which is | ||
3714 | currently only '0' in the PNG 1.2 specification. The 'filters' | ||
3715 | parameter sets which filter(s), if any, should be used for each | ||
3716 | scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS | ||
3717 | to turn filtering on and off, respectively. | ||
3718 | |||
3719 | Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB, | ||
3720 | PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise | ||
3721 | ORed together with '|' to specify one or more filters to use. | ||
3722 | These filters are described in more detail in the PNG specification. | ||
3723 | If you intend to change the filter type during the course of writing | ||
3724 | the image, you should start with flags set for all of the filters | ||
3725 | you intend to use so that libpng can initialize its internal | ||
3726 | structures appropriately for all of the filter types. (Note that this | ||
3727 | means the first row must always be adaptively filtered, because libpng | ||
3728 | currently does not allocate the filter buffers until png_write_row() | ||
3729 | is called for the first time.) | ||
3730 | |||
3731 | filters = PNG_FILTER_NONE | PNG_FILTER_SUB | ||
3732 | PNG_FILTER_UP | PNG_FILTER_AVG | | ||
3733 | PNG_FILTER_PAETH | PNG_ALL_FILTERS; | ||
3734 | |||
3735 | png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE, | ||
3736 | filters); | ||
3737 | The second parameter can also be | ||
3738 | PNG_INTRAPIXEL_DIFFERENCING if you are | ||
3739 | writing a PNG to be embedded in a MNG | ||
3740 | datastream. This parameter must be the | ||
3741 | same as the value of filter_method used | ||
3742 | in png_set_IHDR(). | ||
3743 | |||
3744 | It is also possible to influence how libpng chooses from among the | ||
3745 | available filters. This is done in one or both of two ways - by | ||
3746 | telling it how important it is to keep the same filter for successive | ||
3747 | rows, and by telling it the relative computational costs of the filters. | ||
3748 | |||
3749 | double weights[3] = {1.5, 1.3, 1.1}, | ||
3750 | costs[PNG_FILTER_VALUE_LAST] = | ||
3751 | {1.0, 1.3, 1.3, 1.5, 1.7}; | ||
3752 | |||
3753 | png_set_filter_heuristics(png_ptr, | ||
3754 | PNG_FILTER_HEURISTIC_WEIGHTED, 3, | ||
3755 | weights, costs); | ||
3756 | |||
3757 | The weights are multiplying factors that indicate to libpng that the | ||
3758 | row filter should be the same for successive rows unless another row filter | ||
3759 | is that many times better than the previous filter. In the above example, | ||
3760 | if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a | ||
3761 | "sum of absolute differences" 1.5 x 1.3 times higher than other filters | ||
3762 | and still be chosen, while the NONE filter could have a sum 1.1 times | ||
3763 | higher than other filters and still be chosen. Unspecified weights are | ||
3764 | taken to be 1.0, and the specified weights should probably be declining | ||
3765 | like those above in order to emphasize recent filters over older filters. | ||
3766 | |||
3767 | The filter costs specify for each filter type a relative decoding cost | ||
3768 | to be considered when selecting row filters. This means that filters | ||
3769 | with higher costs are less likely to be chosen over filters with lower | ||
3770 | costs, unless their "sum of absolute differences" is that much smaller. | ||
3771 | The costs do not necessarily reflect the exact computational speeds of | ||
3772 | the various filters, since this would unduly influence the final image | ||
3773 | size. | ||
3774 | |||
3775 | Note that the numbers above were invented purely for this example and | ||
3776 | are given only to help explain the function usage. Little testing has | ||
3777 | been done to find optimum values for either the costs or the weights. | ||
3778 | |||
3779 | Removing unwanted object code | ||
3780 | |||
3781 | There are a bunch of #define's in pngconf.h that control what parts of | ||
3782 | libpng are compiled. All the defines end in _SUPPORTED. If you are | ||
3783 | never going to use a capability, you can change the #define to #undef | ||
3784 | before recompiling libpng and save yourself code and data space, or | ||
3785 | you can turn off individual capabilities with defines that begin with | ||
3786 | PNG_NO_. | ||
3787 | |||
3788 | In libpng-1.5.0 and later, the #define's are in pnglibconf.h instead. | ||
3789 | |||
3790 | You can also turn all of the transforms and ancillary chunk capabilities | ||
3791 | off en masse with compiler directives that define | ||
3792 | PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS, | ||
3793 | or all four, | ||
3794 | along with directives to turn on any of the capabilities that you do | ||
3795 | want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the extra | ||
3796 | transformations but still leave the library fully capable of reading | ||
3797 | and writing PNG files with all known public chunks. Use of the | ||
3798 | PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive produces a library | ||
3799 | that is incapable of reading or writing ancillary chunks. If you are | ||
3800 | not using the progressive reading capability, you can turn that off | ||
3801 | with PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTERLACING | ||
3802 | capability, which you'll still have). | ||
3803 | |||
3804 | All the reading and writing specific code are in separate files, so the | ||
3805 | linker should only grab the files it needs. However, if you want to | ||
3806 | make sure, or if you are building a stand alone library, all the | ||
3807 | reading files start with "pngr" and all the writing files start with "pngw". | ||
3808 | The files that don't match either (like png.c, pngtrans.c, etc.) | ||
3809 | are used for both reading and writing, and always need to be included. | ||
3810 | The progressive reader is in pngpread.c | ||
3811 | |||
3812 | If you are creating or distributing a dynamically linked library (a .so | ||
3813 | or DLL file), you should not remove or disable any parts of the library, | ||
3814 | as this will cause applications linked with different versions of the | ||
3815 | library to fail if they call functions not available in your library. | ||
3816 | The size of the library itself should not be an issue, because only | ||
3817 | those sections that are actually used will be loaded into memory. | ||
3818 | |||
3819 | Requesting debug printout | ||
3820 | |||
3821 | The macro definition PNG_DEBUG can be used to request debugging | ||
3822 | printout. Set it to an integer value in the range 0 to 3. Higher | ||
3823 | numbers result in increasing amounts of debugging information. The | ||
3824 | information is printed to the "stderr" file, unless another file | ||
3825 | name is specified in the PNG_DEBUG_FILE macro definition. | ||
3826 | |||
3827 | When PNG_DEBUG > 0, the following functions (macros) become available: | ||
3828 | |||
3829 | png_debug(level, message) | ||
3830 | png_debug1(level, message, p1) | ||
3831 | png_debug2(level, message, p1, p2) | ||
3832 | |||
3833 | in which "level" is compared to PNG_DEBUG to decide whether to print | ||
3834 | the message, "message" is the formatted string to be printed, | ||
3835 | and p1 and p2 are parameters that are to be embedded in the string | ||
3836 | according to printf-style formatting directives. For example, | ||
3837 | |||
3838 | png_debug1(2, "foo=%d\n", foo); | ||
3839 | |||
3840 | is expanded to | ||
3841 | |||
3842 | if (PNG_DEBUG > 2) | ||
3843 | fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo); | ||
3844 | |||
3845 | When PNG_DEBUG is defined but is zero, the macros aren't defined, but you | ||
3846 | can still use PNG_DEBUG to control your own debugging: | ||
3847 | |||
3848 | #ifdef PNG_DEBUG | ||
3849 | fprintf(stderr, ... | ||
3850 | #endif | ||
3851 | |||
3852 | When PNG_DEBUG = 1, the macros are defined, but only png_debug statements | ||
3853 | having level = 0 will be printed. There aren't any such statements in | ||
3854 | this version of libpng, but if you insert some they will be printed. | ||
3855 | |||
3856 | VI. MNG support | ||
3857 | |||
3858 | The MNG specification (available at http://www.libpng.org/pub/mng) allows | ||
3859 | certain extensions to PNG for PNG images that are embedded in MNG datastreams. | ||
3860 | Libpng can support some of these extensions. To enable them, use the | ||
3861 | png_permit_mng_features() function: | ||
3862 | |||
3863 | feature_set = png_permit_mng_features(png_ptr, mask) | ||
3864 | |||
3865 | mask is a png_uint_32 containing the bitwise OR of the | ||
3866 | features you want to enable. These include | ||
3867 | PNG_FLAG_MNG_EMPTY_PLTE | ||
3868 | PNG_FLAG_MNG_FILTER_64 | ||
3869 | PNG_ALL_MNG_FEATURES | ||
3870 | |||
3871 | feature_set is a png_uint_32 that is the bitwise AND of | ||
3872 | your mask with the set of MNG features that is | ||
3873 | supported by the version of libpng that you are using. | ||
3874 | |||
3875 | It is an error to use this function when reading or writing a standalone | ||
3876 | PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped | ||
3877 | in a MNG datastream. As a minimum, it must have the MNG 8-byte signature | ||
3878 | and the MHDR and MEND chunks. Libpng does not provide support for these | ||
3879 | or any other MNG chunks; your application must provide its own support for | ||
3880 | them. You may wish to consider using libmng (available at | ||
3881 | http://www.libmng.com) instead. | ||
3882 | |||
3883 | VII. Changes to Libpng from version 0.88 | ||
3884 | |||
3885 | It should be noted that versions of libpng later than 0.96 are not | ||
3886 | distributed by the original libpng author, Guy Schalnat, nor by | ||
3887 | Andreas Dilger, who had taken over from Guy during 1996 and 1997, and | ||
3888 | distributed versions 0.89 through 0.96, but rather by another member | ||
3889 | of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are | ||
3890 | still alive and well, but they have moved on to other things. | ||
3891 | |||
3892 | The old libpng functions png_read_init(), png_write_init(), | ||
3893 | png_info_init(), png_read_destroy(), and png_write_destroy() have been | ||
3894 | moved to PNG_INTERNAL in version 0.95 to discourage their use. These | ||
3895 | functions will be removed from libpng version 1.4.0. | ||
3896 | |||
3897 | The preferred method of creating and initializing the libpng structures is | ||
3898 | via the png_create_read_struct(), png_create_write_struct(), and | ||
3899 | png_create_info_struct() because they isolate the size of the structures | ||
3900 | from the application, allow version error checking, and also allow the | ||
3901 | use of custom error handling routines during the initialization, which | ||
3902 | the old functions do not. The functions png_read_destroy() and | ||
3903 | png_write_destroy() do not actually free the memory that libpng | ||
3904 | allocated for these structs, but just reset the data structures, so they | ||
3905 | can be used instead of png_destroy_read_struct() and | ||
3906 | png_destroy_write_struct() if you feel there is too much system overhead | ||
3907 | allocating and freeing the png_struct for each image read. | ||
3908 | |||
3909 | Setting the error callbacks via png_set_message_fn() before | ||
3910 | png_read_init() as was suggested in libpng-0.88 is no longer supported | ||
3911 | because this caused applications that do not use custom error functions | ||
3912 | to fail if the png_ptr was not initialized to zero. It is still possible | ||
3913 | to set the error callbacks AFTER png_read_init(), or to change them with | ||
3914 | png_set_error_fn(), which is essentially the same function, but with a new | ||
3915 | name to force compilation errors with applications that try to use the old | ||
3916 | method. | ||
3917 | |||
3918 | Starting with version 1.0.7, you can find out which version of the library | ||
3919 | you are using at run-time: | ||
3920 | |||
3921 | png_uint_32 libpng_vn = png_access_version_number(); | ||
3922 | |||
3923 | The number libpng_vn is constructed from the major version, minor | ||
3924 | version with leading zero, and release number with leading zero, | ||
3925 | (e.g., libpng_vn for version 1.0.7 is 10007). | ||
3926 | |||
3927 | Note that this function does not take a png_ptr, so you can call it | ||
3928 | before you've created one. | ||
3929 | |||
3930 | You can also check which version of png.h you used when compiling your | ||
3931 | application: | ||
3932 | |||
3933 | png_uint_32 application_vn = PNG_LIBPNG_VER; | ||
3934 | |||
3935 | VIII. Changes to Libpng from version 1.0.x to 1.2.x | ||
3936 | |||
3937 | Support for user memory management was enabled by default. To | ||
3938 | accomplish this, the functions png_create_read_struct_2(), | ||
3939 | png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(), | ||
3940 | png_malloc_default(), and png_free_default() were added. | ||
3941 | |||
3942 | Support for the iTXt chunk has been enabled by default as of | ||
3943 | version 1.2.41. | ||
3944 | |||
3945 | Support for certain MNG features was enabled. | ||
3946 | |||
3947 | Support for numbered error messages was added. However, we never got | ||
3948 | around to actually numbering the error messages. The function | ||
3949 | png_set_strip_error_numbers() was added (Note: the prototype for this | ||
3950 | function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE | ||
3951 | builds of libpng-1.2.15. It was restored in libpng-1.2.36). | ||
3952 | |||
3953 | The png_malloc_warn() function was added at libpng-1.2.3. This issues | ||
3954 | a png_warning and returns NULL instead of aborting when it fails to | ||
3955 | acquire the requested memory allocation. | ||
3956 | |||
3957 | Support for setting user limits on image width and height was enabled | ||
3958 | by default. The functions png_set_user_limits(), png_get_user_width_max(), | ||
3959 | and png_get_user_height_max() were added at libpng-1.2.6. | ||
3960 | |||
3961 | The png_set_add_alpha() function was added at libpng-1.2.7. | ||
3962 | |||
3963 | The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9. | ||
3964 | Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the | ||
3965 | tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is | ||
3966 | deprecated. | ||
3967 | |||
3968 | A number of macro definitions in support of runtime selection of | ||
3969 | assembler code features (especially Intel MMX code support) were | ||
3970 | added at libpng-1.2.0: | ||
3971 | |||
3972 | PNG_ASM_FLAG_MMX_SUPPORT_COMPILED | ||
3973 | PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU | ||
3974 | PNG_ASM_FLAG_MMX_READ_COMBINE_ROW | ||
3975 | PNG_ASM_FLAG_MMX_READ_INTERLACE | ||
3976 | PNG_ASM_FLAG_MMX_READ_FILTER_SUB | ||
3977 | PNG_ASM_FLAG_MMX_READ_FILTER_UP | ||
3978 | PNG_ASM_FLAG_MMX_READ_FILTER_AVG | ||
3979 | PNG_ASM_FLAG_MMX_READ_FILTER_PAETH | ||
3980 | PNG_ASM_FLAGS_INITIALIZED | ||
3981 | PNG_MMX_READ_FLAGS | ||
3982 | PNG_MMX_FLAGS | ||
3983 | PNG_MMX_WRITE_FLAGS | ||
3984 | PNG_MMX_FLAGS | ||
3985 | |||
3986 | We added the following functions in support of runtime | ||
3987 | selection of assembler code features: | ||
3988 | |||
3989 | png_get_mmx_flagmask() | ||
3990 | png_set_mmx_thresholds() | ||
3991 | png_get_asm_flags() | ||
3992 | png_get_mmx_bitdepth_threshold() | ||
3993 | png_get_mmx_rowbytes_threshold() | ||
3994 | png_set_asm_flags() | ||
3995 | |||
3996 | We replaced all of these functions with simple stubs in libpng-1.2.20, | ||
3997 | when the Intel assembler code was removed due to a licensing issue. | ||
3998 | |||
3999 | These macros are deprecated: | ||
4000 | |||
4001 | PNG_READ_TRANSFORMS_NOT_SUPPORTED | ||
4002 | PNG_PROGRESSIVE_READ_NOT_SUPPORTED | ||
4003 | PNG_NO_SEQUENTIAL_READ_SUPPORTED | ||
4004 | PNG_WRITE_TRANSFORMS_NOT_SUPPORTED | ||
4005 | PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED | ||
4006 | PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED | ||
4007 | |||
4008 | They have been replaced, respectively, by: | ||
4009 | |||
4010 | PNG_NO_READ_TRANSFORMS | ||
4011 | PNG_NO_PROGRESSIVE_READ | ||
4012 | PNG_NO_SEQUENTIAL_READ | ||
4013 | PNG_NO_WRITE_TRANSFORMS | ||
4014 | PNG_NO_READ_ANCILLARY_CHUNKS | ||
4015 | PNG_NO_WRITE_ANCILLARY_CHUNKS | ||
4016 | |||
4017 | PNG_MAX_UINT was replaced with PNG_UINT_31_MAX. It has been | ||
4018 | deprecated since libpng-1.0.16 and libpng-1.2.6. | ||
4019 | |||
4020 | The function | ||
4021 | png_check_sig(sig, num) | ||
4022 | was replaced with | ||
4023 | !png_sig_cmp(sig, 0, num) | ||
4024 | It has been deprecated since libpng-0.90. | ||
4025 | |||
4026 | The function | ||
4027 | png_set_gray_1_2_4_to_8() | ||
4028 | which also expands tRNS to alpha was replaced with | ||
4029 | png_set_expand_gray_1_2_4_to_8() | ||
4030 | which does not. It has been deprecated since libpng-1.0.18 and 1.2.9. | ||
4031 | |||
4032 | IX. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x | ||
4033 | |||
4034 | Private libpng prototypes and macro definitions were moved from | ||
4035 | png.h and pngconf.h into a new pngpriv.h header file. | ||
4036 | |||
4037 | Functions png_set_benign_errors(), png_benign_error(), and | ||
4038 | png_chunk_benign_error() were added. | ||
4039 | |||
4040 | Support for setting the maximum amount of memory that the application | ||
4041 | will allocate for reading chunks was added, as a security measure. | ||
4042 | The functions png_set_chunk_cache_max() and png_get_chunk_cache_max() | ||
4043 | were added to the library. | ||
4044 | |||
4045 | We implemented support for I/O states by adding png_ptr member io_state | ||
4046 | and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c | ||
4047 | |||
4048 | We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level | ||
4049 | input transforms. | ||
4050 | |||
4051 | Checking for and reporting of errors in the IHDR chunk is more thorough. | ||
4052 | |||
4053 | Support for global arrays was removed, to improve thread safety. | ||
4054 | |||
4055 | Some obsolete/deprecated macros and functions have been removed. | ||
4056 | |||
4057 | Typecasted NULL definitions such as | ||
4058 | #define png_voidp_NULL (png_voidp)NULL | ||
4059 | were eliminated. If you used these in your application, just use | ||
4060 | NULL instead. | ||
4061 | |||
4062 | The png_struct and info_struct members "trans" and "trans_values" were | ||
4063 | changed to "trans_alpha" and "trans_color", respectively. | ||
4064 | |||
4065 | The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles | ||
4066 | were removed. | ||
4067 | |||
4068 | The PNG_1_0_X and PNG_1_2_X macros were eliminated. | ||
4069 | |||
4070 | The PNG_LEGACY_SUPPORTED macro was eliminated. | ||
4071 | |||
4072 | Many WIN32_WCE #ifdefs were removed. | ||
4073 | |||
4074 | The functions png_read_init(info_ptr), png_write_init(info_ptr), | ||
4075 | png_info_init(info_ptr), png_read_destroy(), and png_write_destroy() | ||
4076 | have been removed. They have been deprecated since libpng-0.95. | ||
4077 | |||
4078 | The png_permit_empty_plte() was removed. It has been deprecated | ||
4079 | since libpng-1.0.9. Use png_permit_mng_features() instead. | ||
4080 | |||
4081 | We removed the obsolete stub functions png_get_mmx_flagmask(), | ||
4082 | png_set_mmx_thresholds(), png_get_asm_flags(), | ||
4083 | png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(), | ||
4084 | png_set_asm_flags(), and png_mmx_supported() | ||
4085 | |||
4086 | We removed the obsolete png_check_sig(), png_memcpy_check(), and | ||
4087 | png_memset_check() functions. Instead use !png_sig_cmp(), memcpy(), | ||
4088 | and memset(), respectively. | ||
4089 | |||
4090 | The function png_set_gray_1_2_4_to_8() was removed. It has been | ||
4091 | deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with | ||
4092 | png_set_expand_gray_1_2_4_to_8() because the former function also | ||
4093 | expanded any tRNS chunk to an alpha channel. | ||
4094 | |||
4095 | Macros for png_get_uint_16, png_get_uint_32, and png_get_int_32 | ||
4096 | were added and are used by default instead of the corresponding | ||
4097 | functions. Unfortunately, | ||
4098 | from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the | ||
4099 | function) incorrectly returned a value of type png_uint_32. | ||
4100 | |||
4101 | We changed the prototype for png_malloc() from | ||
4102 | png_malloc(png_structp png_ptr, png_uint_32 size) | ||
4103 | to | ||
4104 | png_malloc(png_structp png_ptr, png_alloc_size_t size) | ||
4105 | |||
4106 | This also applies to the prototype for the user replacement malloc_fn(). | ||
4107 | |||
4108 | The png_calloc() function was added and is used in place of | ||
4109 | of "png_malloc(); memset();" except in the case in png_read_png() | ||
4110 | where the array consists of pointers; in this case a "for" loop is used | ||
4111 | after the png_malloc() to set the pointers to NULL, to give robust. | ||
4112 | behavior in case the application runs out of memory part-way through | ||
4113 | the process. | ||
4114 | |||
4115 | We changed the prototypes of png_get_compression_buffer_size() and | ||
4116 | png_set_compression_buffer_size() to work with png_size_t instead of | ||
4117 | png_uint_32. | ||
4118 | |||
4119 | Support for numbered error messages was removed by default, since we | ||
4120 | never got around to actually numbering the error messages. The function | ||
4121 | png_set_strip_error_numbers() was removed from the library by default. | ||
4122 | |||
4123 | The png_zalloc() and png_zfree() functions are no longer exported. | ||
4124 | The png_zalloc() function no longer zeroes out the memory that it | ||
4125 | allocates. | ||
4126 | |||
4127 | Support for dithering was disabled by default in libpng-1.4.0, because | ||
4128 | it has not been well tested and doesn't actually "dither". | ||
4129 | The code was not | ||
4130 | removed, however, and could be enabled by building libpng with | ||
4131 | PNG_READ_DITHER_SUPPORTED defined. In libpng-1.4.2, this support | ||
4132 | was reenabled, but the function was renamed png_set_quantize() to | ||
4133 | reflect more accurately what it actually does. At the same time, | ||
4134 | the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to | ||
4135 | PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED | ||
4136 | was renamed to PNG_READ_QUANTIZE_SUPPORTED. | ||
4137 | |||
4138 | We removed the trailing '.' from the warning and error messages. | ||
4139 | |||
4140 | X. Changes to Libpng from version 1.4.x to 1.5.x | ||
4141 | |||
4142 | From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the | ||
4143 | function) incorrectly returned a value of type png_uint_32. | ||
4144 | |||
4145 | A. Changes that affect users of libpng | ||
4146 | |||
4147 | There are no substantial API changes between the non-deprecated parts of | ||
4148 | the 1.4.5 API and the 1.5.0 API, however the ability to directly access | ||
4149 | the main libpng control structures, png_struct and png_info, deprecated | ||
4150 | in earlier versions of libpng, has been completely removed from | ||
4151 | libpng 1.5. | ||
4152 | |||
4153 | We no longer include zlib.h in png.h. Applications that need access | ||
4154 | to information in zlib.h will need to add the '#include "zlib.h"' | ||
4155 | directive. It does not matter whether it is placed prior to or after | ||
4156 | the '"#include png.h"' directive. | ||
4157 | |||
4158 | We moved the png_strcpy(), png_strncpy(), png_strlen(), png_memcpy(), | ||
4159 | png_memcmp(), png_sprintf, and png_memcpy() macros into a private | ||
4160 | header file (pngpriv.h) that is not accessible to applications. | ||
4161 | |||
4162 | In png_get_iCCP, the type of "profile" was changed from png_charpp | ||
4163 | to png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep. | ||
4164 | |||
4165 | There are changes of form in png.h, including new and changed macros to | ||
4166 | declare parts of the API. Some API functions with arguments that are | ||
4167 | pointers to data not modified within the function have been corrected to | ||
4168 | declare these arguments with PNG_CONST. | ||
4169 | |||
4170 | Much of the internal use of C macros to control the library build has also | ||
4171 | changed and some of this is visible in the exported header files, in | ||
4172 | particular the use of macros to control data and API elements visible | ||
4173 | during application compilation may require significant revision to | ||
4174 | application code. (It is extremely rare for an application to do this.) | ||
4175 | |||
4176 | Any program that compiled against libpng 1.4 and did not use deprecated | ||
4177 | features or access internal library structures should compile and work | ||
4178 | against libpng 1.5, except for the change in the prototype for | ||
4179 | png_get_iCCP() and png_set_iCCP() API functions mentioned above. | ||
4180 | |||
4181 | libpng 1.5.0 adds PNG_ PASS macros to help in the reading and writing of | ||
4182 | interlaced images. The macros return the number of rows and columns in | ||
4183 | each pass and information that can be used to de-interlace and (if | ||
4184 | absolutely necessary) interlace an image. | ||
4185 | |||
4186 | libpng 1.5.0 adds an API png_longjmp(png_ptr, value). This API calls | ||
4187 | the application-provided png_longjmp_ptr on the internal, but application | ||
4188 | initialized, longjmp buffer. It is provided as a convenience to avoid | ||
4189 | the need to use the png_jmpbuf macro, which had the unnecessary side | ||
4190 | effect of resetting the internal png_longjmp_ptr value. | ||
4191 | |||
4192 | libpng 1.5.0 includes a complete fixed point API. By default this is | ||
4193 | present along with the corresponding floating point API. In general the | ||
4194 | fixed point API is faster and smaller than the floating point one because | ||
4195 | the PNG file format used fixed point, not floating point. This applies | ||
4196 | even if the library uses floating point in internal calculations. A new | ||
4197 | macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether the library | ||
4198 | uses floating point arithmetic (the default) or fixed point arithmetic | ||
4199 | internally for performance critical calculations such as gamma correction. | ||
4200 | In some cases, the gamma calculations may produce slightly different | ||
4201 | results. This has changed the results in png_rgb_to_gray and in alpha | ||
4202 | composition (png_set_background for example). This applies even if the | ||
4203 | original image was already linear (gamma == 1.0) and, therefore, it is | ||
4204 | not necessary to linearize the image. This is because libpng has *not* | ||
4205 | been changed to optimize that case correctly, yet. | ||
4206 | |||
4207 | Fixed point support for the sCAL chunk comes with an important caveat; | ||
4208 | the sCAL specification uses a decimal encoding of floating point values | ||
4209 | and the accuracy of PNG fixed point values is insufficient for | ||
4210 | representation of these values. Consequently a "string" API | ||
4211 | (png_get_sCAL_s and png_set_sCAL_s) is the only reliable way of reading | ||
4212 | arbitrary sCAL chunks in the absence of either the floating point API or | ||
4213 | internal floating point calculations. | ||
4214 | |||
4215 | Applications no longer need to include the optional distribution header | ||
4216 | file pngusr.h or define the corresponding macros during application | ||
4217 | build in order to see the correct variant of the libpng API. From 1.5.0 | ||
4218 | application code can check for the corresponding _SUPPORTED macro: | ||
4219 | |||
4220 | #ifdef PNG_INCH_CONVERSIONS_SUPPORTED | ||
4221 | /* code that uses the inch conversion APIs. */ | ||
4222 | #endif | ||
4223 | |||
4224 | This macro will only be defined if the inch conversion functions have been | ||
4225 | compiled into libpng. The full set of macros, and whether or not support | ||
4226 | has been compiled in, are available in the header file pnglibconf.h. | ||
4227 | This header file is specific to the libpng build. Notice that prior to | ||
4228 | 1.5.0 the _SUPPORTED macros would always have the default definition unless | ||
4229 | reset by pngusr.h or by explicit settings on the compiler command line. | ||
4230 | These settings may produce compiler warnings or errors in 1.5.0 because | ||
4231 | of macro redefinition. | ||
4232 | |||
4233 | From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the | ||
4234 | function) incorrectly returned a value of type png_uint_32. libpng 1.5.0 | ||
4235 | is consistent with the implementation in 1.4.5 and 1.2.x (where the macro | ||
4236 | did not exist.) | ||
4237 | |||
4238 | Applications can now choose whether to use these macros or to call the | ||
4239 | corresponding function by defining PNG_USE_READ_MACROS or | ||
4240 | PNG_NO_USE_READ_MACROS before including png.h. Notice that this is | ||
4241 | only supported from 1.5.0 -defining PNG_NO_USE_READ_MACROS prior to 1.5.0 | ||
4242 | will lead to a link failure. | ||
4243 | |||
4244 | Prior to libpng-1.5.4, the zlib compressor used the same set of parameters | ||
4245 | when compressing the IDAT data and textual data such as zTXt and iCCP. | ||
4246 | In libpng-1.5.4 we reinitialized the zlib stream for each type of data. | ||
4247 | We added five png_set_text_*() functions for setting the parameters to | ||
4248 | use with textual data. | ||
4249 | |||
4250 | Prior to libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED | ||
4251 | option was off by default, and slightly inaccurate scaling occurred. | ||
4252 | This option can no longer be turned off, and the choice of accurate | ||
4253 | or inaccurate 16-to-8 scaling is by using the new png_set_scale_16_to_8() | ||
4254 | API for accurate scaling or the old png_set_strip_16_to_8() API for simple | ||
4255 | chopping. | ||
4256 | |||
4257 | Prior to libpng-1.5.4, the png_set_user_limits() function could only be | ||
4258 | used to reduce the width and height limits from the value of | ||
4259 | PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said | ||
4260 | that it could be used to override them. Now this function will reduce or | ||
4261 | increase the limits. | ||
4262 | |||
4263 | B. Changes to the build and configuration of libpng | ||
4264 | |||
4265 | Details of internal changes to the library code can be found in the CHANGES | ||
4266 | file and in the GIT repository logs. These will be of no concern to the vast | ||
4267 | majority of library users or builders, however the few who configure libpng | ||
4268 | to a non-default feature set may need to change how this is done. | ||
4269 | |||
4270 | There should be no need for library builders to alter build scripts if | ||
4271 | these use the distributed build support - configure or the makefiles - | ||
4272 | however users of the makefiles may care to update their build scripts | ||
4273 | to build pnglibconf.h where the corresponding makefile does not do so. | ||
4274 | |||
4275 | Building libpng with a non-default configuration has changed completely. | ||
4276 | The old method using pngusr.h should still work correctly even though the | ||
4277 | way pngusr.h is used in the build has been changed; however, library | ||
4278 | builders will probably want to examine the changes to take advantage of | ||
4279 | new capabilities and to simplify their build system. | ||
4280 | |||
4281 | B.1 Specific changes to library configuration capabilities | ||
4282 | |||
4283 | The library now supports a complete fixed point implementation and can | ||
4284 | thus be used on systems that have no floating point support or very | ||
4285 | limited or slow support. Previously gamma correction, an essential part | ||
4286 | of complete PNG support, required reasonably fast floating point. | ||
4287 | |||
4288 | As part of this the choice of internal implementation has been made | ||
4289 | independent of the choice of fixed versus floating point APIs and all the | ||
4290 | missing fixed point APIs have been implemented. | ||
4291 | |||
4292 | The exact mechanism used to control attributes of API functions has | ||
4293 | changed. A single set of operating system independent macro definitions | ||
4294 | is used and operating system specific directives are defined in | ||
4295 | pnglibconf.h | ||
4296 | |||
4297 | As part of this the mechanism used to choose procedure call standards on | ||
4298 | those systems that allow a choice has been changed. At present this only | ||
4299 | affects certain Microsoft (DOS, Windows) and IBM (OS/2) operating systems | ||
4300 | running on Intel processors. As before, PNGAPI is defined where required | ||
4301 | to control the exported API functions; however, two new macros, PNGCBAPI | ||
4302 | and PNGCAPI, are used instead for callback functions (PNGCBAPI) and | ||
4303 | (PNGCAPI) for functions that must match a C library prototype (currently | ||
4304 | only png_longjmp_ptr, which must match the C longjmp function.) The new | ||
4305 | approach is documented in pngconf.h | ||
4306 | |||
4307 | Despite these changes, libpng 1.5.0 only supports the native C function | ||
4308 | calling standard on those platforms tested so far (__cdecl on Microsoft | ||
4309 | Windows). This is because the support requirements for alternative | ||
4310 | calling conventions seem to no longer exist. Developers who find it | ||
4311 | necessary to set PNG_API_RULE to 1 should advise the mailing list | ||
4312 | (png-mng-implement) of this and library builders who use Openwatcom and | ||
4313 | therefore set PNG_API_RULE to 2 should also contact the mailing list. | ||
4314 | |||
4315 | A new test program, pngvalid, is provided in addition to pngtest. | ||
4316 | pngvalid validates the arithmetic accuracy of the gamma correction | ||
4317 | calculations and includes a number of validations of the file format. | ||
4318 | A subset of the full range of tests is run when "make check" is done | ||
4319 | (in the 'configure' build.) pngvalid also allows total allocated memory | ||
4320 | usage to be evaluated and performs additional memory overwrite validation. | ||
4321 | |||
4322 | Many changes to individual feature macros have been made. The following | ||
4323 | are the changes most likely to be noticed by library builders who | ||
4324 | configure libpng: | ||
4325 | |||
4326 | 1) All feature macros now have consistent naming: | ||
4327 | |||
4328 | #define PNG_NO_feature turns the feature off | ||
4329 | #define PNG_feature_SUPPORTED turns the feature on | ||
4330 | |||
4331 | pnglibconf.h contains one line for each feature macro which is either: | ||
4332 | |||
4333 | #define PNG_feature_SUPPORTED | ||
4334 | |||
4335 | if the feature is supported or: | ||
4336 | |||
4337 | /*#undef PNG_feature_SUPPORTED*/ | ||
4338 | |||
4339 | if it is not. Library code consistently checks for the 'SUPPORTED' macro. | ||
4340 | It does not, and libpng applications should not, check for the 'NO' macro | ||
4341 | which will not normally be defined even if the feature is not supported. | ||
4342 | The 'NO' macros are only used internally for setting or not setting the | ||
4343 | corresponding 'SUPPORTED' macros. | ||
4344 | |||
4345 | Compatibility with the old names is provided as follows: | ||
4346 | |||
4347 | PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED | ||
4348 | |||
4349 | And the following definitions disable the corresponding feature: | ||
4350 | |||
4351 | PNG_SETJMP_NOT_SUPPORTED disables SETJMP | ||
4352 | PNG_READ_TRANSFORMS_NOT_SUPPORTED disables READ_TRANSFORMS | ||
4353 | PNG_NO_READ_COMPOSITED_NODIV disables READ_COMPOSITE_NODIV | ||
4354 | PNG_WRITE_TRANSFORMS_NOT_SUPPORTED disables WRITE_TRANSFORMS | ||
4355 | PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED disables READ_ANCILLARY_CHUNKS | ||
4356 | PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables WRITE_ANCILLARY_CHUNKS | ||
4357 | |||
4358 | Library builders should remove use of the above, inconsistent, names. | ||
4359 | |||
4360 | 2) Warning and error message formatting was previously conditional on | ||
4361 | the STDIO feature. The library has been changed to use the | ||
4362 | CONSOLE_IO feature instead. This means that if CONSOLE_IO is disabled | ||
4363 | the library no longer uses the printf(3) functions, even though the | ||
4364 | default read/write implementations use (FILE) style stdio.h functions. | ||
4365 | |||
4366 | 3) Three feature macros now control the fixed/floating point decisions: | ||
4367 | |||
4368 | PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs | ||
4369 | |||
4370 | PNG_FIXED_POINT_SUPPORTED enables the fixed point APIs; however, in | ||
4371 | practice these are normally required internally anyway (because the PNG | ||
4372 | file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT | ||
4373 | merely stops the function from being exported. | ||
4374 | |||
4375 | PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating | ||
4376 | point implementation or the fixed point one. Typically the fixed point | ||
4377 | implementation is larger and slower than the floating point implementation | ||
4378 | on a system that supports floating point, however it may be faster on a | ||
4379 | system which lacks floating point hardware and therefore uses a software | ||
4380 | emulation. | ||
4381 | |||
4382 | 4) Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED. This allows the | ||
4383 | functions to read and write ints to be disabled independently of | ||
4384 | PNG_USE_READ_MACROS, which allows libpng to be built with the functions | ||
4385 | even though the default is to use the macros - this allows applications | ||
4386 | to choose at app buildtime whether or not to use macros (previously | ||
4387 | impossible because the functions weren't in the default build.) | ||
4388 | |||
4389 | B.2 Changes to the configuration mechanism | ||
4390 | |||
4391 | Prior to libpng-1.5.0 library builders who needed to configure libpng | ||
4392 | had either to modify the exported pngconf.h header file to add system | ||
4393 | specific configuration or had to write feature selection macros into | ||
4394 | pngusr.h and cause this to be included into pngconf.h by defining | ||
4395 | PNG_USER_CONFIG. The latter mechanism had the disadvantage that an | ||
4396 | application built without PNG_USER_CONFIG defined would see the | ||
4397 | unmodified, default, libpng API and thus would probably fail to link. | ||
4398 | |||
4399 | These mechanisms still work in the configure build and in any makefile | ||
4400 | build that builds pnglibconf.h, although the feature selection macros | ||
4401 | have changed somewhat as described above. In 1.5.0, however, pngusr.h is | ||
4402 | processed only once, when the exported header file pnglibconf.h is built. | ||
4403 | pngconf.h no longer includes pngusr.h, therefore pngusr.h is ignored after the | ||
4404 | build of pnglibconf.h and it is never included in an application build. | ||
4405 | |||
4406 | The rarely used alternative of adding a list of feature macros to the | ||
4407 | CFLAGS setting in the build also still works, however the macros will be | ||
4408 | copied to pnglibconf.h and this may produce macro redefinition warnings | ||
4409 | when the individual C files are compiled. | ||
4410 | |||
4411 | All configuration now only works if pnglibconf.h is built from | ||
4412 | scripts/pnglibconf.dfa. This requires the program awk. Brian Kernighan | ||
4413 | (the original author of awk) maintains C source code of that awk and this | ||
4414 | and all known later implementations (often called by subtly different | ||
4415 | names - nawk and gawk for example) are adequate to build pnglibconf.h. | ||
4416 | The Sun Microsystems (now Oracle) program 'awk' is an earlier version | ||
4417 | and does not work; this may also apply to other systems that have a | ||
4418 | functioning awk called 'nawk'. | ||
4419 | |||
4420 | Configuration options are now documented in scripts/pnglibconf.dfa. This | ||
4421 | file also includes dependency information that ensures a configuration is | ||
4422 | consistent; that is, if a feature is switched off dependent features are | ||
4423 | also removed. As a recommended alternative to using feature macros in | ||
4424 | pngusr.h a system builder may also define equivalent options in pngusr.dfa | ||
4425 | (or, indeed, any file) and add that to the configuration by setting | ||
4426 | DFA_XTRA to the file name. The makefiles in contrib/pngminim illustrate | ||
4427 | how to do this, and a case where pngusr.h is still required. | ||
4428 | |||
4429 | XI. Detecting libpng | ||
4430 | |||
4431 | The png_get_io_ptr() function has been present since libpng-0.88, has never | ||
4432 | changed, and is unaffected by conditional compilation macros. It is the | ||
4433 | best choice for use in configure scripts for detecting the presence of any | ||
4434 | libpng version since 0.88. In an autoconf "configure.in" you could use | ||
4435 | |||
4436 | AC_CHECK_LIB(png, png_get_io_ptr, ... | ||
4437 | |||
4438 | XII. Source code repository | ||
4439 | |||
4440 | Since about February 2009, version 1.2.34, libpng has been under "git" source | ||
4441 | control. The git repository was built from old libpng-x.y.z.tar.gz files | ||
4442 | going back to version 0.70. You can access the git repository (read only) | ||
4443 | at | ||
4444 | |||
4445 | git://libpng.git.sourceforge.net/gitroot/libpng | ||
4446 | |||
4447 | or you can browse it via "gitweb" at | ||
4448 | |||
4449 | http://libpng.git.sourceforge.net/git/gitweb.cgi?p=libpng | ||
4450 | |||
4451 | Patches can be sent to glennrp at users.sourceforge.net or to | ||
4452 | png-mng-implement at lists.sourceforge.net or you can upload them to | ||
4453 | the libpng bug tracker at | ||
4454 | |||
4455 | http://libpng.sourceforge.net | ||
4456 | |||
4457 | We also accept patches built from the tar or zip distributions, and | ||
4458 | simple verbal discriptions of bug fixes, reported either to the | ||
4459 | SourceForge bug tracker, to the png-mng-implement at lists.sf.net | ||
4460 | mailing list, or directly to glennrp. | ||
4461 | |||
4462 | XIII. Coding style | ||
4463 | |||
4464 | Our coding style is similar to the "Allman" style, with curly | ||
4465 | braces on separate lines: | ||
4466 | |||
4467 | if (condition) | ||
4468 | { | ||
4469 | action; | ||
4470 | } | ||
4471 | |||
4472 | else if (another condition) | ||
4473 | { | ||
4474 | another action; | ||
4475 | } | ||
4476 | |||
4477 | The braces can be omitted from simple one-line actions: | ||
4478 | |||
4479 | if (condition) | ||
4480 | return (0); | ||
4481 | |||
4482 | We use 3-space indentation, except for continued statements which | ||
4483 | are usually indented the same as the first line of the statement | ||
4484 | plus four more spaces. | ||
4485 | |||
4486 | For macro definitions we use 2-space indentation, always leaving the "#" | ||
4487 | in the first column. | ||
4488 | |||
4489 | #ifndef PNG_NO_FEATURE | ||
4490 | # ifndef PNG_FEATURE_SUPPORTED | ||
4491 | # define PNG_FEATURE_SUPPORTED | ||
4492 | # endif | ||
4493 | #endif | ||
4494 | |||
4495 | Comments appear with the leading "/*" at the same indentation as | ||
4496 | the statement that follows the comment: | ||
4497 | |||
4498 | /* Single-line comment */ | ||
4499 | statement; | ||
4500 | |||
4501 | /* This is a multiple-line | ||
4502 | * comment. | ||
4503 | */ | ||
4504 | statement; | ||
4505 | |||
4506 | Very short comments can be placed after the end of the statement | ||
4507 | to which they pertain: | ||
4508 | |||
4509 | statement; /* comment */ | ||
4510 | |||
4511 | We don't use C++ style ("//") comments. We have, however, | ||
4512 | used them in the past in some now-abandoned MMX assembler | ||
4513 | code. | ||
4514 | |||
4515 | Functions and their curly braces are not indented, and | ||
4516 | exported functions are marked with PNGAPI: | ||
4517 | |||
4518 | /* This is a public function that is visible to | ||
4519 | * application programmers. It does thus-and-so. | ||
4520 | */ | ||
4521 | void PNGAPI | ||
4522 | png_exported_function(png_ptr, png_info, foo) | ||
4523 | { | ||
4524 | body; | ||
4525 | } | ||
4526 | |||
4527 | The prototypes for all exported functions appear in png.h, | ||
4528 | above the comment that says | ||
4529 | |||
4530 | /* Maintainer: Put new public prototypes here ... */ | ||
4531 | |||
4532 | We mark all non-exported functions with "/* PRIVATE */"": | ||
4533 | |||
4534 | void /* PRIVATE */ | ||
4535 | png_non_exported_function(png_ptr, png_info, foo) | ||
4536 | { | ||
4537 | body; | ||
4538 | } | ||
4539 | |||
4540 | The prototypes for non-exported functions (except for those in | ||
4541 | pngtest) appear in | ||
4542 | pngpriv.h | ||
4543 | above the comment that says | ||
4544 | |||
4545 | /* Maintainer: Put new private prototypes here ^ and in libpngpf.3 */ | ||
4546 | |||
4547 | To avoid polluting the global namespace, the names of all exported | ||
4548 | functions and variables begin with "png_", and all publicly visible C | ||
4549 | preprocessor macros begin with "PNG_". We request that applications that | ||
4550 | use libpng *not* begin any of their own symbols with either of these strings. | ||
4551 | |||
4552 | We put a space after each comma and after each semicolon | ||
4553 | in "for" statements, and we put spaces before and after each | ||
4554 | C binary operator and after "for" or "while", and before | ||
4555 | "?". We don't put a space between a typecast and the expression | ||
4556 | being cast, nor do we put one between a function name and the | ||
4557 | left parenthesis that follows it: | ||
4558 | |||
4559 | for (i = 2; i > 0; --i) | ||
4560 | y[i] = a(x) + (int)b; | ||
4561 | |||
4562 | We prefer #ifdef and #ifndef to #if defined() and if !defined() | ||
4563 | when there is only one macro being tested. | ||
4564 | |||
4565 | We prefer to express integers that are used as bit masks in hex format, | ||
4566 | with an even number of lower-case hex digits (e.g., 0x00, 0xff, 0x0100). | ||
4567 | |||
4568 | We do not use the TAB character for indentation in the C sources. | ||
4569 | |||
4570 | Lines do not exceed 80 characters. | ||
4571 | |||
4572 | Other rules can be inferred by inspecting the libpng source. | ||
4573 | |||
4574 | XIV. Y2K Compliance in libpng | ||
4575 | |||
4576 | February 18, 2012 | ||
4577 | |||
4578 | Since the PNG Development group is an ad-hoc body, we can't make | ||
4579 | an official declaration. | ||
4580 | |||
4581 | This is your unofficial assurance that libpng from version 0.71 and | ||
4582 | upward through 1.5.9 are Y2K compliant. It is my belief that earlier | ||
4583 | versions were also Y2K compliant. | ||
4584 | |||
4585 | Libpng only has three year fields. One is a 2-byte unsigned integer that | ||
4586 | will hold years up to 65535. The other two hold the date in text | ||
4587 | format, and will hold years up to 9999. | ||
4588 | |||
4589 | The integer is | ||
4590 | "png_uint_16 year" in png_time_struct. | ||
4591 | |||
4592 | The strings are | ||
4593 | "png_charp time_buffer" in png_struct and | ||
4594 | "near_time_buffer", which is a local character string in png.c. | ||
4595 | |||
4596 | There are seven time-related functions: | ||
4597 | |||
4598 | png_convert_to_rfc_1123() in png.c | ||
4599 | (formerly png_convert_to_rfc_1152() in error) | ||
4600 | png_convert_from_struct_tm() in pngwrite.c, called | ||
4601 | in pngwrite.c | ||
4602 | png_convert_from_time_t() in pngwrite.c | ||
4603 | png_get_tIME() in pngget.c | ||
4604 | png_handle_tIME() in pngrutil.c, called in pngread.c | ||
4605 | png_set_tIME() in pngset.c | ||
4606 | png_write_tIME() in pngwutil.c, called in pngwrite.c | ||
4607 | |||
4608 | All appear to handle dates properly in a Y2K environment. The | ||
4609 | png_convert_from_time_t() function calls gmtime() to convert from system | ||
4610 | clock time, which returns (year - 1900), which we properly convert to | ||
4611 | the full 4-digit year. There is a possibility that applications using | ||
4612 | libpng are not passing 4-digit years into the png_convert_to_rfc_1123() | ||
4613 | function, or that they are incorrectly passing only a 2-digit year | ||
4614 | instead of "year - 1900" into the png_convert_from_struct_tm() function, | ||
4615 | but this is not under our control. The libpng documentation has always | ||
4616 | stated that it works with 4-digit years, and the APIs have been | ||
4617 | documented as such. | ||
4618 | |||
4619 | The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned | ||
4620 | integer to hold the year, and can hold years as large as 65535. | ||
4621 | |||
4622 | zlib, upon which libpng depends, is also Y2K compliant. It contains | ||
4623 | no date-related code. | ||
4624 | |||
4625 | |||
4626 | Glenn Randers-Pehrson | ||
4627 | libpng maintainer | ||
4628 | PNG Development Group | ||