Aminet 5/Aminet 5 - March 1995.iso/Aminet/gfx/conv/jpegV5Asrc.lha/jpeg-5a/libjpeg.doc

Text File | 1994-11-11 | 111KB | 2,169 lines

USING THE IJG JPEG LIBRARY

Copyright (C) 1994, 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
    Abbreviated datastreams and multiple images
    Special markers
    Raw (downsampled) image data
    Progress monitoring
    Memory management
    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 and extended-sequential JPEG processes
are supported.  (Our subset includes all features now in common use.)
Unsupported ISO options include:
    * Progressive storage (may be supported in future versions)
    * 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, we anticipate that
Sam Leffler's LIBTIFF library will use this code to support the revised TIFF
JPEG format.)


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.  However,
there are some restrictions on the processing that can be done in this mode.
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" 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).

Note that there is no provision for colormapped input.  You can feed in a
colormapped image by expanding it to full-color format.  However JPEG often
doesn't work very well with colormapped source data, 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 serve this purpose.  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 data is supported.  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" (plus 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.

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.  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, pass a j_common_ptr to jpeg_destroy().)

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
  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.


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 adjust 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 (fully interleaved) 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.

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 won't return more than cinfo.rec_outbuf_height
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 compression, 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).

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 JPEG zigzag 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).


Compression parameters (cinfo fields) include:

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.

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.

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 floating-point method is the most accurate, but may give slightly
    different results on different machines due to varying roundoff
    behavior.  The integer methods should give the same results on all
    machines.  On machines with sufficiently fast FP hardware, the
    floating-point method may also be the fastest.  The IFAST method is
    considerably less accurate than the other two; its use is not
    recommended if high quality is a concern.  JDCT_DEFAULT and
    JDCT_FASTEST are macros configurable by each installation.

unsigned int 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).

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 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 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[].


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 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.

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.


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.


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
    YCCK => CMYK
as well as the null transforms.

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.

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)
    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 flushed 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 a limited solution: it works only in the simplest
operating modes (namely single-pass processing of single-scan JPEG files), and
it has several other restrictions which are documented below.  Furthermore,
nothing is guaranteed about the maximum amount of time spent in any one call
to the library, so a single-threaded application may still have response-time
problems.  If you need multi-pass processing or 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 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 dump out the entire buffer, 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 JPEG compressor does not support 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.
  * 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().
Furthermore, since jpeg_finish_compress() cannot suspend, you cannot request
multi-pass operating modes such as Huffman code optimization or multiple-scan
output.  That would imply that a large amount of data would be written inside
jpeg_finish_compress(), which would certainly trigger a buffer overrun.

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 three places:
  * jpeg_read_header(): will return JPEG_SUSPENDED.
  * 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(),
adjust the passed pointers to reflect any scanlines successfully read.

Just as with compression, the decompressor will typically backtrack to a
convenient restart point before suspending.  The data beyond the current
position of next_input_byte/bytes_in_buffer must NOT be discarded; it will
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 will return FALSE, which will cause a
suspension return.  The surrounding application must then arrange to discard
the right 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 support suspension within jpeg_start_decompress().
This means that you cannot use suspension with any multi-pass processing mode
(eg, two-pass color quantization or multiple-scan JPEG files).  In single-pass
modes, jpeg_start_decompress() reads no data and thus need never suspend.

The decompressor does not attempt to suspend within any JPEG marker; it will
backtrack to the start of the marker.  Hence the input buffer must be large
enough to hold the longest marker in the file.  We recommend at least a 2K
buffer.  The buffer would need to be 64K to allow for arbitrary COM or APPn
markers, but the decompressor does not actually try to read these; it just
skips them by calling skip_input_data().  If you provide a special marker
handling routine that does look at such markers, coping with buffer overflow
is your problem.  Ordinary JPEG markers should normally not exceed a few
hundred bytes each (DHT tables are typically the longest).  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.

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.  Do not discard "completed" buffers in
the empty_output_buffer() or fill_input_buffer() routine, unless you can tell
from the saved pointer/bytecount that the JPEG library will no longer attempt
to backtrack that far.  It's probably simplest to postpone releasing any
buffers until the library returns to its caller; then you can use the final
bytecount to tell how much data has been fully processed, and release buffers
on that basis.


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.


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.  More commonly
you'd want to read the tables from a tables-only file.  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 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.

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 JPEG 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.  Write 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));
Or if you prefer to synthesize the marker byte sequence yourself, you can
just cram it straight into the data destination module.

For decompression, you can supply your own routine to process COM or APPn
markers by calling jpeg_set_marker_processor().  Usually you'd call this
after creating a decompression object and before calling jpeg_read_header(),
because the markers of interest will normally be scanned by jpeg_read_header.
Once you've supplied a routine, it will be used for the life of that
decompression object.  A separate routine may be registered for COM and for
each APPn marker code.

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 simple example of an external COM processor can be found in djpeg.c.


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.

Currently, output suspension is not supported with raw data output: an error
will result if the data destination module tries to suspend.


Decompression with raw data output implies bypassing all postprocessing:
you cannot ask for 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.

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.


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.)

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 not necessarily 1.  Both the step size and the
limit may differ 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, the total_passes value is not trustworthy, because it
depends on the number of scans in the JPEG file, which isn't always known in
advance.  In the current implementation, completed_passes may jump by more
than one when dealing with a multiple-scan input file.  About all that is
really safe to assume is that when completed_passes = total_passes - 1, the
current pass will be the last one.

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.


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.

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 either, since the C
library is supposed to take care of deleting files made with tmpfile().


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.  12-bit lossy JPEG is supported 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.  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!)

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.


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 both ANSI 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.)

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 50 bytes per column for typical sampling ratios (eg, about 32000
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.