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-USING THE IJG JPEG LIBRARY
-
-Copyright (C) 1994-2011, Thomas G. Lane, Guido Vollbeding.
-This file is part of the Independent JPEG Group's software.
-For conditions of distribution and use, see the accompanying README file.
-
-
-This file describes how to use the IJG JPEG library within an application
-program.  Read it if you want to write a program that uses the library.
-
-The file example.c provides heavily commented skeleton code for calling the
-JPEG library.  Also see jpeglib.h (the include file to be used by application
-programs) for full details about data structures and function parameter lists.
-The library source code, of course, is the ultimate reference.
-
-Note that there have been *major* changes from the application interface
-presented by IJG version 4 and earlier versions.  The old design had several
-inherent limitations, and it had accumulated a lot of cruft as we added
-features while trying to minimize application-interface changes.  We have
-sacrificed backward compatibility in the version 5 rewrite, but we think the
-improvements justify this.
-
-
-TABLE OF CONTENTS
------------------
-
-Overview:
-        Functions provided by the library
-        Outline of typical usage
-Basic library usage:
-        Data formats
-        Compression details
-        Decompression details
-        Mechanics of usage: include files, linking, etc
-Advanced features:
-        Compression parameter selection
-        Decompression parameter selection
-        Special color spaces
-        Error handling
-        Compressed data handling (source and destination managers)
-        I/O suspension
-        Progressive JPEG support
-        Buffered-image mode
-        Abbreviated datastreams and multiple images
-        Special markers
-        Raw (downsampled) image data
-        Really raw data: DCT coefficients
-        Progress monitoring
-        Memory management
-        Memory usage
-        Library compile-time options
-        Portability considerations
-        Notes for MS-DOS implementors
-
-You should read at least the overview and basic usage sections before trying
-to program with the library.  The sections on advanced features can be read
-if and when you need them.
-
-
-OVERVIEW
-========
-
-Functions provided by the library
----------------------------------
-
-The IJG JPEG library provides C code to read and write JPEG-compressed image
-files.  The surrounding application program receives or supplies image data a
-scanline at a time, using a straightforward uncompressed image format.  All
-details of color conversion and other preprocessing/postprocessing can be
-handled by the library.
-
-The library includes a substantial amount of code that is not covered by the
-JPEG standard but is necessary for typical applications of JPEG.  These
-functions preprocess the image before JPEG compression or postprocess it after
-decompression.  They include colorspace conversion, downsampling/upsampling,
-and color quantization.  The application indirectly selects use of this code
-by specifying the format in which it wishes to supply or receive image data.
-For example, if colormapped output is requested, then the decompression
-library automatically invokes color quantization.
-
-A wide range of quality vs. speed tradeoffs are possible in JPEG processing,
-and even more so in decompression postprocessing.  The decompression library
-provides multiple implementations that cover most of the useful tradeoffs,
-ranging from very-high-quality down to fast-preview operation.  On the
-compression side we have generally not provided low-quality choices, since
-compression is normally less time-critical.  It should be understood that the
-low-quality modes may not meet the JPEG standard's accuracy requirements;
-nonetheless, they are useful for viewers.
-
-A word about functions *not* provided by the library.  We handle a subset of
-the ISO JPEG standard; most baseline, extended-sequential, and progressive
-JPEG processes are supported.  (Our subset includes all features now in common
-use.)  Unsupported ISO options include:
-        * Hierarchical storage
-        * Lossless JPEG
-        * DNL marker
-        * Nonintegral subsampling ratios
-We support both 8- and 12-bit data precision, but this is a compile-time
-choice rather than a run-time choice; hence it is difficult to use both
-precisions in a single application.
-
-By itself, the library handles only interchange JPEG datastreams --- in
-particular the widely used JFIF file format.  The library can be used by
-surrounding code to process interchange or abbreviated JPEG datastreams that
-are embedded in more complex file formats.  (For example, this library is
-used by the free LIBTIFF library to support JPEG compression in TIFF.)
-
-
-Outline of typical usage
-------------------------
-
-The rough outline of a JPEG compression operation is:
-
-        Allocate and initialize a JPEG compression object
-        Specify the destination for the compressed data (eg, a file)
-        Set parameters for compression, including image size & colorspace
-        jpeg_start_compress(...);
-        while (scan lines remain to be written)
-                jpeg_write_scanlines(...);
-        jpeg_finish_compress(...);
-        Release the JPEG compression object
-
-A JPEG compression object holds parameters and working state for the JPEG
-library.  We make creation/destruction of the object separate from starting
-or finishing compression of an image; the same object can be re-used for a
-series of image compression operations.  This makes it easy to re-use the
-same parameter settings for a sequence of images.  Re-use of a JPEG object
-also has important implications for processing abbreviated JPEG datastreams,
-as discussed later.
-
-The image data to be compressed is supplied to jpeg_write_scanlines() from
-in-memory buffers.  If the application is doing file-to-file compression,
-reading image data from the source file is the application's responsibility.
-The library emits compressed data by calling a "data destination manager",
-which typically will write the data into a file; but the application can
-provide its own destination manager to do something else.
-
-Similarly, the rough outline of a JPEG decompression operation is:
-
-        Allocate and initialize a JPEG decompression object
-        Specify the source of the compressed data (eg, a file)
-        Call jpeg_read_header() to obtain image info
-        Set parameters for decompression
-        jpeg_start_decompress(...);
-        while (scan lines remain to be read)
-                jpeg_read_scanlines(...);
-        jpeg_finish_decompress(...);
-        Release the JPEG decompression object
-
-This is comparable to the compression outline except that reading the
-datastream header is a separate step.  This is helpful because information
-about the image's size, colorspace, etc is available when the application
-selects decompression parameters.  For example, the application can choose an
-output scaling ratio that will fit the image into the available screen size.
-
-The decompression library obtains compressed data by calling a data source
-manager, which typically will read the data from a file; but other behaviors
-can be obtained with a custom source manager.  Decompressed data is delivered
-into in-memory buffers passed to jpeg_read_scanlines().
-
-It is possible to abort an incomplete compression or decompression operation
-by calling jpeg_abort(); or, if you do not need to retain the JPEG object,
-simply release it by calling jpeg_destroy().
-
-JPEG compression and decompression objects are two separate struct types.
-However, they share some common fields, and certain routines such as
-jpeg_destroy() can work on either type of object.
-
-The JPEG library has no static variables: all state is in the compression
-or decompression object.  Therefore it is possible to process multiple
-compression and decompression operations concurrently, using multiple JPEG
-objects.
-
-Both compression and decompression can be done in an incremental memory-to-
-memory fashion, if suitable source/destination managers are used.  See the
-section on "I/O suspension" for more details.
-
-
-BASIC LIBRARY USAGE
-===================
-
-Data formats
-------------
-
-Before diving into procedural details, it is helpful to understand the
-image data format that the JPEG library expects or returns.
-
-The standard input image format is a rectangular array of pixels, with each
-pixel having the same number of "component" or "sample" values (color
-channels).  You must specify how many components there are and the colorspace
-interpretation of the components.  Most applications will use RGB data
-(three components per pixel) or grayscale data (one component per pixel).
-PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE.
-A remarkable number of people manage to miss this, only to find that their
-programs don't work with grayscale JPEG files.
-
-There is no provision for colormapped input.  JPEG files are always full-color
-or full grayscale (or sometimes another colorspace such as CMYK).  You can
-feed in a colormapped image by expanding it to full-color format.  However
-JPEG often doesn't work very well with source data that has been colormapped,
-because of dithering noise.  This is discussed in more detail in the JPEG FAQ
-and the other references mentioned in the README file.
-
-Pixels are stored by scanlines, with each scanline running from left to
-right.  The component values for each pixel are adjacent in the row; for
-example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color.  Each scanline is an
-array of data type JSAMPLE --- which is typically "unsigned char", unless
-you've changed jmorecfg.h.  (You can also change the RGB pixel layout, say
-to B,G,R order, by modifying jmorecfg.h.  But see the restrictions listed in
-that file before doing so.)
-
-A 2-D array of pixels is formed by making a list of pointers to the starts of
-scanlines; so the scanlines need not be physically adjacent in memory.  Even
-if you process just one scanline at a time, you must make a one-element
-pointer array to conform to this structure.  Pointers to JSAMPLE rows are of
-type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY.
-
-The library accepts or supplies one or more complete scanlines per call.
-It is not possible to process part of a row at a time.  Scanlines are always
-processed top-to-bottom.  You can process an entire image in one call if you
-have it all in memory, but usually it's simplest to process one scanline at
-a time.
-
-For best results, source data values should have the precision specified by
-BITS_IN_JSAMPLE (normally 8 bits).  For instance, if you choose to compress
-data that's only 6 bits/channel, you should left-justify each value in a
-byte before passing it to the compressor.  If you need to compress data
-that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 12.
-(See "Library compile-time options", later.)
-
-
-The data format returned by the decompressor is the same in all details,
-except that colormapped output is supported.  (Again, a JPEG file is never
-colormapped.  But you can ask the decompressor to perform on-the-fly color
-quantization to deliver colormapped output.)  If you request colormapped
-output then the returned data array contains a single JSAMPLE per pixel;
-its value is an index into a color map.  The color map is represented as
-a 2-D JSAMPARRAY in which each row holds the values of one color component,
-that is, colormap[i][j] is the value of the i'th color component for pixel
-value (map index) j.  Note that since the colormap indexes are stored in
-JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE
-(ie, at most 256 colors for an 8-bit JPEG library).
-
-
-Compression details
--------------------
-
-Here we revisit the JPEG compression outline given in the overview.
-
-1. Allocate and initialize a JPEG compression object.
-
-A JPEG compression object is a "struct jpeg_compress_struct".  (It also has
-a bunch of subsidiary structures which are allocated via malloc(), but the
-application doesn't control those directly.)  This struct can be just a local
-variable in the calling routine, if a single routine is going to execute the
-whole JPEG compression sequence.  Otherwise it can be static or allocated
-from malloc().
-
-You will also need a structure representing a JPEG error handler.  The part
-of this that the library cares about is a "struct jpeg_error_mgr".  If you
-are providing your own error handler, you'll typically want to embed the
-jpeg_error_mgr struct in a larger structure; this is discussed later under
-"Error handling".  For now we'll assume you are just using the default error
-handler.  The default error handler will print JPEG error/warning messages
-on stderr, and it will call exit() if a fatal error occurs.
-
-You must initialize the error handler structure, store a pointer to it into
-the JPEG object's "err" field, and then call jpeg_create_compress() to
-initialize the rest of the JPEG object.
-
-Typical code for this step, if you are using the default error handler, is
-
-        struct jpeg_compress_struct cinfo;
-        struct jpeg_error_mgr jerr;
-        ...
-        cinfo.err = jpeg_std_error(&jerr);
-        jpeg_create_compress(&cinfo);
-
-jpeg_create_compress allocates a small amount of memory, so it could fail
-if you are out of memory.  In that case it will exit via the error handler;
-that's why the error handler must be initialized first.
-
-
-2. Specify the destination for the compressed data (eg, a file).
-
-As previously mentioned, the JPEG library delivers compressed data to a
-"data destination" module.  The library includes one data destination
-module which knows how to write to a stdio stream.  You can use your own
-destination module if you want to do something else, as discussed later.
-
-If you use the standard destination module, you must open the target stdio
-stream beforehand.  Typical code for this step looks like:
-
-        FILE * outfile;
-        ...
-        if ((outfile = fopen(filename, "wb")) == NULL) {
-            fprintf(stderr, "can't open %s\n", filename);
-            exit(1);
-        }
-        jpeg_stdio_dest(&cinfo, outfile);
-
-where the last line invokes the standard destination module.
-
-WARNING: it is critical that the binary compressed data be delivered to the
-output file unchanged.  On non-Unix systems the stdio library may perform
-newline translation or otherwise corrupt binary data.  To suppress this
-behavior, you may need to use a "b" option to fopen (as shown above), or use
-setmode() or another routine to put the stdio stream in binary mode.  See
-cjpeg.c and djpeg.c for code that has been found to work on many systems.
-
-You can select the data destination after setting other parameters (step 3),
-if that's more convenient.  You may not change the destination between
-calling jpeg_start_compress() and jpeg_finish_compress().
-
-
-3. Set parameters for compression, including image size & colorspace.
-
-You must supply information about the source image by setting the following
-fields in the JPEG object (cinfo structure):
-
-        image_width             Width of image, in pixels
-        image_height            Height of image, in pixels
-        input_components        Number of color channels (samples per pixel)
-        in_color_space          Color space of source image
-
-The image dimensions are, hopefully, obvious.  JPEG supports image dimensions
-of 1 to 64K pixels in either direction.  The input color space is typically
-RGB or grayscale, and input_components is 3 or 1 accordingly.  (See "Special
-color spaces", later, for more info.)  The in_color_space field must be
-assigned one of the J_COLOR_SPACE enum constants, typically JCS_RGB or
-JCS_GRAYSCALE.
-
-JPEG has a large number of compression parameters that determine how the
-image is encoded.  Most applications don't need or want to know about all
-these parameters.  You can set all the parameters to reasonable defaults by
-calling jpeg_set_defaults(); then, if there are particular values you want
-to change, you can do so after that.  The "Compression parameter selection"
-section tells about all the parameters.
-
-You must set in_color_space correctly before calling jpeg_set_defaults(),
-because the defaults depend on the source image colorspace.  However the
-other three source image parameters need not be valid until you call
-jpeg_start_compress().  There's no harm in calling jpeg_set_defaults() more
-than once, if that happens to be convenient.
-
-Typical code for a 24-bit RGB source image is
-
-        cinfo.image_width = Width;      /* image width and height, in pixels */
-        cinfo.image_height = Height;
-        cinfo.input_components = 3;     /* # of color components per pixel */
-        cinfo.in_color_space = JCS_RGB; /* colorspace of input image */
-
-        jpeg_set_defaults(&cinfo);
-        /* Make optional parameter settings here */
-
-
-4. jpeg_start_compress(...);
-
-After you have established the data destination and set all the necessary
-source image info and other parameters, call jpeg_start_compress() to begin
-a compression cycle.  This will initialize internal state, allocate working
-storage, and emit the first few bytes of the JPEG datastream header.
-
-Typical code:
-
-        jpeg_start_compress(&cinfo, TRUE);
-
-The "TRUE" parameter ensures that a complete JPEG interchange datastream
-will be written.  This is appropriate in most cases.  If you think you might
-want to use an abbreviated datastream, read the section on abbreviated
-datastreams, below.
-
-Once you have called jpeg_start_compress(), you may not alter any JPEG
-parameters or other fields of the JPEG object until you have completed
-the compression cycle.
-
-
-5. while (scan lines remain to be written)
-        jpeg_write_scanlines(...);
-
-Now write all the required image data by calling jpeg_write_scanlines()
-one or more times.  You can pass one or more scanlines in each call, up
-to the total image height.  In most applications it is convenient to pass
-just one or a few scanlines at a time.  The expected format for the passed
-data is discussed under "Data formats", above.
-
-Image data should be written in top-to-bottom scanline order.  The JPEG spec
-contains some weasel wording about how top and bottom are application-defined
-terms (a curious interpretation of the English language...) but if you want
-your files to be compatible with everyone else's, you WILL use top-to-bottom
-order.  If the source data must be read in bottom-to-top order, you can use
-the JPEG library's virtual array mechanism to invert the data efficiently.
-Examples of this can be found in the sample application cjpeg.
-
-The library maintains a count of the number of scanlines written so far
-in the next_scanline field of the JPEG object.  Usually you can just use
-this variable as the loop counter, so that the loop test looks like
-"while (cinfo.next_scanline < cinfo.image_height)".
-
-Code for this step depends heavily on the way that you store the source data.
-example.c shows the following code for the case of a full-size 2-D source
-array containing 3-byte RGB pixels:
-
-        JSAMPROW row_pointer[1];        /* pointer to a single row */
-        int row_stride;                 /* physical row width in buffer */
-
-        row_stride = image_width * 3;   /* JSAMPLEs per row in image_buffer */
-
-        while (cinfo.next_scanline < cinfo.image_height) {
-            row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
-            jpeg_write_scanlines(&cinfo, row_pointer, 1);
-        }
-
-jpeg_write_scanlines() returns the number of scanlines actually written.
-This will normally be equal to the number passed in, so you can usually
-ignore the return value.  It is different in just two cases:
-  * If you try to write more scanlines than the declared image height,
-    the additional scanlines are ignored.
-  * If you use a suspending data destination manager, output buffer overrun
-    will cause the compressor to return before accepting all the passed lines.
-    This feature is discussed under "I/O suspension", below.  The normal
-    stdio destination manager will NOT cause this to happen.
-In any case, the return value is the same as the change in the value of
-next_scanline.
-
-
-6. jpeg_finish_compress(...);
-
-After all the image data has been written, call jpeg_finish_compress() to
-complete the compression cycle.  This step is ESSENTIAL to ensure that the
-last bufferload of data is written to the data destination.
-jpeg_finish_compress() also releases working memory associated with the JPEG
-object.
-
-Typical code:
-
-        jpeg_finish_compress(&cinfo);
-
-If using the stdio destination manager, don't forget to close the output
-stdio stream (if necessary) afterwards.
-
-If you have requested a multi-pass operating mode, such as Huffman code
-optimization, jpeg_finish_compress() will perform the additional passes using
-data buffered by the first pass.  In this case jpeg_finish_compress() may take
-quite a while to complete.  With the default compression parameters, this will
-not happen.
-
-It is an error to call jpeg_finish_compress() before writing the necessary
-total number of scanlines.  If you wish to abort compression, call
-jpeg_abort() as discussed below.
-
-After completing a compression cycle, you may dispose of the JPEG object
-as discussed next, or you may use it to compress another image.  In that case
-return to step 2, 3, or 4 as appropriate.  If you do not change the
-destination manager, the new datastream will be written to the same target.
-If you do not change any JPEG parameters, the new datastream will be written
-with the same parameters as before.  Note that you can change the input image
-dimensions freely between cycles, but if you change the input colorspace, you
-should call jpeg_set_defaults() to adjust for the new colorspace; and then
-you'll need to repeat all of step 3.
-
-
-7. Release the JPEG compression object.
-
-When you are done with a JPEG compression object, destroy it by calling
-jpeg_destroy_compress().  This will free all subsidiary memory (regardless of
-the previous state of the object).  Or you can call jpeg_destroy(), which
-works for either compression or decompression objects --- this may be more
-convenient if you are sharing code between compression and decompression
-cases.  (Actually, these routines are equivalent except for the declared type
-of the passed pointer.  To avoid gripes from ANSI C compilers, jpeg_destroy()
-should be passed a j_common_ptr.)
-
-If you allocated the jpeg_compress_struct structure from malloc(), freeing
-it is your responsibility --- jpeg_destroy() won't.  Ditto for the error
-handler structure.
-
-Typical code:
-
-        jpeg_destroy_compress(&cinfo);
-
-
-8. Aborting.
-
-If you decide to abort a compression cycle before finishing, you can clean up
-in either of two ways:
-
-* If you don't need the JPEG object any more, just call
-  jpeg_destroy_compress() or jpeg_destroy() to release memory.  This is
-  legitimate at any point after calling jpeg_create_compress() --- in fact,
-  it's safe even if jpeg_create_compress() fails.
-
-* If you want to re-use the JPEG object, call jpeg_abort_compress(), or call
-  jpeg_abort() which works on both compression and decompression objects.
-  This will return the object to an idle state, releasing any working memory.
-  jpeg_abort() is allowed at any time after successful object creation.
-
-Note that cleaning up the data destination, if required, is your
-responsibility; neither of these routines will call term_destination().
-(See "Compressed data handling", below, for more about that.)
-
-jpeg_destroy() and jpeg_abort() are the only safe calls to make on a JPEG
-object that has reported an error by calling error_exit (see "Error handling"
-for more info).  The internal state of such an object is likely to be out of
-whack.  Either of these two routines will return the object to a known state.
-
-
-Decompression details
----------------------
-
-Here we revisit the JPEG decompression outline given in the overview.
-
-1. Allocate and initialize a JPEG decompression object.
-
-This is just like initialization for compression, as discussed above,
-except that the object is a "struct jpeg_decompress_struct" and you
-call jpeg_create_decompress().  Error handling is exactly the same.
-
-Typical code:
-
-        struct jpeg_decompress_struct cinfo;
-        struct jpeg_error_mgr jerr;
-        ...
-        cinfo.err = jpeg_std_error(&jerr);
-        jpeg_create_decompress(&cinfo);
-
-(Both here and in the IJG code, we usually use variable name "cinfo" for
-both compression and decompression objects.)
-
-
-2. Specify the source of the compressed data (eg, a file).
-
-As previously mentioned, the JPEG library reads compressed data from a "data
-source" module.  The library includes one data source module which knows how
-to read from a stdio stream.  You can use your own source module if you want
-to do something else, as discussed later.
-
-If you use the standard source module, you must open the source stdio stream
-beforehand.  Typical code for this step looks like:
-
-        FILE * infile;
-        ...
-        if ((infile = fopen(filename, "rb")) == NULL) {
-            fprintf(stderr, "can't open %s\n", filename);
-            exit(1);
-        }
-        jpeg_stdio_src(&cinfo, infile);
-
-where the last line invokes the standard source module.
-
-WARNING: it is critical that the binary compressed data be read unchanged.
-On non-Unix systems the stdio library may perform newline translation or
-otherwise corrupt binary data.  To suppress this behavior, you may need to use
-a "b" option to fopen (as shown above), or use setmode() or another routine to
-put the stdio stream in binary mode.  See cjpeg.c and djpeg.c for code that
-has been found to work on many systems.
-
-You may not change the data source between calling jpeg_read_header() and
-jpeg_finish_decompress().  If you wish to read a series of JPEG images from
-a single source file, you should repeat the jpeg_read_header() to
-jpeg_finish_decompress() sequence without reinitializing either the JPEG
-object or the data source module; this prevents buffered input data from
-being discarded.
-
-
-3. Call jpeg_read_header() to obtain image info.
-
-Typical code for this step is just
-
-        jpeg_read_header(&cinfo, TRUE);
-
-This will read the source datastream header markers, up to the beginning
-of the compressed data proper.  On return, the image dimensions and other
-info have been stored in the JPEG object.  The application may wish to
-consult this information before selecting decompression parameters.
-
-More complex code is necessary if
-  * A suspending data source is used --- in that case jpeg_read_header()
-    may return before it has read all the header data.  See "I/O suspension",
-    below.  The normal stdio source manager will NOT cause this to happen.
-  * Abbreviated JPEG files are to be processed --- see the section on
-    abbreviated datastreams.  Standard applications that deal only in
-    interchange JPEG files need not be concerned with this case either.
-
-It is permissible to stop at this point if you just wanted to find out the
-image dimensions and other header info for a JPEG file.  In that case,
-call jpeg_destroy() when you are done with the JPEG object, or call
-jpeg_abort() to return it to an idle state before selecting a new data
-source and reading another header.
-
-
-4. Set parameters for decompression.
-
-jpeg_read_header() sets appropriate default decompression parameters based on
-the properties of the image (in particular, its colorspace).  However, you
-may well want to alter these defaults before beginning the decompression.
-For example, the default is to produce full color output from a color file.
-If you want colormapped output you must ask for it.  Other options allow the
-returned image to be scaled and allow various speed/quality tradeoffs to be
-selected.  "Decompression parameter selection", below, gives details.
-
-If the defaults are appropriate, nothing need be done at this step.
-
-Note that all default values are set by each call to jpeg_read_header().
-If you reuse a decompression object, you cannot expect your parameter
-settings to be preserved across cycles, as you can for compression.
-You must set desired parameter values each time.
-
-
-5. jpeg_start_decompress(...);
-
-Once the parameter values are satisfactory, call jpeg_start_decompress() to
-begin decompression.  This will initialize internal state, allocate working
-memory, and prepare for returning data.
-
-Typical code is just
-
-        jpeg_start_decompress(&cinfo);
-
-If you have requested a multi-pass operating mode, such as 2-pass color
-quantization, jpeg_start_decompress() will do everything needed before data
-output can begin.  In this case jpeg_start_decompress() may take quite a while
-to complete.  With a single-scan (non progressive) JPEG file and default
-decompression parameters, this will not happen; jpeg_start_decompress() will
-return quickly.
-
-After this call, the final output image dimensions, including any requested
-scaling, are available in the JPEG object; so is the selected colormap, if
-colormapped output has been requested.  Useful fields include
-
-        output_width            image width and height, as scaled
-        output_height
-        out_color_components    # of color components in out_color_space
-        output_components       # of color components returned per pixel
-        colormap                the selected colormap, if any
-        actual_number_of_colors         number of entries in colormap
-
-output_components is 1 (a colormap index) when quantizing colors; otherwise it
-equals out_color_components.  It is the number of JSAMPLE values that will be
-emitted per pixel in the output arrays.
-
-Typically you will need to allocate data buffers to hold the incoming image.
-You will need output_width * output_components JSAMPLEs per scanline in your
-output buffer, and a total of output_height scanlines will be returned.
-
-Note: if you are using the JPEG library's internal memory manager to allocate
-data buffers (as djpeg does), then the manager's protocol requires that you
-request large buffers *before* calling jpeg_start_decompress().  This is a
-little tricky since the output_XXX fields are not normally valid then.  You
-can make them valid by calling jpeg_calc_output_dimensions() after setting the
-relevant parameters (scaling, output color space, and quantization flag).
-
-
-6. while (scan lines remain to be read)
-        jpeg_read_scanlines(...);
-
-Now you can read the decompressed image data by calling jpeg_read_scanlines()
-one or more times.  At each call, you pass in the maximum number of scanlines
-to be read (ie, the height of your working buffer); jpeg_read_scanlines()
-will return up to that many lines.  The return value is the number of lines
-actually read.  The format of the returned data is discussed under "Data
-formats", above.  Don't forget that grayscale and color JPEGs will return
-different data formats!
-
-Image data is returned in top-to-bottom scanline order.  If you must write
-out the image in bottom-to-top order, you can use the JPEG library's virtual
-array mechanism to invert the data efficiently.  Examples of this can be
-found in the sample application djpeg.
-
-The library maintains a count of the number of scanlines returned so far
-in the output_scanline field of the JPEG object.  Usually you can just use
-this variable as the loop counter, so that the loop test looks like
-"while (cinfo.output_scanline < cinfo.output_height)".  (Note that the test
-should NOT be against image_height, unless you never use scaling.  The
-image_height field is the height of the original unscaled image.)
-The return value always equals the change in the value of output_scanline.
-
-If you don't use a suspending data source, it is safe to assume that
-jpeg_read_scanlines() reads at least one scanline per call, until the
-bottom of the image has been reached.
-
-If you use a buffer larger than one scanline, it is NOT safe to assume that
-jpeg_read_scanlines() fills it.  (The current implementation returns only a
-few scanlines per call, no matter how large a buffer you pass.)  So you must
-always provide a loop that calls jpeg_read_scanlines() repeatedly until the
-whole image has been read.
-
-
-7. jpeg_finish_decompress(...);
-
-After all the image data has been read, call jpeg_finish_decompress() to
-complete the decompression cycle.  This causes working memory associated
-with the JPEG object to be released.
-
-Typical code:
-
-        jpeg_finish_decompress(&cinfo);
-
-If using the stdio source manager, don't forget to close the source stdio
-stream if necessary.
-
-It is an error to call jpeg_finish_decompress() before reading the correct
-total number of scanlines.  If you wish to abort decompression, call
-jpeg_abort() as discussed below.
-
-After completing a decompression cycle, you may dispose of the JPEG object as
-discussed next, or you may use it to decompress another image.  In that case
-return to step 2 or 3 as appropriate.  If you do not change the source
-manager, the next image will be read from the same source.
-
-
-8. Release the JPEG decompression object.
-
-When you are done with a JPEG decompression object, destroy it by calling
-jpeg_destroy_decompress() or jpeg_destroy().  The previous discussion of
-destroying compression objects applies here too.
-
-Typical code:
-
-        jpeg_destroy_decompress(&cinfo);
-
-
-9. Aborting.
-
-You can abort a decompression cycle by calling jpeg_destroy_decompress() or
-jpeg_destroy() if you don't need the JPEG object any more, or
-jpeg_abort_decompress() or jpeg_abort() if you want to reuse the object.
-The previous discussion of aborting compression cycles applies here too.
-
-
-Mechanics of usage: include files, linking, etc
------------------------------------------------
-
-Applications using the JPEG library should include the header file jpeglib.h
-to obtain declarations of data types and routines.  Before including
-jpeglib.h, include system headers that define at least the typedefs FILE and
-size_t.  On ANSI-conforming systems, including <stdio.h> is sufficient; on
-older Unix systems, you may need <sys/types.h> to define size_t.
-
-If the application needs to refer to individual JPEG library error codes, also
-include jerror.h to define those symbols.
-
-jpeglib.h indirectly includes the files jconfig.h and jmorecfg.h.  If you are
-installing the JPEG header files in a system directory, you will want to
-install all four files: jpeglib.h, jerror.h, jconfig.h, jmorecfg.h.
-
-The most convenient way to include the JPEG code into your executable program
-is to prepare a library file ("libjpeg.a", or a corresponding name on non-Unix
-machines) and reference it at your link step.  If you use only half of the
-library (only compression or only decompression), only that much code will be
-included from the library, unless your linker is hopelessly brain-damaged.
-The supplied makefiles build libjpeg.a automatically (see install.txt).
-
-While you can build the JPEG library as a shared library if the whim strikes
-you, we don't really recommend it.  The trouble with shared libraries is that
-at some point you'll probably try to substitute a new version of the library
-without recompiling the calling applications.  That generally doesn't work
-because the parameter struct declarations usually change with each new
-version.  In other words, the library's API is *not* guaranteed binary
-compatible across versions; we only try to ensure source-code compatibility.
-(In hindsight, it might have been smarter to hide the parameter structs from
-applications and introduce a ton of access functions instead.  Too late now,
-however.)
-
-On some systems your application may need to set up a signal handler to ensure
-that temporary files are deleted if the program is interrupted.  This is most
-critical if you are on MS-DOS and use the jmemdos.c memory manager back end;
-it will try to grab extended memory for temp files, and that space will NOT be
-freed automatically.  See cjpeg.c or djpeg.c for an example signal handler.
-
-It may be worth pointing out that the core JPEG library does not actually
-require the stdio library: only the default source/destination managers and
-error handler need it.  You can use the library in a stdio-less environment
-if you replace those modules and use jmemnobs.c (or another memory manager of
-your own devising).  More info about the minimum system library requirements
-may be found in jinclude.h.
-
-
-ADVANCED FEATURES
-=================
-
-Compression parameter selection
--------------------------------
-
-This section describes all the optional parameters you can set for JPEG
-compression, as well as the "helper" routines provided to assist in this
-task.  Proper setting of some parameters requires detailed understanding
-of the JPEG standard; if you don't know what a parameter is for, it's best
-not to mess with it!  See REFERENCES in the README file for pointers to
-more info about JPEG.
-
-It's a good idea to call jpeg_set_defaults() first, even if you plan to set
-all the parameters; that way your code is more likely to work with future JPEG
-libraries that have additional parameters.  For the same reason, we recommend
-you use a helper routine where one is provided, in preference to twiddling
-cinfo fields directly.
-
-The helper routines are:
-
-jpeg_set_defaults (j_compress_ptr cinfo)
-        This routine sets all JPEG parameters to reasonable defaults, using
-        only the input image's color space (field in_color_space, which must
-        already be set in cinfo).  Many applications will only need to use
-        this routine and perhaps jpeg_set_quality().
-
-jpeg_set_colorspace (j_compress_ptr cinfo, J_COLOR_SPACE colorspace)
-        Sets the JPEG file's colorspace (field jpeg_color_space) as specified,
-        and sets other color-space-dependent parameters appropriately.  See
-        "Special color spaces", below, before using this.  A large number of
-        parameters, including all per-component parameters, are set by this
-        routine; if you want to twiddle individual parameters you should call
-        jpeg_set_colorspace() before rather than after.
-
-jpeg_default_colorspace (j_compress_ptr cinfo)
-        Selects an appropriate JPEG colorspace based on cinfo->in_color_space,
-        and calls jpeg_set_colorspace().  This is actually a subroutine of
-        jpeg_set_defaults().  It's broken out in case you want to change
-        just the colorspace-dependent JPEG parameters.
-
-jpeg_set_quality (j_compress_ptr cinfo, int quality, boolean force_baseline)
-        Constructs JPEG quantization tables appropriate for the indicated
-        quality setting.  The quality value is expressed on the 0..100 scale
-        recommended by IJG (cjpeg's "-quality" switch uses this routine).
-        Note that the exact mapping from quality values to tables may change
-        in future IJG releases as more is learned about DCT quantization.
-        If the force_baseline parameter is TRUE, then the quantization table
-        entries are constrained to the range 1..255 for full JPEG baseline
-        compatibility.  In the current implementation, this only makes a
-        difference for quality settings below 25, and it effectively prevents
-        very small/low quality files from being generated.  The IJG decoder
-        is capable of reading the non-baseline files generated at low quality
-        settings when force_baseline is FALSE, but other decoders may not be.
-
-jpeg_set_linear_quality (j_compress_ptr cinfo, int scale_factor,
-                         boolean force_baseline)
-        Same as jpeg_set_quality() except that the generated tables are the
-        sample tables given in the JPEC spec section K.1, multiplied by the
-        specified scale factor (which is expressed as a percentage; thus
-        scale_factor = 100 reproduces the spec's tables).  Note that larger
-        scale factors give lower quality.  This entry point is useful for
-        conforming to the Adobe PostScript DCT conventions, but we do not
-        recommend linear scaling as a user-visible quality scale otherwise.
-        force_baseline again constrains the computed table entries to 1..255.
-
-int jpeg_quality_scaling (int quality)
-        Converts a value on the IJG-recommended quality scale to a linear
-        scaling percentage.  Note that this routine may change or go away
-        in future releases --- IJG may choose to adopt a scaling method that
-        can't be expressed as a simple scalar multiplier, in which case the
-        premise of this routine collapses.  Caveat user.
-
-jpeg_default_qtables (j_compress_ptr cinfo, boolean force_baseline)
-        Set default quantization tables with linear q_scale_factor[] values
-        (see below).
-
-jpeg_add_quant_table (j_compress_ptr cinfo, int which_tbl,
-                      const unsigned int *basic_table,
-                      int scale_factor, boolean force_baseline)
-        Allows an arbitrary quantization table to be created.  which_tbl
-        indicates which table slot to fill.  basic_table points to an array
-        of 64 unsigned ints given in normal array order.  These values are
-        multiplied by scale_factor/100 and then clamped to the range 1..65535
-        (or to 1..255 if force_baseline is TRUE).
-        CAUTION: prior to library version 6a, jpeg_add_quant_table expected
-        the basic table to be given in JPEG zigzag order.  If you need to
-        write code that works with either older or newer versions of this
-        routine, you must check the library version number.  Something like
-        "#if JPEG_LIB_VERSION >= 61" is the right test.
-
-jpeg_simple_progression (j_compress_ptr cinfo)
-        Generates a default scan script for writing a progressive-JPEG file.
-        This is the recommended method of creating a progressive file,
-        unless you want to make a custom scan sequence.  You must ensure that
-        the JPEG color space is set correctly before calling this routine.
-
-
-Compression parameters (cinfo fields) include:
-
-int block_size
-        Set DCT block size.  All N from 1 to 16 are possible.
-        Default is 8 (baseline format).
-        Larger values produce higher compression,
-        smaller values produce higher quality.
-        An exact DCT stage is possible with 1 or 2.
-        With the default quality of 75 and default Luminance qtable
-        the DCT+Quantization stage is lossless for value 1.
-        Note that values other than 8 require a SmartScale capable decoder,
-        introduced with IJG JPEG 8.  Setting the block_size parameter for
-        compression works with version 8c and later.
-
-J_DCT_METHOD dct_method
-        Selects the algorithm used for the DCT step.  Choices are:
-                JDCT_ISLOW: slow but accurate integer algorithm
-                JDCT_IFAST: faster, less accurate integer method
-                JDCT_FLOAT: floating-point method
-                JDCT_DEFAULT: default method (normally JDCT_ISLOW)
-                JDCT_FASTEST: fastest method (normally JDCT_IFAST)
-        The FLOAT method is very slightly more accurate than the ISLOW method,
-        but may give different results on different machines due to varying
-        roundoff behavior.  The integer methods should give the same results
-        on all machines.  On machines with sufficiently fast FP hardware, the
-        floating-point method may also be the fastest.  The IFAST method is
-        considerably less accurate than the other two; its use is not
-        recommended if high quality is a concern.  JDCT_DEFAULT and
-        JDCT_FASTEST are macros configurable by each installation.
-
-unsigned int scale_num, scale_denom
-        Scale the image by the fraction scale_num/scale_denom.  Default is
-        1/1, or no scaling.  Currently, the supported scaling ratios are
-        M/N with all N from 1 to 16, where M is the destination DCT size,
-        which is 8 by default (see block_size parameter above).
-        (The library design allows for arbitrary scaling ratios but this
-        is not likely to be implemented any time soon.)
-
-J_COLOR_SPACE jpeg_color_space
-int num_components
-        The JPEG color space and corresponding number of components; see
-        "Special color spaces", below, for more info.  We recommend using
-        jpeg_set_color_space() if you want to change these.
-
-boolean optimize_coding
-        TRUE causes the compressor to compute optimal Huffman coding tables
-        for the image.  This requires an extra pass over the data and
-        therefore costs a good deal of space and time.  The default is
-        FALSE, which tells the compressor to use the supplied or default
-        Huffman tables.  In most cases optimal tables save only a few percent
-        of file size compared to the default tables.  Note that when this is
-        TRUE, you need not supply Huffman tables at all, and any you do
-        supply will be overwritten.
-
-unsigned int restart_interval
-int restart_in_rows
-        To emit restart markers in the JPEG file, set one of these nonzero.
-        Set restart_interval to specify the exact interval in MCU blocks.
-        Set restart_in_rows to specify the interval in MCU rows.  (If
-        restart_in_rows is not 0, then restart_interval is set after the
-        image width in MCUs is computed.)  Defaults are zero (no restarts).
-        One restart marker per MCU row is often a good choice.
-        NOTE: the overhead of restart markers is higher in grayscale JPEG
-        files than in color files, and MUCH higher in progressive JPEGs.
-        If you use restarts, you may want to use larger intervals in those
-        cases.
-
-const jpeg_scan_info * scan_info
-int num_scans
-        By default, scan_info is NULL; this causes the compressor to write a
-        single-scan sequential JPEG file.  If not NULL, scan_info points to
-        an array of scan definition records of length num_scans.  The
-        compressor will then write a JPEG file having one scan for each scan
-        definition record.  This is used to generate noninterleaved or
-        progressive JPEG files.  The library checks that the scan array
-        defines a valid JPEG scan sequence.  (jpeg_simple_progression creates
-        a suitable scan definition array for progressive JPEG.)  This is
-        discussed further under "Progressive JPEG support".
-
-boolean do_fancy_downsampling
-        If TRUE, use direct DCT scaling with DCT size > 8 for downsampling
-        of chroma components.
-        If FALSE, use only DCT size <= 8 and simple separate downsampling.
-        Default is TRUE.
-        For better image stability in multiple generation compression cycles
-        it is preferable that this value matches the corresponding
-        do_fancy_upsampling value in decompression.
-
-int smoothing_factor
-        If non-zero, the input image is smoothed; the value should be 1 for
-        minimal smoothing to 100 for maximum smoothing.  Consult jcsample.c
-        for details of the smoothing algorithm.  The default is zero.
-
-boolean write_JFIF_header
-        If TRUE, a JFIF APP0 marker is emitted.  jpeg_set_defaults() and
-        jpeg_set_colorspace() set this TRUE if a JFIF-legal JPEG color space
-        (ie, YCbCr or grayscale) is selected, otherwise FALSE.
-
-UINT8 JFIF_major_version
-UINT8 JFIF_minor_version
-        The version number to be written into the JFIF marker.
-        jpeg_set_defaults() initializes the version to 1.01 (major=minor=1).
-        You should set it to 1.02 (major=1, minor=2) if you plan to write
-        any JFIF 1.02 extension markers.
-
-UINT8 density_unit
-UINT16 X_density
-UINT16 Y_density
-        The resolution information to be written into the JFIF marker;
-        not used otherwise.  density_unit may be 0 for unknown,
-        1 for dots/inch, or 2 for dots/cm.  The default values are 0,1,1
-        indicating square pixels of unknown size.
-
-boolean write_Adobe_marker
-        If TRUE, an Adobe APP14 marker is emitted.  jpeg_set_defaults() and
-        jpeg_set_colorspace() set this TRUE if JPEG color space RGB, CMYK,
-        or YCCK is selected, otherwise FALSE.  It is generally a bad idea
-        to set both write_JFIF_header and write_Adobe_marker.  In fact,
-        you probably shouldn't change the default settings at all --- the
-        default behavior ensures that the JPEG file's color space can be
-        recognized by the decoder.
-
-JQUANT_TBL * quant_tbl_ptrs[NUM_QUANT_TBLS]
-        Pointers to coefficient quantization tables, one per table slot,
-        or NULL if no table is defined for a slot.  Usually these should
-        be set via one of the above helper routines; jpeg_add_quant_table()
-        is general enough to define any quantization table.  The other
-        routines will set up table slot 0 for luminance quality and table
-        slot 1 for chrominance.
-
-int q_scale_factor[NUM_QUANT_TBLS]
-        Linear quantization scaling factors (percentage, initialized 100)
-        for use with jpeg_default_qtables().
-        See rdswitch.c and cjpeg.c for an example of usage.
-        Note that the q_scale_factor[] fields are the "linear" scales, so you
-        have to convert from user-defined ratings via jpeg_quality_scaling().
-        Here is an example code which corresponds to cjpeg -quality 90,70:
-
-                jpeg_set_defaults(cinfo);
-
-                /* Set luminance quality 90. */
-                cinfo->q_scale_factor[0] = jpeg_quality_scaling(90);
-                /* Set chrominance quality 70. */
-                cinfo->q_scale_factor[1] = jpeg_quality_scaling(70);
-
-                jpeg_default_qtables(cinfo, force_baseline);
-
-        CAUTION: You must also set 1x1 subsampling for efficient separate
-        color quality selection, since the default value used by library
-        is 2x2:
-
-                cinfo->comp_info[0].v_samp_factor = 1;
-                cinfo->comp_info[0].h_samp_factor = 1;
-
-JHUFF_TBL * dc_huff_tbl_ptrs[NUM_HUFF_TBLS]
-JHUFF_TBL * ac_huff_tbl_ptrs[NUM_HUFF_TBLS]
-        Pointers to Huffman coding tables, one per table slot, or NULL if
-        no table is defined for a slot.  Slots 0 and 1 are filled with the
-        JPEG sample tables by jpeg_set_defaults().  If you need to allocate
-        more table structures, jpeg_alloc_huff_table() may be used.
-        Note that optimal Huffman tables can be computed for an image
-        by setting optimize_coding, as discussed above; there's seldom
-        any need to mess with providing your own Huffman tables.
-
-
-The actual dimensions of the JPEG image that will be written to the file are
-given by the following fields.  These are computed from the input image
-dimensions and the compression parameters by jpeg_start_compress().  You can
-also call jpeg_calc_jpeg_dimensions() to obtain the values that will result
-from the current parameter settings.  This can be useful if you are trying
-to pick a scaling ratio that will get close to a desired target size.
-
-JDIMENSION jpeg_width           Actual dimensions of output image.
-JDIMENSION jpeg_height
-
-
-Per-component parameters are stored in the struct cinfo.comp_info[i] for
-component number i.  Note that components here refer to components of the
-JPEG color space, *not* the source image color space.  A suitably large
-comp_info[] array is allocated by jpeg_set_defaults(); if you choose not
-to use that routine, it's up to you to allocate the array.
-
-int component_id
-        The one-byte identifier code to be recorded in the JPEG file for
-        this component.  For the standard color spaces, we recommend you
-        leave the default values alone.
-
-int h_samp_factor
-int v_samp_factor
-        Horizontal and vertical sampling factors for the component; must
-        be 1..4 according to the JPEG standard.  Note that larger sampling
-        factors indicate a higher-resolution component; many people find
-        this behavior quite unintuitive.  The default values are 2,2 for
-        luminance components and 1,1 for chrominance components, except
-        for grayscale where 1,1 is used.
-
-int quant_tbl_no
-        Quantization table number for component.  The default value is
-        0 for luminance components and 1 for chrominance components.
-
-int dc_tbl_no
-int ac_tbl_no
-        DC and AC entropy coding table numbers.  The default values are
-        0 for luminance components and 1 for chrominance components.
-
-int component_index
-        Must equal the component's index in comp_info[].  (Beginning in
-        release v6, the compressor library will fill this in automatically;
-        you don't have to.)
-
-
-Decompression parameter selection
----------------------------------
-
-Decompression parameter selection is somewhat simpler than compression
-parameter selection, since all of the JPEG internal parameters are
-recorded in the source file and need not be supplied by the application.
-(Unless you are working with abbreviated files, in which case see
-"Abbreviated datastreams", below.)  Decompression parameters control
-the postprocessing done on the image to deliver it in a format suitable
-for the application's use.  Many of the parameters control speed/quality
-tradeoffs, in which faster decompression may be obtained at the price of
-a poorer-quality image.  The defaults select the highest quality (slowest)
-processing.
-
-The following fields in the JPEG object are set by jpeg_read_header() and
-may be useful to the application in choosing decompression parameters:
-
-JDIMENSION image_width                  Width and height of image
-JDIMENSION image_height
-int num_components                      Number of color components
-J_COLOR_SPACE jpeg_color_space          Colorspace of image
-boolean saw_JFIF_marker                 TRUE if a JFIF APP0 marker was seen
-  UINT8 JFIF_major_version              Version information from JFIF marker
-  UINT8 JFIF_minor_version
-  UINT8 density_unit                    Resolution data from JFIF marker
-  UINT16 X_density
-  UINT16 Y_density
-boolean saw_Adobe_marker                TRUE if an Adobe APP14 marker was seen
-  UINT8 Adobe_transform                 Color transform code from Adobe marker
-
-The JPEG color space, unfortunately, is something of a guess since the JPEG
-standard proper does not provide a way to record it.  In practice most files
-adhere to the JFIF or Adobe conventions, and the decoder will recognize these
-correctly.  See "Special color spaces", below, for more info.
-
-
-The decompression parameters that determine the basic properties of the
-returned image are:
-
-J_COLOR_SPACE out_color_space
-        Output color space.  jpeg_read_header() sets an appropriate default
-        based on jpeg_color_space; typically it will be RGB or grayscale.
-        The application can change this field to request output in a different
-        colorspace.  For example, set it to JCS_GRAYSCALE to get grayscale
-        output from a color file.  (This is useful for previewing: grayscale
-        output is faster than full color since the color components need not
-        be processed.)  Note that not all possible color space transforms are
-        currently implemented; you may need to extend jdcolor.c if you want an
-        unusual conversion.
-
-unsigned int scale_num, scale_denom
-        Scale the image by the fraction scale_num/scale_denom.  Currently,
-        the supported scaling ratios are M/N with all M from 1 to 16, where
-        N is the source DCT size, which is 8 for baseline JPEG.  (The library
-        design allows for arbitrary scaling ratios but this is not likely
-        to be implemented any time soon.)  The values are initialized by
-        jpeg_read_header() with the source DCT size.  For baseline JPEG
-        this is 8/8.  If you change only the scale_num value while leaving
-        the other unchanged, then this specifies the DCT scaled size to be
-        applied on the given input.  For baseline JPEG this is equivalent
-        to M/8 scaling, since the source DCT size for baseline JPEG is 8.
-        Smaller scaling ratios permit significantly faster decoding since
-        fewer pixels need be processed and a simpler IDCT method can be used.
-
-boolean quantize_colors
-        If set TRUE, colormapped output will be delivered.  Default is FALSE,
-        meaning that full-color output will be delivered.
-
-The next three parameters are relevant only if quantize_colors is TRUE.
-
-int desired_number_of_colors
-        Maximum number of colors to use in generating a library-supplied color
-        map (the actual number of colors is returned in a different field).
-        Default 256.  Ignored when the application supplies its own color map.
-
-boolean two_pass_quantize
-        If TRUE, an extra pass over the image is made to select a custom color
-        map for the image.  This usually looks a lot better than the one-size-
-        fits-all colormap that is used otherwise.  Default is TRUE.  Ignored
-        when the application supplies its own color map.
-
-J_DITHER_MODE dither_mode
-        Selects color dithering method.  Supported values are:
-                JDITHER_NONE    no dithering: fast, very low quality
-                JDITHER_ORDERED ordered dither: moderate speed and quality
-                JDITHER_FS      Floyd-Steinberg dither: slow, high quality
-        Default is JDITHER_FS.  (At present, ordered dither is implemented
-        only in the single-pass, standard-colormap case.  If you ask for
-        ordered dither when two_pass_quantize is TRUE or when you supply
-        an external color map, you'll get F-S dithering.)
-
-When quantize_colors is TRUE, the target color map is described by the next
-two fields.  colormap is set to NULL by jpeg_read_header().  The application
-can supply a color map by setting colormap non-NULL and setting
-actual_number_of_colors to the map size.  Otherwise, jpeg_start_decompress()
-selects a suitable color map and sets these two fields itself.
-[Implementation restriction: at present, an externally supplied colormap is
-only accepted for 3-component output color spaces.]
-
-JSAMPARRAY colormap
-        The color map, represented as a 2-D pixel array of out_color_components
-        rows and actual_number_of_colors columns.  Ignored if not quantizing.
-        CAUTION: if the JPEG library creates its own colormap, the storage
-        pointed to by this field is released by jpeg_finish_decompress().
-        Copy the colormap somewhere else first, if you want to save it.
-
-int actual_number_of_colors
-        The number of colors in the color map.
-
-Additional decompression parameters that the application may set include:
-
-J_DCT_METHOD dct_method
-        Selects the algorithm used for the DCT step.  Choices are the same
-        as described above for compression.
-
-boolean do_fancy_upsampling
-        If TRUE, use direct DCT scaling with DCT size > 8 for upsampling
-        of chroma components.
-        If FALSE, use only DCT size <= 8 and simple separate upsampling.
-        Default is TRUE.
-        For better image stability in multiple generation compression cycles
-        it is preferable that this value matches the corresponding
-        do_fancy_downsampling value in compression.
-
-boolean do_block_smoothing
-        If TRUE, interblock smoothing is applied in early stages of decoding
-        progressive JPEG files; if FALSE, not.  Default is TRUE.  Early
-        progression stages look "fuzzy" with smoothing, "blocky" without.
-        In any case, block smoothing ceases to be applied after the first few
-        AC coefficients are known to full accuracy, so it is relevant only
-        when using buffered-image mode for progressive images.
-
-boolean enable_1pass_quant
-boolean enable_external_quant
-boolean enable_2pass_quant
-        These are significant only in buffered-image mode, which is
-        described in its own section below.
-
-
-The output image dimensions are given by the following fields.  These are
-computed from the source image dimensions and the decompression parameters
-by jpeg_start_decompress().  You can also call jpeg_calc_output_dimensions()
-to obtain the values that will result from the current parameter settings.
-This can be useful if you are trying to pick a scaling ratio that will get
-close to a desired target size.  It's also important if you are using the
-JPEG library's memory manager to allocate output buffer space, because you
-are supposed to request such buffers *before* jpeg_start_decompress().
-
-JDIMENSION output_width         Actual dimensions of output image.
-JDIMENSION output_height
-int out_color_components        Number of color components in out_color_space.
-int output_components           Number of color components returned.
-int rec_outbuf_height           Recommended height of scanline buffer.
-
-When quantizing colors, output_components is 1, indicating a single color map
-index per pixel.  Otherwise it equals out_color_components.  The output arrays
-are required to be output_width * output_components JSAMPLEs wide.
-
-rec_outbuf_height is the recommended minimum height (in scanlines) of the
-buffer passed to jpeg_read_scanlines().  If the buffer is smaller, the
-library will still work, but time will be wasted due to unnecessary data
-copying.  In high-quality modes, rec_outbuf_height is always 1, but some
-faster, lower-quality modes set it to larger values (typically 2 to 4).
-If you are going to ask for a high-speed processing mode, you may as well
-go to the trouble of honoring rec_outbuf_height so as to avoid data copying.
-(An output buffer larger than rec_outbuf_height lines is OK, but won't
-provide any material speed improvement over that height.)
-
-
-Special color spaces
---------------------
-
-The JPEG standard itself is "color blind" and doesn't specify any particular
-color space.  It is customary to convert color data to a luminance/chrominance
-color space before compressing, since this permits greater compression.  The
-existing de-facto JPEG file format standards specify YCbCr or grayscale data
-(JFIF), or grayscale, RGB, YCbCr, CMYK, or YCCK (Adobe).  For special
-applications such as multispectral images, other color spaces can be used,
-but it must be understood that such files will be unportable.
-
-The JPEG library can handle the most common colorspace conversions (namely
-RGB <=> YCbCr and CMYK <=> YCCK).  It can also deal with data of an unknown
-color space, passing it through without conversion.  If you deal extensively
-with an unusual color space, you can easily extend the library to understand
-additional color spaces and perform appropriate conversions.
-
-For compression, the source data's color space is specified by field
-in_color_space.  This is transformed to the JPEG file's color space given
-by jpeg_color_space.  jpeg_set_defaults() chooses a reasonable JPEG color
-space depending on in_color_space, but you can override this by calling
-jpeg_set_colorspace().  Of course you must select a supported transformation.
-jccolor.c currently supports the following transformations:
-        RGB => YCbCr
-        RGB => GRAYSCALE
-        YCbCr => GRAYSCALE
-        CMYK => YCCK
-plus the null transforms: GRAYSCALE => GRAYSCALE, RGB => RGB,
-YCbCr => YCbCr, CMYK => CMYK, YCCK => YCCK, and UNKNOWN => UNKNOWN.
-
-The de-facto file format standards (JFIF and Adobe) specify APPn markers that
-indicate the color space of the JPEG file.  It is important to ensure that
-these are written correctly, or omitted if the JPEG file's color space is not
-one of the ones supported by the de-facto standards.  jpeg_set_colorspace()
-will set the compression parameters to include or omit the APPn markers
-properly, so long as it is told the truth about the JPEG color space.
-For example, if you are writing some random 3-component color space without
-conversion, don't try to fake out the library by setting in_color_space and
-jpeg_color_space to JCS_YCbCr; use JCS_UNKNOWN.  You may want to write an
-APPn marker of your own devising to identify the colorspace --- see "Special
-markers", below.
-
-When told that the color space is UNKNOWN, the library will default to using
-luminance-quality compression parameters for all color components.  You may
-well want to change these parameters.  See the source code for
-jpeg_set_colorspace(), in jcparam.c, for details.
-
-For decompression, the JPEG file's color space is given in jpeg_color_space,
-and this is transformed to the output color space out_color_space.
-jpeg_read_header's setting of jpeg_color_space can be relied on if the file
-conforms to JFIF or Adobe conventions, but otherwise it is no better than a
-guess.  If you know the JPEG file's color space for certain, you can override
-jpeg_read_header's guess by setting jpeg_color_space.  jpeg_read_header also
-selects a default output color space based on (its guess of) jpeg_color_space;
-set out_color_space to override this.  Again, you must select a supported
-transformation.  jdcolor.c currently supports
-        YCbCr => RGB
-        YCbCr => GRAYSCALE
-        RGB => GRAYSCALE
-        GRAYSCALE => RGB
-        YCCK => CMYK
-as well as the null transforms.  (Since GRAYSCALE=>RGB is provided, an
-application can force grayscale JPEGs to look like color JPEGs if it only
-wants to handle one case.)
-
-The two-pass color quantizer, jquant2.c, is specialized to handle RGB data
-(it weights distances appropriately for RGB colors).  You'll need to modify
-the code if you want to use it for non-RGB output color spaces.  Note that
-jquant2.c is used to map to an application-supplied colormap as well as for
-the normal two-pass colormap selection process.
-
-CAUTION: it appears that Adobe Photoshop writes inverted data in CMYK JPEG
-files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect.
-This is arguably a bug in Photoshop, but if you need to work with Photoshop
-CMYK files, you will have to deal with it in your application.  We cannot
-"fix" this in the library by inverting the data during the CMYK<=>YCCK
-transform, because that would break other applications, notably Ghostscript.
-Photoshop versions prior to 3.0 write EPS files containing JPEG-encoded CMYK
-data in the same inverted-YCCK representation used in bare JPEG files, but
-the surrounding PostScript code performs an inversion using the PS image
-operator.  I am told that Photoshop 3.0 will write uninverted YCCK in
-EPS/JPEG files, and will omit the PS-level inversion.  (But the data
-polarity used in bare JPEG files will not change in 3.0.)  In either case,
-the JPEG library must not invert the data itself, or else Ghostscript would
-read these EPS files incorrectly.
-
-
-Error handling
---------------
-
-When the default error handler is used, any error detected inside the JPEG
-routines will cause a message to be printed on stderr, followed by exit().
-You can supply your own error handling routines to override this behavior
-and to control the treatment of nonfatal warnings and trace/debug messages.
-The file example.c illustrates the most common case, which is to have the
-application regain control after an error rather than exiting.
-
-The JPEG library never writes any message directly; it always goes through
-the error handling routines.  Three classes of messages are recognized:
-  * Fatal errors: the library cannot continue.
-  * Warnings: the library can continue, but the data is corrupt, and a
-    damaged output image is likely to result.
-  * Trace/informational messages.  These come with a trace level indicating
-    the importance of the message; you can control the verbosity of the
-    program by adjusting the maximum trace level that will be displayed.
-
-You may, if you wish, simply replace the entire JPEG error handling module
-(jerror.c) with your own code.  However, you can avoid code duplication by
-only replacing some of the routines depending on the behavior you need.
-This is accomplished by calling jpeg_std_error() as usual, but then overriding
-some of the method pointers in the jpeg_error_mgr struct, as illustrated by
-example.c.
-
-All of the error handling routines will receive a pointer to the JPEG object
-(a j_common_ptr which points to either a jpeg_compress_struct or a
-jpeg_decompress_struct; if you need to tell which, test the is_decompressor
-field).  This struct includes a pointer to the error manager struct in its
-"err" field.  Frequently, custom error handler routines will need to access
-additional data which is not known to the JPEG library or the standard error
-handler.  The most convenient way to do this is to embed either the JPEG
-object or the jpeg_error_mgr struct in a larger structure that contains
-additional fields; then casting the passed pointer provides access to the
-additional fields.  Again, see example.c for one way to do it.  (Beginning
-with IJG version 6b, there is also a void pointer "client_data" in each
-JPEG object, which the application can also use to find related data.
-The library does not touch client_data at all.)
-
-The individual methods that you might wish to override are:
-
-error_exit (j_common_ptr cinfo)
-        Receives control for a fatal error.  Information sufficient to
-        generate the error message has been stored in cinfo->err; call
-        output_message to display it.  Control must NOT return to the caller;
-        generally this routine will exit() or longjmp() somewhere.
-        Typically you would override this routine to get rid of the exit()
-        default behavior.  Note that if you continue processing, you should
-        clean up the JPEG object with jpeg_abort() or jpeg_destroy().
-
-output_message (j_common_ptr cinfo)
-        Actual output of any JPEG message.  Override this to send messages
-        somewhere other than stderr.  Note that this method does not know
-        how to generate a message, only where to send it.
-
-format_message (j_common_ptr cinfo, char * buffer)
-        Constructs a readable error message string based on the error info
-        stored in cinfo->err.  This method is called by output_message.  Few
-        applications should need to override this method.  One possible
-        reason for doing so is to implement dynamic switching of error message
-        language.
-
-emit_message (j_common_ptr cinfo, int msg_level)
-        Decide whether or not to emit a warning or trace message; if so,
-        calls output_message.  The main reason for overriding this method
-        would be to abort on warnings.  msg_level is -1 for warnings,
-        0 and up for trace messages.
-
-Only error_exit() and emit_message() are called from the rest of the JPEG
-library; the other two are internal to the error handler.
-
-The actual message texts are stored in an array of strings which is pointed to
-by the field err->jpeg_message_table.  The messages are numbered from 0 to
-err->last_jpeg_message, and it is these code numbers that are used in the
-JPEG library code.  You could replace the message texts (for instance, with
-messages in French or German) by changing the message table pointer.  See
-jerror.h for the default texts.  CAUTION: this table will almost certainly
-change or grow from one library version to the next.
-
-It may be useful for an application to add its own message texts that are
-handled by the same mechanism.  The error handler supports a second "add-on"
-message table for this purpose.  To define an addon table, set the pointer
-err->addon_message_table and the message numbers err->first_addon_message and
-err->last_addon_message.  If you number the addon messages beginning at 1000
-or so, you won't have to worry about conflicts with the library's built-in
-messages.  See the sample applications cjpeg/djpeg for an example of using
-addon messages (the addon messages are defined in cderror.h).
-
-Actual invocation of the error handler is done via macros defined in jerror.h:
-        ERREXITn(...)   for fatal errors
-        WARNMSn(...)    for corrupt-data warnings
-        TRACEMSn(...)   for trace and informational messages.
-These macros store the message code and any additional parameters into the
-error handler struct, then invoke the error_exit() or emit_message() method.
-The variants of each macro are for varying numbers of additional parameters.
-The additional parameters are inserted into the generated message using
-standard printf() format codes.
-
-See jerror.h and jerror.c for further details.
-
-
-Compressed data handling (source and destination managers)
-----------------------------------------------------------
-
-The JPEG compression library sends its compressed data to a "destination
-manager" module.  The default destination manager just writes the data to a
-memory buffer or to a stdio stream, but you can provide your own manager to
-do something else.  Similarly, the decompression library calls a "source
-manager" to obtain the compressed data; you can provide your own source
-manager if you want the data to come from somewhere other than a memory
-buffer or a stdio stream.
-
-In both cases, compressed data is processed a bufferload at a time: the
-destination or source manager provides a work buffer, and the library invokes
-the manager only when the buffer is filled or emptied.  (You could define a
-one-character buffer to force the manager to be invoked for each byte, but
-that would be rather inefficient.)  The buffer's size and location are
-controlled by the manager, not by the library.  For example, the memory
-source manager just makes the buffer pointer and length point to the original
-data in memory.  In this case the buffer-reload procedure will be invoked
-only if the decompressor ran off the end of the datastream, which would
-indicate an erroneous datastream.
-
-The work buffer is defined as an array of datatype JOCTET, which is generally
-"char" or "unsigned char".  On a machine where char is not exactly 8 bits
-wide, you must define JOCTET as a wider data type and then modify the data
-source and destination modules to transcribe the work arrays into 8-bit units
-on external storage.
-
-A data destination manager struct contains a pointer and count defining the
-next byte to write in the work buffer and the remaining free space:
-
-        JOCTET * next_output_byte;  /* => next byte to write in buffer */
-        size_t free_in_buffer;      /* # of byte spaces remaining in buffer */
-
-The library increments the pointer and decrements the count until the buffer
-is filled.  The manager's empty_output_buffer method must reset the pointer
-and count.  The manager is expected to remember the buffer's starting address
-and total size in private fields not visible to the library.
-
-A data destination manager provides three methods:
-
-init_destination (j_compress_ptr cinfo)
-        Initialize destination.  This is called by jpeg_start_compress()
-        before any data is actually written.  It must initialize
-        next_output_byte and free_in_buffer.  free_in_buffer must be
-        initialized to a positive value.
-
-empty_output_buffer (j_compress_ptr cinfo)
-        This is called whenever the buffer has filled (free_in_buffer
-        reaches zero).  In typical applications, it should write out the
-        *entire* buffer (use the saved start address and buffer length;
-        ignore the current state of next_output_byte and free_in_buffer).
-        Then reset the pointer & count to the start of the buffer, and
-        return TRUE indicating that the buffer has been dumped.
-        free_in_buffer must be set to a positive value when TRUE is
-        returned.  A FALSE return should only be used when I/O suspension is
-        desired (this operating mode is discussed in the next section).
-
-term_destination (j_compress_ptr cinfo)
-        Terminate destination --- called by jpeg_finish_compress() after all
-        data has been written.  In most applications, this must flush any
-        data remaining in the buffer.  Use either next_output_byte or
-        free_in_buffer to determine how much data is in the buffer.
-
-term_destination() is NOT called by jpeg_abort() or jpeg_destroy().  If you
-want the destination manager to be cleaned up during an abort, you must do it
-yourself.
-
-You will also need code to create a jpeg_destination_mgr struct, fill in its
-method pointers, and insert a pointer to the struct into the "dest" field of
-the JPEG compression object.  This can be done in-line in your setup code if
-you like, but it's probably cleaner to provide a separate routine similar to
-the jpeg_stdio_dest() or jpeg_mem_dest() routines of the supplied destination
-managers.
-
-Decompression source managers follow a parallel design, but with some
-additional frammishes.  The source manager struct contains a pointer and count
-defining the next byte to read from the work buffer and the number of bytes
-remaining:
-
-        const JOCTET * next_input_byte; /* => next byte to read from buffer */
-        size_t bytes_in_buffer;         /* # of bytes remaining in buffer */
-
-The library increments the pointer and decrements the count until the buffer
-is emptied.  The manager's fill_input_buffer method must reset the pointer and
-count.  In most applications, the manager must remember the buffer's starting
-address and total size in private fields not visible to the library.
-
-A data source manager provides five methods:
-
-init_source (j_decompress_ptr cinfo)
-        Initialize source.  This is called by jpeg_read_header() before any
-        data is actually read.  Unlike init_destination(), it may leave
-        bytes_in_buffer set to 0 (in which case a fill_input_buffer() call
-        will occur immediately).
-
-fill_input_buffer (j_decompress_ptr cinfo)
-        This is called whenever bytes_in_buffer has reached zero and more
-        data is wanted.  In typical applications, it should read fresh data
-        into the buffer (ignoring the current state of next_input_byte and
-        bytes_in_buffer), reset the pointer & count to the start of the
-        buffer, and return TRUE indicating that the buffer has been reloaded.
-        It is not necessary to fill the buffer entirely, only to obtain at
-        least one more byte.  bytes_in_buffer MUST be set to a positive value
-        if TRUE is returned.  A FALSE return should only be used when I/O
-        suspension is desired (this mode is discussed in the next section).
-
-skip_input_data (j_decompress_ptr cinfo, long num_bytes)
-        Skip num_bytes worth of data.  The buffer pointer and count should
-        be advanced over num_bytes input bytes, refilling the buffer as
-        needed.  This is used to skip over a potentially large amount of
-        uninteresting data (such as an APPn marker).  In some applications
-        it may be possible to optimize away the reading of the skipped data,
-        but it's not clear that being smart is worth much trouble; large
-        skips are uncommon.  bytes_in_buffer may be zero on return.
-        A zero or negative skip count should be treated as a no-op.
-
-resync_to_restart (j_decompress_ptr cinfo, int desired)
-        This routine is called only when the decompressor has failed to find
-        a restart (RSTn) marker where one is expected.  Its mission is to
-        find a suitable point for resuming decompression.  For most
-        applications, we recommend that you just use the default resync
-        procedure, jpeg_resync_to_restart().  However, if you are able to back
-        up in the input data stream, or if you have a-priori knowledge about
-        the likely location of restart markers, you may be able to do better.
-        Read the read_restart_marker() and jpeg_resync_to_restart() routines
-        in jdmarker.c if you think you'd like to implement your own resync
-        procedure.
-
-term_source (j_decompress_ptr cinfo)
-        Terminate source --- called by jpeg_finish_decompress() after all
-        data has been read.  Often a no-op.
-
-For both fill_input_buffer() and skip_input_data(), there is no such thing
-as an EOF return.  If the end of the file has been reached, the routine has
-a choice of exiting via ERREXIT() or inserting fake data into the buffer.
-In most cases, generating a warning message and inserting a fake EOI marker
-is the best course of action --- this will allow the decompressor to output
-however much of the image is there.  In pathological cases, the decompressor
-may swallow the EOI and again demand data ... just keep feeding it fake EOIs.
-jdatasrc.c illustrates the recommended error recovery behavior.
-
-term_source() is NOT called by jpeg_abort() or jpeg_destroy().  If you want
-the source manager to be cleaned up during an abort, you must do it yourself.
-
-You will also need code to create a jpeg_source_mgr struct, fill in its method
-pointers, and insert a pointer to the struct into the "src" field of the JPEG
-decompression object.  This can be done in-line in your setup code if you
-like, but it's probably cleaner to provide a separate routine similar to the
-jpeg_stdio_src() or jpeg_mem_src() routines of the supplied source managers.
-
-For more information, consult the memory and stdio source and destination
-managers in jdatasrc.c and jdatadst.c.
-
-
-I/O suspension
---------------
-
-Some applications need to use the JPEG library as an incremental memory-to-
-memory filter: when the compressed data buffer is filled or emptied, they want
-control to return to the outer loop, rather than expecting that the buffer can
-be emptied or reloaded within the data source/destination manager subroutine.
-The library supports this need by providing an "I/O suspension" mode, which we
-describe in this section.
-
-The I/O suspension mode is not a panacea: nothing is guaranteed about the
-maximum amount of time spent in any one call to the library, so it will not
-eliminate response-time problems in single-threaded applications.  If you
-need guaranteed response time, we suggest you "bite the bullet" and implement
-a real multi-tasking capability.
-
-To use I/O suspension, cooperation is needed between the calling application
-and the data source or destination manager; you will always need a custom
-source/destination manager.  (Please read the previous section if you haven't
-already.)  The basic idea is that the empty_output_buffer() or
-fill_input_buffer() routine is a no-op, merely returning FALSE to indicate
-that it has done nothing.  Upon seeing this, the JPEG library suspends
-operation and returns to its caller.  The surrounding application is
-responsible for emptying or refilling the work buffer before calling the
-JPEG library again.
-
-Compression suspension:
-
-For compression suspension, use an empty_output_buffer() routine that returns
-FALSE; typically it will not do anything else.  This will cause the
-compressor to return to the caller of jpeg_write_scanlines(), with the return
-value indicating that not all the supplied scanlines have been accepted.
-The application must make more room in the output buffer, adjust the output
-buffer pointer/count appropriately, and then call jpeg_write_scanlines()
-again, pointing to the first unconsumed scanline.
-
-When forced to suspend, the compressor will backtrack to a convenient stopping
-point (usually the start of the current MCU); it will regenerate some output
-data when restarted.  Therefore, although empty_output_buffer() is only
-called when the buffer is filled, you should NOT write out the entire buffer
-after a suspension.  Write only the data up to the current position of
-next_output_byte/free_in_buffer.  The data beyond that point will be
-regenerated after resumption.
-
-Because of the backtracking behavior, a good-size output buffer is essential
-for efficiency; you don't want the compressor to suspend often.  (In fact, an
-overly small buffer could lead to infinite looping, if a single MCU required
-more data than would fit in the buffer.)  We recommend a buffer of at least
-several Kbytes.  You may want to insert explicit code to ensure that you don't
-call jpeg_write_scanlines() unless there is a reasonable amount of space in
-the output buffer; in other words, flush the buffer before trying to compress
-more data.
-
-The compressor does not allow suspension while it is trying to write JPEG
-markers at the beginning and end of the file.  This means that:
-  * At the beginning of a compression operation, there must be enough free
-    space in the output buffer to hold the header markers (typically 600 or
-    so bytes).  The recommended buffer size is bigger than this anyway, so
-    this is not a problem as long as you start with an empty buffer.  However,
-    this restriction might catch you if you insert large special markers, such
-    as a JFIF thumbnail image, without flushing the buffer afterwards.
-  * When you call jpeg_finish_compress(), there must be enough space in the
-    output buffer to emit any buffered data and the final EOI marker.  In the
-    current implementation, half a dozen bytes should suffice for this, but
-    for safety's sake we recommend ensuring that at least 100 bytes are free
-    before calling jpeg_finish_compress().
-
-A more significant restriction is that jpeg_finish_compress() cannot suspend.
-This means you cannot use suspension with multi-pass operating modes, namely
-Huffman code optimization and multiple-scan output.  Those modes write the
-whole file during jpeg_finish_compress(), which will certainly result in
-buffer overrun.  (Note that this restriction applies only to compression,
-not decompression.  The decompressor supports input suspension in all of its
-operating modes.)
-
-Decompression suspension:
-
-For decompression suspension, use a fill_input_buffer() routine that simply
-returns FALSE (except perhaps during error recovery, as discussed below).
-This will cause the decompressor to return to its caller with an indication
-that suspension has occurred.  This can happen at four places:
-  * jpeg_read_header(): will return JPEG_SUSPENDED.
-  * jpeg_start_decompress(): will return FALSE, rather than its usual TRUE.
-  * jpeg_read_scanlines(): will return the number of scanlines already
-        completed (possibly 0).
-  * jpeg_finish_decompress(): will return FALSE, rather than its usual TRUE.
-The surrounding application must recognize these cases, load more data into
-the input buffer, and repeat the call.  In the case of jpeg_read_scanlines(),
-increment the passed pointers past any scanlines successfully read.
-
-Just as with compression, the decompressor will typically backtrack to a
-convenient restart point before suspending.  When fill_input_buffer() is
-called, next_input_byte/bytes_in_buffer point to the current restart point,
-which is where the decompressor will backtrack to if FALSE is returned.
-The data beyond that position must NOT be discarded if you suspend; it needs
-to be re-read upon resumption.  In most implementations, you'll need to shift
-this data down to the start of your work buffer and then load more data after
-it.  Again, this behavior means that a several-Kbyte work buffer is essential
-for decent performance; furthermore, you should load a reasonable amount of
-new data before resuming decompression.  (If you loaded, say, only one new
-byte each time around, you could waste a LOT of cycles.)
-
-The skip_input_data() source manager routine requires special care in a
-suspension scenario.  This routine is NOT granted the ability to suspend the
-decompressor; it can decrement bytes_in_buffer to zero, but no more.  If the
-requested skip distance exceeds the amount of data currently in the input
-buffer, then skip_input_data() must set bytes_in_buffer to zero and record the
-additional skip distance somewhere else.  The decompressor will immediately
-call fill_input_buffer(), which should return FALSE, which will cause a
-suspension return.  The surrounding application must then arrange to discard
-the recorded number of bytes before it resumes loading the input buffer.
-(Yes, this design is rather baroque, but it avoids complexity in the far more
-common case where a non-suspending source manager is used.)
-
-If the input data has been exhausted, we recommend that you emit a warning
-and insert dummy EOI markers just as a non-suspending data source manager
-would do.  This can be handled either in the surrounding application logic or
-within fill_input_buffer(); the latter is probably more efficient.  If
-fill_input_buffer() knows that no more data is available, it can set the
-pointer/count to point to a dummy EOI marker and then return TRUE just as
-though it had read more data in a non-suspending situation.
-
-The decompressor does not attempt to suspend within standard JPEG markers;
-instead it will backtrack to the start of the marker and reprocess the whole
-marker next time.  Hence the input buffer must be large enough to hold the
-longest standard marker in the file.  Standard JPEG markers should normally
-not exceed a few hundred bytes each (DHT tables are typically the longest).
-We recommend at least a 2K buffer for performance reasons, which is much
-larger than any correct marker is likely to be.  For robustness against
-damaged marker length counts, you may wish to insert a test in your
-application for the case that the input buffer is completely full and yet
-the decoder has suspended without consuming any data --- otherwise, if this
-situation did occur, it would lead to an endless loop.  (The library can't
-provide this test since it has no idea whether "the buffer is full", or
-even whether there is a fixed-size input buffer.)
-
-The input buffer would need to be 64K to allow for arbitrary COM or APPn
-markers, but these are handled specially: they are either saved into allocated
-memory, or skipped over by calling skip_input_data().  In the former case,
-suspension is handled correctly, and in the latter case, the problem of
-buffer overrun is placed on skip_input_data's shoulders, as explained above.
-Note that if you provide your own marker handling routine for large markers,
-you should consider how to deal with buffer overflow.
-
-Multiple-buffer management:
-
-In some applications it is desirable to store the compressed data in a linked
-list of buffer areas, so as to avoid data copying.  This can be handled by
-having empty_output_buffer() or fill_input_buffer() set the pointer and count
-to reference the next available buffer; FALSE is returned only if no more
-buffers are available.  Although seemingly straightforward, there is a
-pitfall in this approach: the backtrack that occurs when FALSE is returned
-could back up into an earlier buffer.  For example, when fill_input_buffer()
-is called, the current pointer & count indicate the backtrack restart point.
-Since fill_input_buffer() will set the pointer and count to refer to a new
-buffer, the restart position must be saved somewhere else.  Suppose a second
-call to fill_input_buffer() occurs in the same library call, and no
-additional input data is available, so fill_input_buffer must return FALSE.
-If the JPEG library has not moved the pointer/count forward in the current
-buffer, then *the correct restart point is the saved position in the prior
-buffer*.  Prior buffers may be discarded only after the library establishes
-a restart point within a later buffer.  Similar remarks apply for output into
-a chain of buffers.
-
-The library will never attempt to backtrack over a skip_input_data() call,
-so any skipped data can be permanently discarded.  You still have to deal
-with the case of skipping not-yet-received data, however.
-
-It's much simpler to use only a single buffer; when fill_input_buffer() is
-called, move any unconsumed data (beyond the current pointer/count) down to
-the beginning of this buffer and then load new data into the remaining buffer
-space.  This approach requires a little more data copying but is far easier
-to get right.
-
-
-Progressive JPEG support
-------------------------
-
-Progressive JPEG rearranges the stored data into a series of scans of
-increasing quality.  In situations where a JPEG file is transmitted across a
-slow communications link, a decoder can generate a low-quality image very
-quickly from the first scan, then gradually improve the displayed quality as
-more scans are received.  The final image after all scans are complete is
-identical to that of a regular (sequential) JPEG file of the same quality
-setting.  Progressive JPEG files are often slightly smaller than equivalent
-sequential JPEG files, but the possibility of incremental display is the main
-reason for using progressive JPEG.
-
-The IJG encoder library generates progressive JPEG files when given a
-suitable "scan script" defining how to divide the data into scans.
-Creation of progressive JPEG files is otherwise transparent to the encoder.
-Progressive JPEG files can also be read transparently by the decoder library.
-If the decoding application simply uses the library as defined above, it
-will receive a final decoded image without any indication that the file was
-progressive.  Of course, this approach does not allow incremental display.
-To perform incremental display, an application needs to use the decoder
-library's "buffered-image" mode, in which it receives a decoded image
-multiple times.
-
-Each displayed scan requires about as much work to decode as a full JPEG
-image of the same size, so the decoder must be fairly fast in relation to the
-data transmission rate in order to make incremental display useful.  However,
-it is possible to skip displaying the image and simply add the incoming bits
-to the decoder's coefficient buffer.  This is fast because only Huffman
-decoding need be done, not IDCT, upsampling, colorspace conversion, etc.
-The IJG decoder library allows the application to switch dynamically between
-displaying the image and simply absorbing the incoming bits.  A properly
-coded application can automatically adapt the number of display passes to
-suit the time available as the image is received.  Also, a final
-higher-quality display cycle can be performed from the buffered data after
-the end of the file is reached.
-
-Progressive compression:
-
-To create a progressive JPEG file (or a multiple-scan sequential JPEG file),
-set the scan_info cinfo field to point to an array of scan descriptors, and
-perform compression as usual.  Instead of constructing your own scan list,
-you can call the jpeg_simple_progression() helper routine to create a
-recommended progression sequence; this method should be used by all
-applications that don't want to get involved in the nitty-gritty of
-progressive scan sequence design.  (If you want to provide user control of
-scan sequences, you may wish to borrow the scan script reading code found
-in rdswitch.c, so that you can read scan script files just like cjpeg's.)
-When scan_info is not NULL, the compression library will store DCT'd data
-into a buffer array as jpeg_write_scanlines() is called, and will emit all
-the requested scans during jpeg_finish_compress().  This implies that
-multiple-scan output cannot be created with a suspending data destination
-manager, since jpeg_finish_compress() does not support suspension.  We
-should also note that the compressor currently forces Huffman optimization
-mode when creating a progressive JPEG file, because the default Huffman
-tables are unsuitable for progressive files.
-
-Progressive decompression:
-
-When buffered-image mode is not used, the decoder library will read all of
-a multi-scan file during jpeg_start_decompress(), so that it can provide a
-final decoded image.  (Here "multi-scan" means either progressive or
-multi-scan sequential.)  This makes multi-scan files transparent to the
-decoding application.  However, existing applications that used suspending
-input with version 5 of the IJG library will need to be modified to check
-for a suspension return from jpeg_start_decompress().
-
-To perform incremental display, an application must use the library's
-buffered-image mode.  This is described in the next section.
-
-
-Buffered-image mode
--------------------
-
-In buffered-image mode, the library stores the partially decoded image in a
-coefficient buffer, from which it can be read out as many times as desired.
-This mode is typically used for incremental display of progressive JPEG files,
-but it can be used with any JPEG file.  Each scan of a progressive JPEG file
-adds more data (more detail) to the buffered image.  The application can
-display in lockstep with the source file (one display pass per input scan),
-or it can allow input processing to outrun display processing.  By making
-input and display processing run independently, it is possible for the
-application to adapt progressive display to a wide range of data transmission
-rates.
-
-The basic control flow for buffered-image decoding is
-
-        jpeg_create_decompress()
-        set data source
-        jpeg_read_header()
-        set overall decompression parameters
-        cinfo.buffered_image = TRUE;    /* select buffered-image mode */
-        jpeg_start_decompress()
-        for (each output pass) {
-            adjust output decompression parameters if required
-            jpeg_start_output()         /* start a new output pass */
-            for (all scanlines in image) {
-                jpeg_read_scanlines()
-                display scanlines
-            }
-            jpeg_finish_output()        /* terminate output pass */
-        }
-        jpeg_finish_decompress()
-        jpeg_destroy_decompress()
-
-This differs from ordinary unbuffered decoding in that there is an additional
-level of looping.  The application can choose how many output passes to make
-and how to display each pass.
-
-The simplest approach to displaying progressive images is to do one display
-pass for each scan appearing in the input file.  In this case the outer loop
-condition is typically
-        while (! jpeg_input_complete(&cinfo))
-and the start-output call should read
-        jpeg_start_output(&cinfo, cinfo.input_scan_number);
-The second parameter to jpeg_start_output() indicates which scan of the input
-file is to be displayed; the scans are numbered starting at 1 for this
-purpose.  (You can use a loop counter starting at 1 if you like, but using
-the library's input scan counter is easier.)  The library automatically reads
-data as necessary to complete each requested scan, and jpeg_finish_output()
-advances to the next scan or end-of-image marker (hence input_scan_number
-will be incremented by the time control arrives back at jpeg_start_output()).
-With this technique, data is read from the input file only as needed, and
-input and output processing run in lockstep.
-
-After reading the final scan and reaching the end of the input file, the
-buffered image remains available; it can be read additional times by
-repeating the jpeg_start_output()/jpeg_read_scanlines()/jpeg_finish_output()
-sequence.  For example, a useful technique is to use fast one-pass color
-quantization for display passes made while the image is arriving, followed by
-a final display pass using two-pass quantization for highest quality.  This
-is done by changing the library parameters before the final output pass.
-Changing parameters between passes is discussed in detail below.
-
-In general the last scan of a progressive file cannot be recognized as such
-until after it is read, so a post-input display pass is the best approach if
-you want special processing in the final pass.
-
-When done with the image, be sure to call jpeg_finish_decompress() to release
-the buffered image (or just use jpeg_destroy_decompress()).
-
-If input data arrives faster than it can be displayed, the application can
-cause the library to decode input data in advance of what's needed to produce
-output.  This is done by calling the routine jpeg_consume_input().
-The return value is one of the following:
-        JPEG_REACHED_SOS:    reached an SOS marker (the start of a new scan)
-        JPEG_REACHED_EOI:    reached the EOI marker (end of image)
-        JPEG_ROW_COMPLETED:  completed reading one MCU row of compressed data
-        JPEG_SCAN_COMPLETED: completed reading last MCU row of current scan
-        JPEG_SUSPENDED:      suspended before completing any of the above
-(JPEG_SUSPENDED can occur only if a suspending data source is used.)  This
-routine can be called at any time after initializing the JPEG object.  It
-reads some additional data and returns when one of the indicated significant
-events occurs.  (If called after the EOI marker is reached, it will
-immediately return JPEG_REACHED_EOI without attempting to read more data.)
-
-The library's output processing will automatically call jpeg_consume_input()
-whenever the output processing overtakes the input; thus, simple lockstep
-display requires no direct calls to jpeg_consume_input().  But by adding
-calls to jpeg_consume_input(), you can absorb data in advance of what is
-being displayed.  This has two benefits:
-  * You can limit buildup of unprocessed data in your input buffer.
-  * You can eliminate extra display passes by paying attention to the
-    state of the library's input processing.
-
-The first of these benefits only requires interspersing calls to
-jpeg_consume_input() with your display operations and any other processing
-you may be doing.  To avoid wasting cycles due to backtracking, it's best to
-call jpeg_consume_input() only after a hundred or so new bytes have arrived.
-This is discussed further under "I/O suspension", above.  (Note: the JPEG
-library currently is not thread-safe.  You must not call jpeg_consume_input()
-from one thread of control if a different library routine is working on the
-same JPEG object in another thread.)
-
-When input arrives fast enough that more than one new scan is available
-before you start a new output pass, you may as well skip the output pass
-corresponding to the completed scan.  This occurs for free if you pass
-cinfo.input_scan_number as the target scan number to jpeg_start_output().
-The input_scan_number field is simply the index of the scan currently being
-consumed by the input processor.  You can ensure that this is up-to-date by
-emptying the input buffer just before calling jpeg_start_output(): call
-jpeg_consume_input() repeatedly until it returns JPEG_SUSPENDED or
-JPEG_REACHED_EOI.
-
-The target scan number passed to jpeg_start_output() is saved in the
-cinfo.output_scan_number field.  The library's output processing calls
-jpeg_consume_input() whenever the current input scan number and row within
-that scan is less than or equal to the current output scan number and row.
-Thus, input processing can "get ahead" of the output processing but is not
-allowed to "fall behind".  You can achieve several different effects by
-manipulating this interlock rule.  For example, if you pass a target scan
-number greater than the current input scan number, the output processor will
-wait until that scan starts to arrive before producing any output.  (To avoid
-an infinite loop, the target scan number is automatically reset to the last
-scan number when the end of image is reached.  Thus, if you specify a large
-target scan number, the library will just absorb the entire input file and
-then perform an output pass.  This is effectively the same as what
-jpeg_start_decompress() does when you don't select buffered-image mode.)
-When you pass a target scan number equal to the current input scan number,
-the image is displayed no faster than the current input scan arrives.  The
-final possibility is to pass a target scan number less than the current input
-scan number; this disables the input/output interlock and causes the output
-processor to simply display whatever it finds in the image buffer, without
-waiting for input.  (However, the library will not accept a target scan
-number less than one, so you can't avoid waiting for the first scan.)
-
-When data is arriving faster than the output display processing can advance
-through the image, jpeg_consume_input() will store data into the buffered
-image beyond the point at which the output processing is reading data out
-again.  If the input arrives fast enough, it may "wrap around" the buffer to
-the point where the input is more than one whole scan ahead of the output.
-If the output processing simply proceeds through its display pass without
-paying attention to the input, the effect seen on-screen is that the lower
-part of the image is one or more scans better in quality than the upper part.
-Then, when the next output scan is started, you have a choice of what target
-scan number to use.  The recommended choice is to use the current input scan
-number at that time, which implies that you've skipped the output scans
-corresponding to the input scans that were completed while you processed the
-previous output scan.  In this way, the decoder automatically adapts its
-speed to the arriving data, by skipping output scans as necessary to keep up
-with the arriving data.
-
-When using this strategy, you'll want to be sure that you perform a final
-output pass after receiving all the data; otherwise your last display may not
-be full quality across the whole screen.  So the right outer loop logic is
-something like this:
-        do {
-            absorb any waiting input by calling jpeg_consume_input()
-            final_pass = jpeg_input_complete(&cinfo);
-            adjust output decompression parameters if required
-            jpeg_start_output(&cinfo, cinfo.input_scan_number);
-            ...
-            jpeg_finish_output()
-        } while (! final_pass);
-rather than quitting as soon as jpeg_input_complete() returns TRUE.  This
-arrangement makes it simple to use higher-quality decoding parameters
-for the final pass.  But if you don't want to use special parameters for
-the final pass, the right loop logic is like this:
-        for (;;) {
-            absorb any waiting input by calling jpeg_consume_input()
-            jpeg_start_output(&cinfo, cinfo.input_scan_number);
-            ...
-            jpeg_finish_output()
-            if (jpeg_input_complete(&cinfo) &&
-                cinfo.input_scan_number == cinfo.output_scan_number)
-              break;
-        }
-In this case you don't need to know in advance whether an output pass is to
-be the last one, so it's not necessary to have reached EOF before starting
-the final output pass; rather, what you want to test is whether the output
-pass was performed in sync with the final input scan.  This form of the loop
-will avoid an extra output pass whenever the decoder is able (or nearly able)
-to keep up with the incoming data.
-
-When the data transmission speed is high, you might begin a display pass,
-then find that much or all of the file has arrived before you can complete
-the pass.  (You can detect this by noting the JPEG_REACHED_EOI return code
-from jpeg_consume_input(), or equivalently by testing jpeg_input_complete().)
-In this situation you may wish to abort the current display pass and start a
-new one using the newly arrived information.  To do so, just call
-jpeg_finish_output() and then start a new pass with jpeg_start_output().
-
-A variant strategy is to abort and restart display if more than one complete
-scan arrives during an output pass; this can be detected by noting
-JPEG_REACHED_SOS returns and/or examining cinfo.input_scan_number.  This
-idea should be employed with caution, however, since the display process
-might never get to the bottom of the image before being aborted, resulting
-in the lower part of the screen being several passes worse than the upper.
-In most cases it's probably best to abort an output pass only if the whole
-file has arrived and you want to begin the final output pass immediately.
-
-When receiving data across a communication link, we recommend always using
-the current input scan number for the output target scan number; if a
-higher-quality final pass is to be done, it should be started (aborting any
-incomplete output pass) as soon as the end of file is received.  However,
-many other strategies are possible.  For example, the application can examine
-the parameters of the current input scan and decide whether to display it or
-not.  If the scan contains only chroma data, one might choose not to use it
-as the target scan, expecting that the scan will be small and will arrive
-quickly.  To skip to the next scan, call jpeg_consume_input() until it
-returns JPEG_REACHED_SOS or JPEG_REACHED_EOI.  Or just use the next higher
-number as the target scan for jpeg_start_output(); but that method doesn't
-let you inspect the next scan's parameters before deciding to display it.
-
-
-In buffered-image mode, jpeg_start_decompress() never performs input and
-thus never suspends.  An application that uses input suspension with
-buffered-image mode must be prepared for suspension returns from these
-routines:
-* jpeg_start_output() performs input only if you request 2-pass quantization
-  and the target scan isn't fully read yet.  (This is discussed below.)
-* jpeg_read_scanlines(), as always, returns the number of scanlines that it
-  was able to produce before suspending.
-* jpeg_finish_output() will read any markers following the target scan,
-  up to the end of the file or the SOS marker that begins another scan.
-  (But it reads no input if jpeg_consume_input() has already reached the
-  end of the file or a SOS marker beyond the target output scan.)
-* jpeg_finish_decompress() will read until the end of file, and thus can
-  suspend if the end hasn't already been reached (as can be tested by
-  calling jpeg_input_complete()).
-jpeg_start_output(), jpeg_finish_output(), and jpeg_finish_decompress()
-all return TRUE if they completed their tasks, FALSE if they had to suspend.
-In the event of a FALSE return, the application must load more input data
-and repeat the call.  Applications that use non-suspending data sources need
-not check the return values of these three routines.
-
-
-It is possible to change decoding parameters between output passes in the
-buffered-image mode.  The decoder library currently supports only very
-limited changes of parameters.  ONLY THE FOLLOWING parameter changes are
-allowed after jpeg_start_decompress() is called:
-* dct_method can be changed before each call to jpeg_start_output().
-  For example, one could use a fast DCT method for early scans, changing
-  to a higher quality method for the final scan.
-* dither_mode can be changed before each call to jpeg_start_output();
-  of course this has no impact if not using color quantization.  Typically
-  one would use ordered dither for initial passes, then switch to
-  Floyd-Steinberg dither for the final pass.  Caution: changing dither mode
-  can cause more memory to be allocated by the library.  Although the amount
-  of memory involved is not large (a scanline or so), it may cause the
-  initial max_memory_to_use specification to be exceeded, which in the worst
-  case would result in an out-of-memory failure.
-* do_block_smoothing can be changed before each call to jpeg_start_output().
-  This setting is relevant only when decoding a progressive JPEG image.
-  During the first DC-only scan, block smoothing provides a very "fuzzy" look
-  instead of the very "blocky" look seen without it; which is better seems a
-  matter of personal taste.  But block smoothing is nearly always a win
-  during later stages, especially when decoding a successive-approximation
-  image: smoothing helps to hide the slight blockiness that otherwise shows
-  up on smooth gradients until the lowest coefficient bits are sent.
-* Color quantization mode can be changed under the rules described below.
-  You *cannot* change between full-color and quantized output (because that
-  would alter the required I/O buffer sizes), but you can change which
-  quantization method is used.
-
-When generating color-quantized output, changing quantization method is a
-very useful way of switching between high-speed and high-quality display.
-The library allows you to change among its three quantization methods:
-1. Single-pass quantization to a fixed color cube.
-   Selected by cinfo.two_pass_quantize = FALSE and cinfo.colormap = NULL.
-2. Single-pass quantization to an application-supplied colormap.
-   Selected by setting cinfo.colormap to point to the colormap (the value of
-   two_pass_quantize is ignored); also set cinfo.actual_number_of_colors.
-3. Two-pass quantization to a colormap chosen specifically for the image.
-   Selected by cinfo.two_pass_quantize = TRUE and cinfo.colormap = NULL.
-   (This is the default setting selected by jpeg_read_header, but it is
-   probably NOT what you want for the first pass of progressive display!)
-These methods offer successively better quality and lesser speed.  However,
-only the first method is available for quantizing in non-RGB color spaces.
-
-IMPORTANT: because the different quantizer methods have very different
-working-storage requirements, the library requires you to indicate which
-one(s) you intend to use before you call jpeg_start_decompress().  (If we did
-not require this, the max_memory_to_use setting would be a complete fiction.)
-You do this by setting one or more of these three cinfo fields to TRUE:
-        enable_1pass_quant              Fixed color cube colormap
-        enable_external_quant           Externally-supplied colormap
-        enable_2pass_quant              Two-pass custom colormap
-All three are initialized FALSE by jpeg_read_header().  But
-jpeg_start_decompress() automatically sets TRUE the one selected by the
-current two_pass_quantize and colormap settings, so you only need to set the
-enable flags for any other quantization methods you plan to change to later.
-
-After setting the enable flags correctly at jpeg_start_decompress() time, you
-can change to any enabled quantization method by setting two_pass_quantize
-and colormap properly just before calling jpeg_start_output().  The following
-special rules apply:
-1. You must explicitly set cinfo.colormap to NULL when switching to 1-pass
-   or 2-pass mode from a different mode, or when you want the 2-pass
-   quantizer to be re-run to generate a new colormap.
-2. To switch to an external colormap, or to change to a different external
-   colormap than was used on the prior pass, you must call
-   jpeg_new_colormap() after setting cinfo.colormap.
-NOTE: if you want to use the same colormap as was used in the prior pass,
-you should not do either of these things.  This will save some nontrivial
-switchover costs.
-(These requirements exist because cinfo.colormap will always be non-NULL
-after completing a prior output pass, since both the 1-pass and 2-pass
-quantizers set it to point to their output colormaps.  Thus you have to
-do one of these two things to notify the library that something has changed.
-Yup, it's a bit klugy, but it's necessary to do it this way for backwards
-compatibility.)
-
-Note that in buffered-image mode, the library generates any requested colormap
-during jpeg_start_output(), not during jpeg_start_decompress().
-
-When using two-pass quantization, jpeg_start_output() makes a pass over the
-buffered image to determine the optimum color map; it therefore may take a
-significant amount of time, whereas ordinarily it does little work.  The
-progress monitor hook is called during this pass, if defined.  It is also
-important to realize that if the specified target scan number is greater than
-or equal to the current input scan number, jpeg_start_output() will attempt
-to consume input as it makes this pass.  If you use a suspending data source,
-you need to check for a FALSE return from jpeg_start_output() under these
-conditions.  The combination of 2-pass quantization and a not-yet-fully-read
-target scan is the only case in which jpeg_start_output() will consume input.
-
-
-Application authors who support buffered-image mode may be tempted to use it
-for all JPEG images, even single-scan ones.  This will work, but it is
-inefficient: there is no need to create an image-sized coefficient buffer for
-single-scan images.  Requesting buffered-image mode for such an image wastes
-memory.  Worse, it can cost time on large images, since the buffered data has
-to be swapped out or written to a temporary file.  If you are concerned about
-maximum performance on baseline JPEG files, you should use buffered-image
-mode only when the incoming file actually has multiple scans.  This can be
-tested by calling jpeg_has_multiple_scans(), which will return a correct
-result at any time after jpeg_read_header() completes.
-
-It is also worth noting that when you use jpeg_consume_input() to let input
-processing get ahead of output processing, the resulting pattern of access to
-the coefficient buffer is quite nonsequential.  It's best to use the memory
-manager jmemnobs.c if you can (ie, if you have enough real or virtual main
-memory).  If not, at least make sure that max_memory_to_use is set as high as
-possible.  If the JPEG memory manager has to use a temporary file, you will
-probably see a lot of disk traffic and poor performance.  (This could be
-improved with additional work on the memory manager, but we haven't gotten
-around to it yet.)
-
-In some applications it may be convenient to use jpeg_consume_input() for all
-input processing, including reading the initial markers; that is, you may
-wish to call jpeg_consume_input() instead of jpeg_read_header() during
-startup.  This works, but note that you must check for JPEG_REACHED_SOS and
-JPEG_REACHED_EOI return codes as the equivalent of jpeg_read_header's codes.
-Once the first SOS marker has been reached, you must call
-jpeg_start_decompress() before jpeg_consume_input() will consume more input;
-it'll just keep returning JPEG_REACHED_SOS until you do.  If you read a
-tables-only file this way, jpeg_consume_input() will return JPEG_REACHED_EOI
-without ever returning JPEG_REACHED_SOS; be sure to check for this case.
-If this happens, the decompressor will not read any more input until you call
-jpeg_abort() to reset it.  It is OK to call jpeg_consume_input() even when not
-using buffered-image mode, but in that case it's basically a no-op after the
-initial markers have been read: it will just return JPEG_SUSPENDED.
-
-
-Abbreviated datastreams and multiple images
--------------------------------------------
-
-A JPEG compression or decompression object can be reused to process multiple
-images.  This saves a small amount of time per image by eliminating the
-"create" and "destroy" operations, but that isn't the real purpose of the
-feature.  Rather, reuse of an object provides support for abbreviated JPEG
-datastreams.  Object reuse can also simplify processing a series of images in
-a single input or output file.  This section explains these features.
-
-A JPEG file normally contains several hundred bytes worth of quantization
-and Huffman tables.  In a situation where many images will be stored or
-transmitted with identical tables, this may represent an annoying overhead.
-The JPEG standard therefore permits tables to be omitted.  The standard
-defines three classes of JPEG datastreams:
-  * "Interchange" datastreams contain an image and all tables needed to decode
-     the image.  These are the usual kind of JPEG file.
-  * "Abbreviated image" datastreams contain an image, but are missing some or
-    all of the tables needed to decode that image.
-  * "Abbreviated table specification" (henceforth "tables-only") datastreams
-    contain only table specifications.
-To decode an abbreviated image, it is necessary to load the missing table(s)
-into the decoder beforehand.  This can be accomplished by reading a separate
-tables-only file.  A variant scheme uses a series of images in which the first
-image is an interchange (complete) datastream, while subsequent ones are
-abbreviated and rely on the tables loaded by the first image.  It is assumed
-that once the decoder has read a table, it will remember that table until a
-new definition for the same table number is encountered.
-
-It is the application designer's responsibility to figure out how to associate
-the correct tables with an abbreviated image.  While abbreviated datastreams
-can be useful in a closed environment, their use is strongly discouraged in
-any situation where data exchange with other applications might be needed.
-Caveat designer.
-
-The JPEG library provides support for reading and writing any combination of
-tables-only datastreams and abbreviated images.  In both compression and
-decompression objects, a quantization or Huffman table will be retained for
-the lifetime of the object, unless it is overwritten by a new table definition.
-
-
-To create abbreviated image datastreams, it is only necessary to tell the
-compressor not to emit some or all of the tables it is using.  Each
-quantization and Huffman table struct contains a boolean field "sent_table",
-which normally is initialized to FALSE.  For each table used by the image, the
-header-writing process emits the table and sets sent_table = TRUE unless it is
-already TRUE.  (In normal usage, this prevents outputting the same table
-definition multiple times, as would otherwise occur because the chroma
-components typically share tables.)  Thus, setting this field to TRUE before
-calling jpeg_start_compress() will prevent the table from being written at
-all.
-
-If you want to create a "pure" abbreviated image file containing no tables,
-just call "jpeg_suppress_tables(&cinfo, TRUE)" after constructing all the
-tables.  If you want to emit some but not all tables, you'll need to set the
-individual sent_table fields directly.
-
-To create an abbreviated image, you must also call jpeg_start_compress()
-with a second parameter of FALSE, not TRUE.  Otherwise jpeg_start_compress()
-will force all the sent_table fields to FALSE.  (This is a safety feature to
-prevent abbreviated images from being created accidentally.)
-
-To create a tables-only file, perform the same parameter setup that you
-normally would, but instead of calling jpeg_start_compress() and so on, call
-jpeg_write_tables(&cinfo).  This will write an abbreviated datastream
-containing only SOI, DQT and/or DHT markers, and EOI.  All the quantization
-and Huffman tables that are currently defined in the compression object will
-be emitted unless their sent_tables flag is already TRUE, and then all the
-sent_tables flags will be set TRUE.
-
-A sure-fire way to create matching tables-only and abbreviated image files
-is to proceed as follows:
-
-        create JPEG compression object
-        set JPEG parameters
-        set destination to tables-only file
-        jpeg_write_tables(&cinfo);
-        set destination to image file
-        jpeg_start_compress(&cinfo, FALSE);
-        write data...
-        jpeg_finish_compress(&cinfo);
-
-Since the JPEG parameters are not altered between writing the table file and
-the abbreviated image file, the same tables are sure to be used.  Of course,
-you can repeat the jpeg_start_compress() ... jpeg_finish_compress() sequence
-many times to produce many abbreviated image files matching the table file.
-
-You cannot suppress output of the computed Huffman tables when Huffman
-optimization is selected.  (If you could, there'd be no way to decode the
-image...)  Generally, you don't want to set optimize_coding = TRUE when
-you are trying to produce abbreviated files.
-
-In some cases you might want to compress an image using tables which are
-not stored in the application, but are defined in an interchange or
-tables-only file readable by the application.  This can be done by setting up
-a JPEG decompression object to read the specification file, then copying the
-tables into your compression object.  See jpeg_copy_critical_parameters()
-for an example of copying quantization tables.
-
-
-To read abbreviated image files, you simply need to load the proper tables
-into the decompression object before trying to read the abbreviated image.
-If the proper tables are stored in the application program, you can just
-allocate the table structs and fill in their contents directly.  For example,
-to load a fixed quantization table into table slot "n":
-
-    if (cinfo.quant_tbl_ptrs[n] == NULL)
-      cinfo.quant_tbl_ptrs[n] = jpeg_alloc_quant_table((j_common_ptr) &cinfo);
-    quant_ptr = cinfo.quant_tbl_ptrs[n];        /* quant_ptr is JQUANT_TBL* */
-    for (i = 0; i < 64; i++) {
-      /* Qtable[] is desired quantization table, in natural array order */
-      quant_ptr->quantval[i] = Qtable[i];
-    }
-
-Code to load a fixed Huffman table is typically (for AC table "n"):
-
-    if (cinfo.ac_huff_tbl_ptrs[n] == NULL)
-      cinfo.ac_huff_tbl_ptrs[n] = jpeg_alloc_huff_table((j_common_ptr) &cinfo);
-    huff_ptr = cinfo.ac_huff_tbl_ptrs[n];       /* huff_ptr is JHUFF_TBL* */
-    for (i = 1; i <= 16; i++) {
-      /* counts[i] is number of Huffman codes of length i bits, i=1..16 */
-      huff_ptr->bits[i] = counts[i];
-    }
-    for (i = 0; i < 256; i++) {
-      /* symbols[] is the list of Huffman symbols, in code-length order */
-      huff_ptr->huffval[i] = symbols[i];
-    }
-
-(Note that trying to set cinfo.quant_tbl_ptrs[n] to point directly at a
-constant JQUANT_TBL object is not safe.  If the incoming file happened to
-contain a quantization table definition, your master table would get
-overwritten!  Instead allocate a working table copy and copy the master table
-into it, as illustrated above.  Ditto for Huffman tables, of course.)
-
-You might want to read the tables from a tables-only file, rather than
-hard-wiring them into your application.  The jpeg_read_header() call is
-sufficient to read a tables-only file.  You must pass a second parameter of
-FALSE to indicate that you do not require an image to be present.  Thus, the
-typical scenario is
-
-        create JPEG decompression object
-        set source to tables-only file
-        jpeg_read_header(&cinfo, FALSE);
-        set source to abbreviated image file
-        jpeg_read_header(&cinfo, TRUE);
-        set decompression parameters
-        jpeg_start_decompress(&cinfo);
-        read data...
-        jpeg_finish_decompress(&cinfo);
-
-In some cases, you may want to read a file without knowing whether it contains
-an image or just tables.  In that case, pass FALSE and check the return value
-from jpeg_read_header(): it will be JPEG_HEADER_OK if an image was found,
-JPEG_HEADER_TABLES_ONLY if only tables were found.  (A third return value,
-JPEG_SUSPENDED, is possible when using a suspending data source manager.)
-Note that jpeg_read_header() will not complain if you read an abbreviated
-image for which you haven't loaded the missing tables; the missing-table check
-occurs later, in jpeg_start_decompress().
-
-
-It is possible to read a series of images from a single source file by
-repeating the jpeg_read_header() ... jpeg_finish_decompress() sequence,
-without releasing/recreating the JPEG object or the data source module.
-(If you did reinitialize, any partial bufferload left in the data source
-buffer at the end of one image would be discarded, causing you to lose the
-start of the next image.)  When you use this method, stored tables are
-automatically carried forward, so some of the images can be abbreviated images
-that depend on tables from earlier images.
-
-If you intend to write a series of images into a single destination file,
-you might want to make a specialized data destination module that doesn't
-flush the output buffer at term_destination() time.  This would speed things
-up by some trifling amount.  Of course, you'd need to remember to flush the
-buffer after the last image.  You can make the later images be abbreviated
-ones by passing FALSE to jpeg_start_compress().
-
-
-Special markers
----------------
-
-Some applications may need to insert or extract special data in the JPEG
-datastream.  The JPEG standard provides marker types "COM" (comment) and
-"APP0" through "APP15" (application) to hold application-specific data.
-Unfortunately, the use of these markers is not specified by the standard.
-COM markers are fairly widely used to hold user-supplied text.  The JFIF file
-format spec uses APP0 markers with specified initial strings to hold certain
-data.  Adobe applications use APP14 markers beginning with the string "Adobe"
-for miscellaneous data.  Other APPn markers are rarely seen, but might
-contain almost anything.
-
-If you wish to store user-supplied text, we recommend you use COM markers
-and place readable 7-bit ASCII text in them.  Newline conventions are not
-standardized --- expect to find LF (Unix style), CR/LF (DOS style), or CR
-(Mac style).  A robust COM reader should be able to cope with random binary
-garbage, including nulls, since some applications generate COM markers
-containing non-ASCII junk.  (But yours should not be one of them.)
-
-For program-supplied data, use an APPn marker, and be sure to begin it with an
-identifying string so that you can tell whether the marker is actually yours.
-It's probably best to avoid using APP0 or APP14 for any private markers.
-(NOTE: the upcoming SPIFF standard will use APP8 markers; we recommend you
-not use APP8 markers for any private purposes, either.)
-
-Keep in mind that at most 65533 bytes can be put into one marker, but you
-can have as many markers as you like.
-
-By default, the IJG compression library will write a JFIF APP0 marker if the
-selected JPEG colorspace is grayscale or YCbCr, or an Adobe APP14 marker if
-the selected colorspace is RGB, CMYK, or YCCK.  You can disable this, but
-we don't recommend it.  The decompression library will recognize JFIF and
-Adobe markers and will set the JPEG colorspace properly when one is found.
-
-
-You can write special markers immediately following the datastream header by
-calling jpeg_write_marker() after jpeg_start_compress() and before the first
-call to jpeg_write_scanlines().  When you do this, the markers appear after
-the SOI and the JFIF APP0 and Adobe APP14 markers (if written), but before
-all else.  Specify the marker type parameter as "JPEG_COM" for COM or
-"JPEG_APP0 + n" for APPn.  (Actually, jpeg_write_marker will let you write
-any marker type, but we don't recommend writing any other kinds of marker.)
-For example, to write a user comment string pointed to by comment_text:
-        jpeg_write_marker(cinfo, JPEG_COM, comment_text, strlen(comment_text));
-
-If it's not convenient to store all the marker data in memory at once,
-you can instead call jpeg_write_m_header() followed by multiple calls to
-jpeg_write_m_byte().  If you do it this way, it's your responsibility to
-call jpeg_write_m_byte() exactly the number of times given in the length
-parameter to jpeg_write_m_header().  (This method lets you empty the
-output buffer partway through a marker, which might be important when
-using a suspending data destination module.  In any case, if you are using
-a suspending destination, you should flush its buffer after inserting
-any special markers.  See "I/O suspension".)
-
-Or, if you prefer to synthesize the marker byte sequence yourself,
-you can just cram it straight into the data destination module.
-
-If you are writing JFIF 1.02 extension markers (thumbnail images), don't
-forget to set cinfo.JFIF_minor_version = 2 so that the encoder will write the
-correct JFIF version number in the JFIF header marker.  The library's default
-is to write version 1.01, but that's wrong if you insert any 1.02 extension
-markers.  (We could probably get away with just defaulting to 1.02, but there
-used to be broken decoders that would complain about unknown minor version
-numbers.  To reduce compatibility risks it's safest not to write 1.02 unless
-you are actually using 1.02 extensions.)
-
-
-When reading, two methods of handling special markers are available:
-1. You can ask the library to save the contents of COM and/or APPn markers
-into memory, and then examine them at your leisure afterwards.
-2. You can supply your own routine to process COM and/or APPn markers
-on-the-fly as they are read.
-The first method is simpler to use, especially if you are using a suspending
-data source; writing a marker processor that copes with input suspension is
-not easy (consider what happens if the marker is longer than your available
-input buffer).  However, the second method conserves memory since the marker
-data need not be kept around after it's been processed.
-
-For either method, you'd normally set up marker handling after creating a
-decompression object and before calling jpeg_read_header(), because the
-markers of interest will typically be near the head of the file and so will
-be scanned by jpeg_read_header.  Once you've established a marker handling
-method, it will be used for the life of that decompression object
-(potentially many datastreams), unless you change it.  Marker handling is
-determined separately for COM markers and for each APPn marker code.
-
-
-To save the contents of special markers in memory, call
-        jpeg_save_markers(cinfo, marker_code, length_limit)
-where marker_code is the marker type to save, JPEG_COM or JPEG_APP0+n.
-(To arrange to save all the special marker types, you need to call this
-routine 17 times, for COM and APP0-APP15.)  If the incoming marker is longer
-than length_limit data bytes, only length_limit bytes will be saved; this
-parameter allows you to avoid chewing up memory when you only need to see the
-first few bytes of a potentially large marker.  If you want to save all the
-data, set length_limit to 0xFFFF; that is enough since marker lengths are only
-16 bits.  As a special case, setting length_limit to 0 prevents that marker
-type from being saved at all.  (That is the default behavior, in fact.)
-
-After jpeg_read_header() completes, you can examine the special markers by
-following the cinfo->marker_list pointer chain.  All the special markers in
-the file appear in this list, in order of their occurrence in the file (but
-omitting any markers of types you didn't ask for).  Both the original data
-length and the saved data length are recorded for each list entry; the latter
-will not exceed length_limit for the particular marker type.  Note that these
-lengths exclude the marker length word, whereas the stored representation
-within the JPEG file includes it.  (Hence the maximum data length is really
-only 65533.)
-
-It is possible that additional special markers appear in the file beyond the
-SOS marker at which jpeg_read_header stops; if so, the marker list will be
-extended during reading of the rest of the file.  This is not expected to be
-common, however.  If you are short on memory you may want to reset the length
-limit to zero for all marker types after finishing jpeg_read_header, to
-ensure that the max_memory_to_use setting cannot be exceeded due to addition
-of later markers.
-
-The marker list remains stored until you call jpeg_finish_decompress or
-jpeg_abort, at which point the memory is freed and the list is set to empty.
-(jpeg_destroy also releases the storage, of course.)
-
-Note that the library is internally interested in APP0 and APP14 markers;
-if you try to set a small nonzero length limit on these types, the library
-will silently force the length up to the minimum it wants.  (But you can set
-a zero length limit to prevent them from being saved at all.)  Also, in a
-16-bit environment, the maximum length limit may be constrained to less than
-65533 by malloc() limitations.  It is therefore best not to assume that the
-effective length limit is exactly what you set it to be.
-
-
-If you want to supply your own marker-reading routine, you do it by calling
-jpeg_set_marker_processor().  A marker processor routine must have the
-signature
-        boolean jpeg_marker_parser_method (j_decompress_ptr cinfo)
-Although the marker code is not explicitly passed, the routine can find it
-in cinfo->unread_marker.  At the time of call, the marker proper has been
-read from the data source module.  The processor routine is responsible for
-reading the marker length word and the remaining parameter bytes, if any.
-Return TRUE to indicate success.  (FALSE should be returned only if you are
-using a suspending data source and it tells you to suspend.  See the standard
-marker processors in jdmarker.c for appropriate coding methods if you need to
-use a suspending data source.)
-
-If you override the default APP0 or APP14 processors, it is up to you to
-recognize JFIF and Adobe markers if you want colorspace recognition to occur
-properly.  We recommend copying and extending the default processors if you
-want to do that.  (A better idea is to save these marker types for later
-examination by calling jpeg_save_markers(); that method doesn't interfere
-with the library's own processing of these markers.)
-
-jpeg_set_marker_processor() and jpeg_save_markers() are mutually exclusive
---- if you call one it overrides any previous call to the other, for the
-particular marker type specified.
-
-A simple example of an external COM processor can be found in djpeg.c.
-Also, see jpegtran.c for an example of using jpeg_save_markers.
-
-
-Raw (downsampled) image data
-----------------------------
-
-Some applications need to supply already-downsampled image data to the JPEG
-compressor, or to receive raw downsampled data from the decompressor.  The
-library supports this requirement by allowing the application to write or
-read raw data, bypassing the normal preprocessing or postprocessing steps.
-The interface is different from the standard one and is somewhat harder to
-use.  If your interest is merely in bypassing color conversion, we recommend
-that you use the standard interface and simply set jpeg_color_space =
-in_color_space (or jpeg_color_space = out_color_space for decompression).
-The mechanism described in this section is necessary only to supply or
-receive downsampled image data, in which not all components have the same
-dimensions.
-
-
-To compress raw data, you must supply the data in the colorspace to be used
-in the JPEG file (please read the earlier section on Special color spaces)
-and downsampled to the sampling factors specified in the JPEG parameters.
-You must supply the data in the format used internally by the JPEG library,
-namely a JSAMPIMAGE array.  This is an array of pointers to two-dimensional
-arrays, each of type JSAMPARRAY.  Each 2-D array holds the values for one
-color component.  This structure is necessary since the components are of
-different sizes.  If the image dimensions are not a multiple of the MCU size,
-you must also pad the data correctly (usually, this is done by replicating
-the last column and/or row).  The data must be padded to a multiple of a DCT
-block in each component: that is, each downsampled row must contain a
-multiple of 8 valid samples, and there must be a multiple of 8 sample rows
-for each component.  (For applications such as conversion of digital TV
-images, the standard image size is usually a multiple of the DCT block size,
-so that no padding need actually be done.)
-
-The procedure for compression of raw data is basically the same as normal
-compression, except that you call jpeg_write_raw_data() in place of
-jpeg_write_scanlines().  Before calling jpeg_start_compress(), you must do
-the following:
-  * Set cinfo->raw_data_in to TRUE.  (It is set FALSE by jpeg_set_defaults().)
-    This notifies the library that you will be supplying raw data.
-    Furthermore, set cinfo->do_fancy_downsampling to FALSE if you want to use
-    real downsampled data.  (It is set TRUE by jpeg_set_defaults().)
-  * Ensure jpeg_color_space is correct --- an explicit jpeg_set_colorspace()
-    call is a good idea.  Note that since color conversion is bypassed,
-    in_color_space is ignored, except that jpeg_set_defaults() uses it to
-    choose the default jpeg_color_space setting.
-  * Ensure the sampling factors, cinfo->comp_info[i].h_samp_factor and
-    cinfo->comp_info[i].v_samp_factor, are correct.  Since these indicate the
-    dimensions of the data you are supplying, it's wise to set them
-    explicitly, rather than assuming the library's defaults are what you want.
-
-To pass raw data to the library, call jpeg_write_raw_data() in place of
-jpeg_write_scanlines().  The two routines work similarly except that
-jpeg_write_raw_data takes a JSAMPIMAGE data array rather than JSAMPARRAY.
-The scanlines count passed to and returned from jpeg_write_raw_data is
-measured in terms of the component with the largest v_samp_factor.
-
-jpeg_write_raw_data() processes one MCU row per call, which is to say
-v_samp_factor*DCTSIZE sample rows of each component.  The passed num_lines
-value must be at least max_v_samp_factor*DCTSIZE, and the return value will
-be exactly that amount (or possibly some multiple of that amount, in future
-library versions).  This is true even on the last call at the bottom of the
-image; don't forget to pad your data as necessary.
-
-The required dimensions of the supplied data can be computed for each
-component as
-        cinfo->comp_info[i].width_in_blocks*DCTSIZE  samples per row
-        cinfo->comp_info[i].height_in_blocks*DCTSIZE rows in image
-after jpeg_start_compress() has initialized those fields.  If the valid data
-is smaller than this, it must be padded appropriately.  For some sampling
-factors and image sizes, additional dummy DCT blocks are inserted to make
-the image a multiple of the MCU dimensions.  The library creates such dummy
-blocks itself; it does not read them from your supplied data.  Therefore you
-need never pad by more than DCTSIZE samples.  An example may help here.
-Assume 2h2v downsampling of YCbCr data, that is
-        cinfo->comp_info[0].h_samp_factor = 2           for Y
-        cinfo->comp_info[0].v_samp_factor = 2
-        cinfo->comp_info[1].h_samp_factor = 1           for Cb
-        cinfo->comp_info[1].v_samp_factor = 1
-        cinfo->comp_info[2].h_samp_factor = 1           for Cr
-        cinfo->comp_info[2].v_samp_factor = 1
-and suppose that the nominal image dimensions (cinfo->image_width and
-cinfo->image_height) are 101x101 pixels.  Then jpeg_start_compress() will
-compute downsampled_width = 101 and width_in_blocks = 13 for Y,
-downsampled_width = 51 and width_in_blocks = 7 for Cb and Cr (and the same
-for the height fields).  You must pad the Y data to at least 13*8 = 104
-columns and rows, the Cb/Cr data to at least 7*8 = 56 columns and rows.  The
-MCU height is max_v_samp_factor = 2 DCT rows so you must pass at least 16
-scanlines on each call to jpeg_write_raw_data(), which is to say 16 actual
-sample rows of Y and 8 each of Cb and Cr.  A total of 7 MCU rows are needed,
-so you must pass a total of 7*16 = 112 "scanlines".  The last DCT block row
-of Y data is dummy, so it doesn't matter what you pass for it in the data
-arrays, but the scanlines count must total up to 112 so that all of the Cb
-and Cr data gets passed.
-
-Output suspension is supported with raw-data compression: if the data
-destination module suspends, jpeg_write_raw_data() will return 0.
-In this case the same data rows must be passed again on the next call.
-
-
-Decompression with raw data output implies bypassing all postprocessing.
-You must deal with the color space and sampling factors present in the
-incoming file.  If your application only handles, say, 2h1v YCbCr data,
-you must check for and fail on other color spaces or other sampling factors.
-The library will not convert to a different color space for you.
-
-To obtain raw data output, set cinfo->raw_data_out = TRUE before
-jpeg_start_decompress() (it is set FALSE by jpeg_read_header()).  Be sure to
-verify that the color space and sampling factors are ones you can handle.
-Furthermore, set cinfo->do_fancy_upsampling = FALSE if you want to get real
-downsampled data (it is set TRUE by jpeg_read_header()).
-Then call jpeg_read_raw_data() in place of jpeg_read_scanlines().  The
-decompression process is otherwise the same as usual.
-
-jpeg_read_raw_data() returns one MCU row per call, and thus you must pass a
-buffer of at least max_v_samp_factor*DCTSIZE scanlines (scanline counting is
-the same as for raw-data compression).  The buffer you pass must be large
-enough to hold the actual data plus padding to DCT-block boundaries.  As with
-compression, any entirely dummy DCT blocks are not processed so you need not
-allocate space for them, but the total scanline count includes them.  The
-above example of computing buffer dimensions for raw-data compression is
-equally valid for decompression.
-
-Input suspension is supported with raw-data decompression: if the data source
-module suspends, jpeg_read_raw_data() will return 0.  You can also use
-buffered-image mode to read raw data in multiple passes.
-
-
-Really raw data: DCT coefficients
----------------------------------
-
-It is possible to read or write the contents of a JPEG file as raw DCT
-coefficients.  This facility is mainly intended for use in lossless
-transcoding between different JPEG file formats.  Other possible applications
-include lossless cropping of a JPEG image, lossless reassembly of a
-multi-strip or multi-tile TIFF/JPEG file into a single JPEG datastream, etc.
-
-To read the contents of a JPEG file as DCT coefficients, open the file and do
-jpeg_read_header() as usual.  But instead of calling jpeg_start_decompress()
-and jpeg_read_scanlines(), call jpeg_read_coefficients().  This will read the
-entire image into a set of virtual coefficient-block arrays, one array per
-component.  The return value is a pointer to an array of virtual-array
-descriptors.  Each virtual array can be accessed directly using the JPEG
-memory manager's access_virt_barray method (see Memory management, below,
-and also read structure.txt's discussion of virtual array handling).  Or,
-for simple transcoding to a different JPEG file format, the array list can
-just be handed directly to jpeg_write_coefficients().
-
-Each block in the block arrays contains quantized coefficient values in
-normal array order (not JPEG zigzag order).  The block arrays contain only
-DCT blocks containing real data; any entirely-dummy blocks added to fill out
-interleaved MCUs at the right or bottom edges of the image are discarded
-during reading and are not stored in the block arrays.  (The size of each
-block array can be determined from the width_in_blocks and height_in_blocks
-fields of the component's comp_info entry.)  This is also the data format
-expected by jpeg_write_coefficients().
-
-When you are done using the virtual arrays, call jpeg_finish_decompress()
-to release the array storage and return the decompression object to an idle
-state; or just call jpeg_destroy() if you don't need to reuse the object.
-
-If you use a suspending data source, jpeg_read_coefficients() will return
-NULL if it is forced to suspend; a non-NULL return value indicates successful
-completion.  You need not test for a NULL return value when using a
-non-suspending data source.
-
-It is also possible to call jpeg_read_coefficients() to obtain access to the
-decoder's coefficient arrays during a normal decode cycle in buffered-image
-mode.  This frammish might be useful for progressively displaying an incoming
-image and then re-encoding it without loss.  To do this, decode in buffered-
-image mode as discussed previously, then call jpeg_read_coefficients() after
-the last jpeg_finish_output() call.  The arrays will be available for your use
-until you call jpeg_finish_decompress().
-
-
-To write the contents of a JPEG file as DCT coefficients, you must provide
-the DCT coefficients stored in virtual block arrays.  You can either pass
-block arrays read from an input JPEG file by jpeg_read_coefficients(), or
-allocate virtual arrays from the JPEG compression object and fill them
-yourself.  In either case, jpeg_write_coefficients() is substituted for
-jpeg_start_compress() and jpeg_write_scanlines().  Thus the sequence is
-  * Create compression object
-  * Set all compression parameters as necessary
-  * Request virtual arrays if needed
-  * jpeg_write_coefficients()
-  * jpeg_finish_compress()
-  * Destroy or re-use compression object
-jpeg_write_coefficients() is passed a pointer to an array of virtual block
-array descriptors; the number of arrays is equal to cinfo.num_components.
-
-The virtual arrays need only have been requested, not realized, before
-jpeg_write_coefficients() is called.  A side-effect of
-jpeg_write_coefficients() is to realize any virtual arrays that have been
-requested from the compression object's memory manager.  Thus, when obtaining
-the virtual arrays from the compression object, you should fill the arrays
-after calling jpeg_write_coefficients().  The data is actually written out
-when you call jpeg_finish_compress(); jpeg_write_coefficients() only writes
-the file header.
-
-When writing raw DCT coefficients, it is crucial that the JPEG quantization
-tables and sampling factors match the way the data was encoded, or the
-resulting file will be invalid.  For transcoding from an existing JPEG file,
-we recommend using jpeg_copy_critical_parameters().  This routine initializes
-all the compression parameters to default values (like jpeg_set_defaults()),
-then copies the critical information from a source decompression object.
-The decompression object should have just been used to read the entire
-JPEG input file --- that is, it should be awaiting jpeg_finish_decompress().
-
-jpeg_write_coefficients() marks all tables stored in the compression object
-as needing to be written to the output file (thus, it acts like
-jpeg_start_compress(cinfo, TRUE)).  This is for safety's sake, to avoid
-emitting abbreviated JPEG files by accident.  If you really want to emit an
-abbreviated JPEG file, call jpeg_suppress_tables(), or set the tables'
-individual sent_table flags, between calling jpeg_write_coefficients() and
-jpeg_finish_compress().
-
-
-Progress monitoring
--------------------
-
-Some applications may need to regain control from the JPEG library every so
-often.  The typical use of this feature is to produce a percent-done bar or
-other progress display.  (For a simple example, see cjpeg.c or djpeg.c.)
-Although you do get control back frequently during the data-transferring pass
-(the jpeg_read_scanlines or jpeg_write_scanlines loop), any additional passes
-will occur inside jpeg_finish_compress or jpeg_start_decompress; those
-routines may take a long time to execute, and you don't get control back
-until they are done.
-
-You can define a progress-monitor routine which will be called periodically
-by the library.  No guarantees are made about how often this call will occur,
-so we don't recommend you use it for mouse tracking or anything like that.
-At present, a call will occur once per MCU row, scanline, or sample row
-group, whichever unit is convenient for the current processing mode; so the
-wider the image, the longer the time between calls.  During the data
-transferring pass, only one call occurs per call of jpeg_read_scanlines or
-jpeg_write_scanlines, so don't pass a large number of scanlines at once if
-you want fine resolution in the progress count.  (If you really need to use
-the callback mechanism for time-critical tasks like mouse tracking, you could
-insert additional calls inside some of the library's inner loops.)
-
-To establish a progress-monitor callback, create a struct jpeg_progress_mgr,
-fill in its progress_monitor field with a pointer to your callback routine,
-and set cinfo->progress to point to the struct.  The callback will be called
-whenever cinfo->progress is non-NULL.  (This pointer is set to NULL by
-jpeg_create_compress or jpeg_create_decompress; the library will not change
-it thereafter.  So if you allocate dynamic storage for the progress struct,
-make sure it will live as long as the JPEG object does.  Allocating from the
-JPEG memory manager with lifetime JPOOL_PERMANENT will work nicely.)  You
-can use the same callback routine for both compression and decompression.
-
-The jpeg_progress_mgr struct contains four fields which are set by the library:
-        long pass_counter;      /* work units completed in this pass */
-        long pass_limit;        /* total number of work units in this pass */
-        int completed_passes;   /* passes completed so far */
-        int total_passes;       /* total number of passes expected */
-During any one pass, pass_counter increases from 0 up to (not including)
-pass_limit; the step size is usually but not necessarily 1.  The pass_limit
-value may change from one pass to another.  The expected total number of
-passes is in total_passes, and the number of passes already completed is in
-completed_passes.  Thus the fraction of work completed may be estimated as
-                completed_passes + (pass_counter/pass_limit)
-                --------------------------------------------
-                                total_passes
-ignoring the fact that the passes may not be equal amounts of work.
-
-When decompressing, pass_limit can even change within a pass, because it
-depends on the number of scans in the JPEG file, which isn't always known in
-advance.  The computed fraction-of-work-done may jump suddenly (if the library
-discovers it has overestimated the number of scans) or even decrease (in the
-opposite case).  It is not wise to put great faith in the work estimate.
-
-When using the decompressor's buffered-image mode, the progress monitor work
-estimate is likely to be completely unhelpful, because the library has no way
-to know how many output passes will be demanded of it.  Currently, the library
-sets total_passes based on the assumption that there will be one more output
-pass if the input file end hasn't yet been read (jpeg_input_complete() isn't
-TRUE), but no more output passes if the file end has been reached when the
-output pass is started.  This means that total_passes will rise as additional
-output passes are requested.  If you have a way of determining the input file
-size, estimating progress based on the fraction of the file that's been read
-will probably be more useful than using the library's value.
-
-
-Memory management
------------------
-
-This section covers some key facts about the JPEG library's built-in memory
-manager.  For more info, please read structure.txt's section about the memory
-manager, and consult the source code if necessary.
-
-All memory and temporary file allocation within the library is done via the
-memory manager.  If necessary, you can replace the "back end" of the memory
-manager to control allocation yourself (for example, if you don't want the
-library to use malloc() and free() for some reason).
-
-Some data is allocated "permanently" and will not be freed until the JPEG
-object is destroyed.  Most data is allocated "per image" and is freed by
-jpeg_finish_compress, jpeg_finish_decompress, or jpeg_abort.  You can call the
-memory manager yourself to allocate structures that will automatically be
-freed at these times.  Typical code for this is
-  ptr = (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, size);
-Use JPOOL_PERMANENT to get storage that lasts as long as the JPEG object.
-Use alloc_large instead of alloc_small for anything bigger than a few Kbytes.
-There are also alloc_sarray and alloc_barray routines that automatically
-build 2-D sample or block arrays.
-
-The library's minimum space requirements to process an image depend on the
-image's width, but not on its height, because the library ordinarily works
-with "strip" buffers that are as wide as the image but just a few rows high.
-Some operating modes (eg, two-pass color quantization) require full-image
-buffers.  Such buffers are treated as "virtual arrays": only the current strip
-need be in memory, and the rest can be swapped out to a temporary file.
-
-If you use the simplest memory manager back end (jmemnobs.c), then no
-temporary files are used; virtual arrays are simply malloc()'d.  Images bigger
-than memory can be processed only if your system supports virtual memory.
-The other memory manager back ends support temporary files of various flavors
-and thus work in machines without virtual memory.  They may also be useful on
-Unix machines if you need to process images that exceed available swap space.
-
-When using temporary files, the library will make the in-memory buffers for
-its virtual arrays just big enough to stay within a "maximum memory" setting.
-Your application can set this limit by setting cinfo->mem->max_memory_to_use
-after creating the JPEG object.  (Of course, there is still a minimum size for
-the buffers, so the max-memory setting is effective only if it is bigger than
-the minimum space needed.)  If you allocate any large structures yourself, you
-must allocate them before jpeg_start_compress() or jpeg_start_decompress() in
-order to have them counted against the max memory limit.  Also keep in mind
-that space allocated with alloc_small() is ignored, on the assumption that
-it's too small to be worth worrying about; so a reasonable safety margin
-should be left when setting max_memory_to_use.
-
-If you use the jmemname.c or jmemdos.c memory manager back end, it is
-important to clean up the JPEG object properly to ensure that the temporary
-files get deleted.  (This is especially crucial with jmemdos.c, where the
-"temporary files" may be extended-memory segments; if they are not freed,
-DOS will require a reboot to recover the memory.)  Thus, with these memory
-managers, it's a good idea to provide a signal handler that will trap any
-early exit from your program.  The handler should call either jpeg_abort()
-or jpeg_destroy() for any active JPEG objects.  A handler is not needed with
-jmemnobs.c, and shouldn't be necessary with jmemansi.c or jmemmac.c either,
-since the C library is supposed to take care of deleting files made with
-tmpfile().
-
-
-Memory usage
-------------
-
-Working memory requirements while performing compression or decompression
-depend on image dimensions, image characteristics (such as colorspace and
-JPEG process), and operating mode (application-selected options).
-
-As of v6b, the decompressor requires:
- 1. About 24K in more-or-less-fixed-size data.  This varies a bit depending
-    on operating mode and image characteristics (particularly color vs.
-    grayscale), but it doesn't depend on image dimensions.
- 2. Strip buffers (of size proportional to the image width) for IDCT and
-    upsampling results.  The worst case for commonly used sampling factors
-    is about 34 bytes * width in pixels for a color image.  A grayscale image
-    only needs about 8 bytes per pixel column.
- 3. A full-image DCT coefficient buffer is needed to decode a multi-scan JPEG
-    file (including progressive JPEGs), or whenever you select buffered-image
-    mode.  This takes 2 bytes/coefficient.  At typical 2x2 sampling, that's
-    3 bytes per pixel for a color image.  Worst case (1x1 sampling) requires
-    6 bytes/pixel.  For grayscale, figure 2 bytes/pixel.
- 4. To perform 2-pass color quantization, the decompressor also needs a
-    128K color lookup table and a full-image pixel buffer (3 bytes/pixel).
-This does not count any memory allocated by the application, such as a
-buffer to hold the final output image.
-
-The above figures are valid for 8-bit JPEG data precision and a machine with
-32-bit ints.  For 12-bit JPEG data, double the size of the strip buffers and
-quantization pixel buffer.  The "fixed-size" data will be somewhat smaller
-with 16-bit ints, larger with 64-bit ints.  Also, CMYK or other unusual
-color spaces will require different amounts of space.
-
-The full-image coefficient and pixel buffers, if needed at all, do not
-have to be fully RAM resident; you can have the library use temporary
-files instead when the total memory usage would exceed a limit you set.
-(But if your OS supports virtual memory, it's probably better to just use
-jmemnobs and let the OS do the swapping.)
-
-The compressor's memory requirements are similar, except that it has no need
-for color quantization.  Also, it needs a full-image DCT coefficient buffer
-if Huffman-table optimization is asked for, even if progressive mode is not
-requested.
-
-If you need more detailed information about memory usage in a particular
-situation, you can enable the MEM_STATS code in jmemmgr.c.
-
-
-Library compile-time options
-----------------------------
-
-A number of compile-time options are available by modifying jmorecfg.h.
-
-The JPEG standard provides for both the baseline 8-bit DCT process and
-a 12-bit DCT process.  The IJG code supports 12-bit JPEG if you define
-BITS_IN_JSAMPLE as 12 rather than 8.  Note that this causes JSAMPLE to be
-larger than a char, so it affects the surrounding application's image data.
-The sample applications cjpeg and djpeg can support 12-bit mode only for PPM
-and GIF file formats; you must disable the other file formats to compile a
-12-bit cjpeg or djpeg.  (install.txt has more information about that.)
-At present, a 12-bit library can handle *only* 12-bit images, not both
-precisions.  (If you need to include both 8- and 12-bit libraries in a single
-application, you could probably do it by defining NEED_SHORT_EXTERNAL_NAMES
-for just one of the copies.  You'd have to access the 8-bit and 12-bit copies
-from separate application source files.  This is untested ... if you try it,
-we'd like to hear whether it works!)
-
-Note that a 12-bit library always compresses in Huffman optimization mode,
-in order to generate valid Huffman tables.  This is necessary because our
-default Huffman tables only cover 8-bit data.  If you need to output 12-bit
-files in one pass, you'll have to supply suitable default Huffman tables.
-You may also want to supply your own DCT quantization tables; the existing
-quality-scaling code has been developed for 8-bit use, and probably doesn't
-generate especially good tables for 12-bit.
-
-The maximum number of components (color channels) in the image is determined
-by MAX_COMPONENTS.  The JPEG standard allows up to 255 components, but we
-expect that few applications will need more than four or so.
-
-On machines with unusual data type sizes, you may be able to improve
-performance or reduce memory space by tweaking the various typedefs in
-jmorecfg.h.  In particular, on some RISC CPUs, access to arrays of "short"s
-is quite slow; consider trading memory for speed by making JCOEF, INT16, and
-UINT16 be "int" or "unsigned int".  UINT8 is also a candidate to become int.
-You probably don't want to make JSAMPLE be int unless you have lots of memory
-to burn.
-
-You can reduce the size of the library by compiling out various optional
-functions.  To do this, undefine xxx_SUPPORTED symbols as necessary.
-
-You can also save a few K by not having text error messages in the library;
-the standard error message table occupies about 5Kb.  This is particularly
-reasonable for embedded applications where there's no good way to display 
-a message anyway.  To do this, remove the creation of the message table
-(jpeg_std_message_table[]) from jerror.c, and alter format_message to do
-something reasonable without it.  You could output the numeric value of the
-message code number, for example.  If you do this, you can also save a couple
-more K by modifying the TRACEMSn() macros in jerror.h to expand to nothing;
-you don't need trace capability anyway, right?
-
-
-Portability considerations
---------------------------
-
-The JPEG library has been written to be extremely portable; the sample
-applications cjpeg and djpeg are slightly less so.  This section summarizes
-the design goals in this area.  (If you encounter any bugs that cause the
-library to be less portable than is claimed here, we'd appreciate hearing
-about them.)
-
-The code works fine on ANSI C, C++, and pre-ANSI C compilers, using any of
-the popular system include file setups, and some not-so-popular ones too.
-See install.txt for configuration procedures.
-
-The code is not dependent on the exact sizes of the C data types.  As
-distributed, we make the assumptions that
-        char    is at least 8 bits wide
-        short   is at least 16 bits wide
-        int     is at least 16 bits wide
-        long    is at least 32 bits wide
-(These are the minimum requirements of the ANSI C standard.)  Wider types will
-work fine, although memory may be used inefficiently if char is much larger
-than 8 bits or short is much bigger than 16 bits.  The code should work
-equally well with 16- or 32-bit ints.
-
-In a system where these assumptions are not met, you may be able to make the
-code work by modifying the typedefs in jmorecfg.h.  However, you will probably
-have difficulty if int is less than 16 bits wide, since references to plain
-int abound in the code.
-
-char can be either signed or unsigned, although the code runs faster if an
-unsigned char type is available.  If char is wider than 8 bits, you will need
-to redefine JOCTET and/or provide custom data source/destination managers so
-that JOCTET represents exactly 8 bits of data on external storage.
-
-The JPEG library proper does not assume ASCII representation of characters.
-But some of the image file I/O modules in cjpeg/djpeg do have ASCII
-dependencies in file-header manipulation; so does cjpeg's select_file_type()
-routine.
-
-The JPEG library does not rely heavily on the C library.  In particular, C
-stdio is used only by the data source/destination modules and the error
-handler, all of which are application-replaceable.  (cjpeg/djpeg are more
-heavily dependent on stdio.)  malloc and free are called only from the memory
-manager "back end" module, so you can use a different memory allocator by
-replacing that one file.
-
-The code generally assumes that C names must be unique in the first 15
-characters.  However, global function names can be made unique in the
-first 6 characters by defining NEED_SHORT_EXTERNAL_NAMES.
-
-More info about porting the code may be gleaned by reading jconfig.txt,
-jmorecfg.h, and jinclude.h.
-
-
-Notes for MS-DOS implementors
------------------------------
-
-The IJG code is designed to work efficiently in 80x86 "small" or "medium"
-memory models (i.e., data pointers are 16 bits unless explicitly declared
-"far"; code pointers can be either size).  You may be able to use small
-model to compile cjpeg or djpeg by itself, but you will probably have to use
-medium model for any larger application.  This won't make much difference in
-performance.  You *will* take a noticeable performance hit if you use a
-large-data memory model (perhaps 10%-25%), and you should avoid "huge" model
-if at all possible.
-
-The JPEG library typically needs 2Kb-3Kb of stack space.  It will also
-malloc about 20K-30K of near heap space while executing (and lots of far
-heap, but that doesn't count in this calculation).  This figure will vary
-depending on selected operating mode, and to a lesser extent on image size.
-There is also about 5Kb-6Kb of constant data which will be allocated in the
-near data segment (about 4Kb of this is the error message table).
-Thus you have perhaps 20K available for other modules' static data and near
-heap space before you need to go to a larger memory model.  The C library's
-static data will account for several K of this, but that still leaves a good
-deal for your needs.  (If you are tight on space, you could reduce the sizes
-of the I/O buffers allocated by jdatasrc.c and jdatadst.c, say from 4K to
-1K.  Another possibility is to move the error message table to far memory;
-this should be doable with only localized hacking on jerror.c.)
-
-About 2K of the near heap space is "permanent" memory that will not be
-released until you destroy the JPEG object.  This is only an issue if you
-save a JPEG object between compression or decompression operations.
-
-Far data space may also be a tight resource when you are dealing with large
-images.  The most memory-intensive case is decompression with two-pass color
-quantization, or single-pass quantization to an externally supplied color
-map.  This requires a 128Kb color lookup table plus strip buffers amounting
-to about 40 bytes per column for typical sampling ratios (eg, about 25600
-bytes for a 640-pixel-wide image).  You may not be able to process wide
-images if you have large data structures of your own.
-
-Of course, all of these concerns vanish if you use a 32-bit flat-memory-model
-compiler, such as DJGPP or Watcom C.  We highly recommend flat model if you
-can use it; the JPEG library is significantly faster in flat model.