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1 | .TH CJPEG 1 "28 August 2011" | ||
2 | .SH NAME | ||
3 | cjpeg \- compress an image file to a JPEG file | ||
4 | .SH SYNOPSIS | ||
5 | .B cjpeg | ||
6 | [ | ||
7 | .I options | ||
8 | ] | ||
9 | [ | ||
10 | .I filename | ||
11 | ] | ||
12 | .LP | ||
13 | .SH DESCRIPTION | ||
14 | .LP | ||
15 | .B cjpeg | ||
16 | compresses the named image file, or the standard input if no file is | ||
17 | named, and produces a JPEG/JFIF file on the standard output. | ||
18 | The currently supported input file formats are: PPM (PBMPLUS color | ||
19 | format), PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster | ||
20 | Toolkit format). (RLE is supported only if the URT library is available.) | ||
21 | .SH OPTIONS | ||
22 | All switch names may be abbreviated; for example, | ||
23 | .B \-grayscale | ||
24 | may be written | ||
25 | .B \-gray | ||
26 | or | ||
27 | .BR \-gr . | ||
28 | Most of the "basic" switches can be abbreviated to as little as one letter. | ||
29 | Upper and lower case are equivalent (thus | ||
30 | .B \-BMP | ||
31 | is the same as | ||
32 | .BR \-bmp ). | ||
33 | British spellings are also accepted (e.g., | ||
34 | .BR \-greyscale ), | ||
35 | though for brevity these are not mentioned below. | ||
36 | .PP | ||
37 | The basic switches are: | ||
38 | .TP | ||
39 | .BI \-quality " N[,...]" | ||
40 | Scale quantization tables to adjust image quality. Quality is 0 (worst) to | ||
41 | 100 (best); default is 75. (See below for more info.) | ||
42 | .TP | ||
43 | .B \-grayscale | ||
44 | Create monochrome JPEG file from color input. Be sure to use this switch when | ||
45 | compressing a grayscale BMP file, because | ||
46 | .B cjpeg | ||
47 | isn't bright enough to notice whether a BMP file uses only shades of gray. | ||
48 | By saying | ||
49 | .BR \-grayscale , | ||
50 | you'll get a smaller JPEG file that takes less time to process. | ||
51 | .TP | ||
52 | .B \-rgb | ||
53 | Create RGB JPEG file. | ||
54 | Using this switch suppresses the conversion from RGB | ||
55 | colorspace input to the default YCbCr JPEG colorspace. | ||
56 | Use this switch in combination with the | ||
57 | .BI \-block " N" | ||
58 | switch (see below) for lossless JPEG coding. | ||
59 | .TP | ||
60 | .B \-optimize | ||
61 | Perform optimization of entropy encoding parameters. Without this, default | ||
62 | encoding parameters are used. | ||
63 | .B \-optimize | ||
64 | usually makes the JPEG file a little smaller, but | ||
65 | .B cjpeg | ||
66 | runs somewhat slower and needs much more memory. Image quality and speed of | ||
67 | decompression are unaffected by | ||
68 | .BR \-optimize . | ||
69 | .TP | ||
70 | .B \-progressive | ||
71 | Create progressive JPEG file (see below). | ||
72 | .TP | ||
73 | .BI \-scale " M/N" | ||
74 | Scale the output image by a factor M/N. Currently supported scale factors are | ||
75 | M/N with all N from 1 to 16, where M is the destination DCT size, which is 8 | ||
76 | by default (see | ||
77 | .BI \-block " N" | ||
78 | switch below). | ||
79 | .TP | ||
80 | .B \-targa | ||
81 | Input file is Targa format. Targa files that contain an "identification" | ||
82 | field will not be automatically recognized by | ||
83 | .BR cjpeg ; | ||
84 | for such files you must specify | ||
85 | .B \-targa | ||
86 | to make | ||
87 | .B cjpeg | ||
88 | treat the input as Targa format. | ||
89 | For most Targa files, you won't need this switch. | ||
90 | .PP | ||
91 | The | ||
92 | .B \-quality | ||
93 | switch lets you trade off compressed file size against quality of the | ||
94 | reconstructed image: the higher the quality setting, the larger the JPEG file, | ||
95 | and the closer the output image will be to the original input. Normally you | ||
96 | want to use the lowest quality setting (smallest file) that decompresses into | ||
97 | something visually indistinguishable from the original image. For this | ||
98 | purpose the quality setting should be between 50 and 95; the default of 75 is | ||
99 | often about right. If you see defects at | ||
100 | .B \-quality | ||
101 | 75, then go up 5 or 10 counts at a time until you are happy with the output | ||
102 | image. (The optimal setting will vary from one image to another.) | ||
103 | .PP | ||
104 | .B \-quality | ||
105 | 100 will generate a quantization table of all 1's, minimizing loss in the | ||
106 | quantization step (but there is still information loss in subsampling, as well | ||
107 | as roundoff error). This setting is mainly of interest for experimental | ||
108 | purposes. Quality values above about 95 are | ||
109 | .B not | ||
110 | recommended for normal use; the compressed file size goes up dramatically for | ||
111 | hardly any gain in output image quality. | ||
112 | .PP | ||
113 | In the other direction, quality values below 50 will produce very small files | ||
114 | of low image quality. Settings around 5 to 10 might be useful in preparing an | ||
115 | index of a large image library, for example. Try | ||
116 | .B \-quality | ||
117 | 2 (or so) for some amusing Cubist effects. (Note: quality | ||
118 | values below about 25 generate 2-byte quantization tables, which are | ||
119 | considered optional in the JPEG standard. | ||
120 | .B cjpeg | ||
121 | emits a warning message when you give such a quality value, because some | ||
122 | other JPEG programs may be unable to decode the resulting file. Use | ||
123 | .B \-baseline | ||
124 | if you need to ensure compatibility at low quality values.) | ||
125 | .PP | ||
126 | The | ||
127 | .B \-quality | ||
128 | option has been extended in IJG version 7 for support of separate quality | ||
129 | settings for luminance and chrominance (or in general, for every provided | ||
130 | quantization table slot). This feature is useful for high-quality | ||
131 | applications which cannot accept the damage of color data by coarse | ||
132 | subsampling settings. You can now easily reduce the color data amount more | ||
133 | smoothly with finer control without separate subsampling. The resulting file | ||
134 | is fully compliant with standard JPEG decoders. | ||
135 | Note that the | ||
136 | .B \-quality | ||
137 | ratings refer to the quantization table slots, and that the last value is | ||
138 | replicated if there are more q-table slots than parameters. The default | ||
139 | q-table slots are 0 for luminance and 1 for chrominance with default tables as | ||
140 | given in the JPEG standard. This is compatible with the old behaviour in case | ||
141 | that only one parameter is given, which is then used for both luminance and | ||
142 | chrominance (slots 0 and 1). More or custom quantization tables can be set | ||
143 | with | ||
144 | .B \-qtables | ||
145 | and assigned to components with | ||
146 | .B \-qslots | ||
147 | parameter (see the "wizard" switches below). | ||
148 | .B Caution: | ||
149 | You must explicitly add | ||
150 | .BI \-sample " 1x1" | ||
151 | for efficient separate color | ||
152 | quality selection, since the default value used by library is 2x2! | ||
153 | .PP | ||
154 | The | ||
155 | .B \-progressive | ||
156 | switch creates a "progressive JPEG" file. In this type of JPEG file, the data | ||
157 | is stored in multiple scans of increasing quality. If the file is being | ||
158 | transmitted over a slow communications link, the decoder can use the first | ||
159 | scan to display a low-quality image very quickly, and can then improve the | ||
160 | display with each subsequent scan. The final image is exactly equivalent to a | ||
161 | standard JPEG file of the same quality setting, and the total file size is | ||
162 | about the same --- often a little smaller. | ||
163 | .PP | ||
164 | Switches for advanced users: | ||
165 | .TP | ||
166 | .B \-arithmetic | ||
167 | Use arithmetic coding. | ||
168 | .B Caution: | ||
169 | arithmetic coded JPEG is not yet widely implemented, so many decoders will be | ||
170 | unable to view an arithmetic coded JPEG file at all. | ||
171 | .TP | ||
172 | .BI \-block " N" | ||
173 | Set DCT block size. All N from 1 to 16 are possible. | ||
174 | Default is 8 (baseline format). | ||
175 | Larger values produce higher compression, | ||
176 | smaller values produce higher quality | ||
177 | (exact DCT stage possible with 1 or 2; with the default quality of 75 and | ||
178 | default Luminance qtable the DCT+Quantization stage is lossless for N=1). | ||
179 | .B Caution: | ||
180 | An implementation of the JPEG SmartScale extension is required for this | ||
181 | feature. SmartScale enabled JPEG is not yet widely implemented, so many | ||
182 | decoders will be unable to view a SmartScale extended JPEG file at all. | ||
183 | .TP | ||
184 | .B \-dct int | ||
185 | Use integer DCT method (default). | ||
186 | .TP | ||
187 | .B \-dct fast | ||
188 | Use fast integer DCT (less accurate). | ||
189 | .TP | ||
190 | .B \-dct float | ||
191 | Use floating-point DCT method. | ||
192 | The float method is very slightly more accurate than the int method, but is | ||
193 | much slower unless your machine has very fast floating-point hardware. Also | ||
194 | note that results of the floating-point method may vary slightly across | ||
195 | machines, while the integer methods should give the same results everywhere. | ||
196 | The fast integer method is much less accurate than the other two. | ||
197 | .TP | ||
198 | .B \-nosmooth | ||
199 | Don't use high-quality downsampling. | ||
200 | .TP | ||
201 | .BI \-restart " N" | ||
202 | Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is | ||
203 | attached to the number. | ||
204 | .B \-restart 0 | ||
205 | (the default) means no restart markers. | ||
206 | .TP | ||
207 | .BI \-smooth " N" | ||
208 | Smooth the input image to eliminate dithering noise. N, ranging from 1 to | ||
209 | 100, indicates the strength of smoothing. 0 (the default) means no smoothing. | ||
210 | .TP | ||
211 | .BI \-maxmemory " N" | ||
212 | Set limit for amount of memory to use in processing large images. Value is | ||
213 | in thousands of bytes, or millions of bytes if "M" is attached to the | ||
214 | number. For example, | ||
215 | .B \-max 4m | ||
216 | selects 4000000 bytes. If more space is needed, temporary files will be used. | ||
217 | .TP | ||
218 | .BI \-outfile " name" | ||
219 | Send output image to the named file, not to standard output. | ||
220 | .TP | ||
221 | .B \-verbose | ||
222 | Enable debug printout. More | ||
223 | .BR \-v 's | ||
224 | give more output. Also, version information is printed at startup. | ||
225 | .TP | ||
226 | .B \-debug | ||
227 | Same as | ||
228 | .BR \-verbose . | ||
229 | .PP | ||
230 | The | ||
231 | .B \-restart | ||
232 | option inserts extra markers that allow a JPEG decoder to resynchronize after | ||
233 | a transmission error. Without restart markers, any damage to a compressed | ||
234 | file will usually ruin the image from the point of the error to the end of the | ||
235 | image; with restart markers, the damage is usually confined to the portion of | ||
236 | the image up to the next restart marker. Of course, the restart markers | ||
237 | occupy extra space. We recommend | ||
238 | .B \-restart 1 | ||
239 | for images that will be transmitted across unreliable networks such as Usenet. | ||
240 | .PP | ||
241 | The | ||
242 | .B \-smooth | ||
243 | option filters the input to eliminate fine-scale noise. This is often useful | ||
244 | when converting dithered images to JPEG: a moderate smoothing factor of 10 to | ||
245 | 50 gets rid of dithering patterns in the input file, resulting in a smaller | ||
246 | JPEG file and a better-looking image. Too large a smoothing factor will | ||
247 | visibly blur the image, however. | ||
248 | .PP | ||
249 | Switches for wizards: | ||
250 | .TP | ||
251 | .B \-baseline | ||
252 | Force baseline-compatible quantization tables to be generated. This clamps | ||
253 | quantization values to 8 bits even at low quality settings. (This switch is | ||
254 | poorly named, since it does not ensure that the output is actually baseline | ||
255 | JPEG. For example, you can use | ||
256 | .B \-baseline | ||
257 | and | ||
258 | .B \-progressive | ||
259 | together.) | ||
260 | .TP | ||
261 | .BI \-qtables " file" | ||
262 | Use the quantization tables given in the specified text file. | ||
263 | .TP | ||
264 | .BI \-qslots " N[,...]" | ||
265 | Select which quantization table to use for each color component. | ||
266 | .TP | ||
267 | .BI \-sample " HxV[,...]" | ||
268 | Set JPEG sampling factors for each color component. | ||
269 | .TP | ||
270 | .BI \-scans " file" | ||
271 | Use the scan script given in the specified text file. | ||
272 | .PP | ||
273 | The "wizard" switches are intended for experimentation with JPEG. If you | ||
274 | don't know what you are doing, \fBdon't use them\fR. These switches are | ||
275 | documented further in the file wizard.txt. | ||
276 | .SH EXAMPLES | ||
277 | .LP | ||
278 | This example compresses the PPM file foo.ppm with a quality factor of | ||
279 | 60 and saves the output as foo.jpg: | ||
280 | .IP | ||
281 | .B cjpeg \-quality | ||
282 | .I 60 foo.ppm | ||
283 | .B > | ||
284 | .I foo.jpg | ||
285 | .SH HINTS | ||
286 | Color GIF files are not the ideal input for JPEG; JPEG is really intended for | ||
287 | compressing full-color (24-bit) images. In particular, don't try to convert | ||
288 | cartoons, line drawings, and other images that have only a few distinct | ||
289 | colors. GIF works great on these, JPEG does not. If you want to convert a | ||
290 | GIF to JPEG, you should experiment with | ||
291 | .BR cjpeg 's | ||
292 | .B \-quality | ||
293 | and | ||
294 | .B \-smooth | ||
295 | options to get a satisfactory conversion. | ||
296 | .B \-smooth 10 | ||
297 | or so is often helpful. | ||
298 | .PP | ||
299 | Avoid running an image through a series of JPEG compression/decompression | ||
300 | cycles. Image quality loss will accumulate; after ten or so cycles the image | ||
301 | may be noticeably worse than it was after one cycle. It's best to use a | ||
302 | lossless format while manipulating an image, then convert to JPEG format when | ||
303 | you are ready to file the image away. | ||
304 | .PP | ||
305 | The | ||
306 | .B \-optimize | ||
307 | option to | ||
308 | .B cjpeg | ||
309 | is worth using when you are making a "final" version for posting or archiving. | ||
310 | It's also a win when you are using low quality settings to make very small | ||
311 | JPEG files; the percentage improvement is often a lot more than it is on | ||
312 | larger files. (At present, | ||
313 | .B \-optimize | ||
314 | mode is always selected when generating progressive JPEG files.) | ||
315 | .SH ENVIRONMENT | ||
316 | .TP | ||
317 | .B JPEGMEM | ||
318 | If this environment variable is set, its value is the default memory limit. | ||
319 | The value is specified as described for the | ||
320 | .B \-maxmemory | ||
321 | switch. | ||
322 | .B JPEGMEM | ||
323 | overrides the default value specified when the program was compiled, and | ||
324 | itself is overridden by an explicit | ||
325 | .BR \-maxmemory . | ||
326 | .SH SEE ALSO | ||
327 | .BR djpeg (1), | ||
328 | .BR jpegtran (1), | ||
329 | .BR rdjpgcom (1), | ||
330 | .BR wrjpgcom (1) | ||
331 | .br | ||
332 | .BR ppm (5), | ||
333 | .BR pgm (5) | ||
334 | .br | ||
335 | Wallace, Gregory K. "The JPEG Still Picture Compression Standard", | ||
336 | Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44. | ||
337 | .SH AUTHOR | ||
338 | Independent JPEG Group | ||
339 | .SH BUGS | ||
340 | GIF input files are no longer supported, to avoid the Unisys LZW patent. | ||
341 | (Conversion of GIF files to JPEG is usually a bad idea anyway.) | ||
342 | .PP | ||
343 | Not all variants of BMP and Targa file formats are supported. | ||
344 | .PP | ||
345 | The | ||
346 | .B \-targa | ||
347 | switch is not a bug, it's a feature. (It would be a bug if the Targa format | ||
348 | designers had not been clueless.) | ||