--- /dev/null
+USING THE IJG JPEG LIBRARY
+
+Copyright (C) 1994-1998, Thomas G. Lane.
+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
+ * Arithmetic entropy coding (unsupported for legal reasons)
+ * 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.doc).
+
+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_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:
+
+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.
+
+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".
+
+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.
+
+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.
+
+There are some additional cinfo fields which are not documented here
+because you currently can't change them; for example, you can't set
+arith_code TRUE because arithmetic coding is unsupported.
+
+
+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. Default is
+ 1/1, or no scaling. Currently, the only supported scaling ratios
+ are 1/1, 1/2, 1/4, and 1/8. (The library design allows for arbitrary
+ scaling ratios but this is not likely to be implemented any time soon.)
+ 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, do careful upsampling of chroma components. If FALSE,
+ a faster but sloppier method is used. Default is TRUE. The visual
+ impact of the sloppier method is often very small.
+
+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 => GRAYSCALE
+ YCbCr => RGB
+ 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
+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 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, if you desired to
+decompress a JPEG datastream that was all in memory, you could just make the
+buffer pointer and length point to the original data in memory. Then the
+buffer-reload procedure would 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() routine of the supplied destination manager.
+
+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() routine of the supplied source manager.
+
+For more information, consult the 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.
+ * 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 cannot ask for rescaling or color quantization, for instance. More
+seriously, 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.
+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.doc'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.doc'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 lossy 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.doc 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.doc 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.doc,
+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.
projects / synfig.git / blobdiff