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Edit File: libjpeg.txt

USING THE IJG JPEG LIBRARY

This file was part of the Independent JPEG Group's software:
Copyright (C) 1994-2013, Thomas G. Lane, Guido Vollbeding.
libjpeg-turbo Modifications:
Copyright (C) 2010, 2014-2018, 2020, D. R. Commander.
Copyright (C) 2015, Google, Inc.
For conditions of distribution and use, see the accompanying README.ijg 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.txt 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
        Partial image decompression
        Mechanics of usage: include files, linking, etc
Advanced features:
        Compression parameter selection
        Decompression parameter selection
        Special color spaces
        Error handling
        Compressed data handling (source and destination managers)
        I/O suspension
        Progressive JPEG support
        Buffered-image mode
        Abbreviated datastreams and multiple images
        Special markers
        ICC profiles
        Raw (downsampled) image data
        Really raw data: DCT coefficients
        Progress monitoring
        Memory management
        Memory usage
        Library compile-time options
        Portability considerations

You should read at least the overview and basic usage sections before trying
to program with the library.  The sections on advanced features can be read
if and when you need them.

OVERVIEW
========

Functions provided by the library
---------------------------------

The IJG JPEG library provides C code to read and write JPEG-compressed image
files.  The surrounding application program receives or supplies image data a
scanline at a time, using a straightforward uncompressed image format.  All
details of color conversion and other preprocessing/postprocessing can be
handled by the library.

The library includes a substantial amount of code that is not covered by the
JPEG standard but is necessary for typical applications of JPEG.  These
functions preprocess the image before JPEG compression or postprocess it after
decompression.  They include colorspace conversion, downsampling/upsampling,
and color quantization.  The application indirectly selects use of this code
by specifying the format in which it wishes to supply or receive image data.
For example, if colormapped output is requested, then the decompression
library automatically invokes color quantization.

A wide range of quality vs. speed tradeoffs are possible in JPEG processing,
and even more so in decompression postprocessing.  The decompression library
provides multiple implementations that cover most of the useful tradeoffs,
ranging from very-high-quality down to fast-preview operation.  On the
compression side we have generally not provided low-quality choices, since
compression is normally less time-critical.  It should be understood that the
low-quality modes may not meet the JPEG standard's accuracy requirements;
nonetheless, they are useful for viewers.

A word about functions *not* provided by the library.  We handle a subset of
the ISO JPEG standard; most baseline, extended-sequential, and progressive
JPEG processes are supported.  (Our subset includes all features now in common
use.)  Unsupported ISO options include:
        * Hierarchical storage
        * Lossless JPEG
        * DNL marker
        * Nonintegral subsampling ratios
We support both 8- and 12-bit data precision, but this is a compile-time
choice rather than a run-time choice; hence it is difficult to use both
precisions in a single application.

By itself, the library handles only interchange JPEG datastreams --- in
particular the widely used JFIF file format.  The library can be used by
surrounding code to process interchange or abbreviated JPEG datastreams that
are embedded in more complex file formats.  (For example, this library is
used by the free LIBTIFF library to support JPEG compression in TIFF.)

Outline of typical usage
------------------------

The rough outline of a JPEG compression operation is:

Allocate and initialize a JPEG compression object
        Specify the destination for the compressed data (eg, a file)
        Set parameters for compression, including image size & colorspace
        jpeg_start_compress(...);
        while (scan lines remain to be written)
                jpeg_write_scanlines(...);
        jpeg_finish_compress(...);
        Release the JPEG compression object

A JPEG compression object holds parameters and working state for the JPEG
library.  We make creation/destruction of the object separate from starting
or finishing compression of an image; the same object can be re-used for a
series of image compression operations.  This makes it easy to re-use the
same parameter settings for a sequence of images.  Re-use of a JPEG object
also has important implications for processing abbreviated JPEG datastreams,
as discussed later.

The image data to be compressed is supplied to jpeg_write_scanlines() from
in-memory buffers.  If the application is doing file-to-file compression,
reading image data from the source file is the application's responsibility.
The library emits compressed data by calling a "data destination manager",
which typically will write the data into a file; but the application can
provide its own destination manager to do something else.

Similarly, the rough outline of a JPEG decompression operation is:

Allocate and initialize a JPEG decompression object
        Specify the source of the compressed data (eg, a file)
        Call jpeg_read_header() to obtain image info
        Set parameters for decompression
        jpeg_start_decompress(...);
        while (scan lines remain to be read)
                jpeg_read_scanlines(...);
        jpeg_finish_decompress(...);
        Release the JPEG decompression object

This is comparable to the compression outline except that reading the
datastream header is a separate step.  This is helpful because information
about the image's size, colorspace, etc is available when the application
selects decompression parameters.  For example, the application can choose an
output scaling ratio that will fit the image into the available screen size.

The decompression library obtains compressed data by calling a data source
manager, which typically will read the data from a file; but other behaviors
can be obtained with a custom source manager.  Decompressed data is delivered
into in-memory buffers passed to jpeg_read_scanlines().

It is possible to abort an incomplete compression or decompression operation
by calling jpeg_abort(); or, if you do not need to retain the JPEG object,
simply release it by calling jpeg_destroy().

JPEG compression and decompression objects are two separate struct types.
However, they share some common fields, and certain routines such as
jpeg_destroy() can work on either type of object.

The JPEG library has no static variables: all state is in the compression
or decompression object.  Therefore it is possible to process multiple
compression and decompression operations concurrently, using multiple JPEG
objects.

Both compression and decompression can be done in an incremental memory-to-
memory fashion, if suitable source/destination managers are used.  See the
section on "I/O suspension" for more details.

BASIC LIBRARY USAGE
===================

Data formats
------------

Before diving into procedural details, it is helpful to understand the
image data format that the JPEG library expects or returns.

The standard input image format is a rectangular array of pixels, with each
pixel having the same number of "component" or "sample" values (color
channels).  You must specify how many components there are and the colorspace
interpretation of the components.  Most applications will use RGB data
(three components per pixel) or grayscale data (one component per pixel).
PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE.
A remarkable number of people manage to miss this, only to find that their
programs don't work with grayscale JPEG files.

There is no provision for colormapped input.  JPEG files are always full-color
or full grayscale (or sometimes another colorspace such as CMYK).  You can
feed in a colormapped image by expanding it to full-color format.  However
JPEG often doesn't work very well with source data that has been colormapped,
because of dithering noise.  This is discussed in more detail in the JPEG FAQ
and the other references mentioned in the README.ijg file.

Pixels are stored by scanlines, with each scanline running from left to
right.  The component values for each pixel are adjacent in the row; for
example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color.  Each scanline is an
array of data type JSAMPLE --- which is typically "unsigned char", unless
you've changed jmorecfg.h.  (You can also change the RGB pixel layout, say
to B,G,R order, by modifying jmorecfg.h.  But see the restrictions listed in
that file before doing so.)

A 2-D array of pixels is formed by making a list of pointers to the starts of
scanlines; so the scanlines need not be physically adjacent in memory.  Even
if you process just one scanline at a time, you must make a one-element
pointer array to conform to this structure.  Pointers to JSAMPLE rows are of
type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY.

The library accepts or supplies one or more complete scanlines per call.
It is not possible to process part of a row at a time.  Scanlines are always
processed top-to-bottom.  You can process an entire image in one call if you
have it all in memory, but usually it's simplest to process one scanline at
a time.

For best results, source data values should have the precision specified by
BITS_IN_JSAMPLE (normally 8 bits).  For instance, if you choose to compress
data that's only 6 bits/channel, you should left-justify each value in a
byte before passing it to the compressor.  If you need to compress data
that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 12.
(See "Library compile-time options", later.)

The data format returned by the decompressor is the same in all details,
except that colormapped output is supported.  (Again, a JPEG file is never
colormapped.  But you can ask the decompressor to perform on-the-fly color
quantization to deliver colormapped output.)  If you request colormapped
output then the returned data array contains a single JSAMPLE per pixel;
its value is an index into a color map.  The color map is represented as
a 2-D JSAMPARRAY in which each row holds the values of one color component,
that is, colormap[i][j] is the value of the i'th color component for pixel
value (map index) j.  Note that since the colormap indexes are stored in
JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE
(ie, at most 256 colors for an 8-bit JPEG library).

Compression details
-------------------

Here we revisit the JPEG compression outline given in the overview.

1. Allocate and initialize a JPEG compression object.

A JPEG compression object is a "struct jpeg_compress_struct".  (It also has
a bunch of subsidiary structures which are allocated via malloc(), but the
application doesn't control those directly.)  This struct can be just a local
variable in the calling routine, if a single routine is going to execute the
whole JPEG compression sequence.  Otherwise it can be static or allocated
from malloc().

You will also need a structure representing a JPEG error handler.  The part
of this that the library cares about is a "struct jpeg_error_mgr".  If you
are providing your own error handler, you'll typically want to embed the
jpeg_error_mgr struct in a larger structure; this is discussed later under
"Error handling".  For now we'll assume you are just using the default error
handler.  The default error handler will print JPEG error/warning messages
on stderr, and it will call exit() if a fatal error occurs.

You must initialize the error handler structure, store a pointer to it into
the JPEG object's "err" field, and then call jpeg_create_compress() to
initialize the rest of the JPEG object.

Typical code for this step, if you are using the default error handler, is

struct jpeg_compress_struct cinfo;
        struct jpeg_error_mgr jerr;
        ...
        cinfo.err = jpeg_std_error(&jerr);
        jpeg_create_compress(&cinfo);

jpeg_create_compress allocates a small amount of memory, so it could fail
if you are out of memory.  In that case it will exit via the error handler;
that's why the error handler must be initialized first.

2. Specify the destination for the compressed data (eg, a file).

As previously mentioned, the JPEG library delivers compressed data to a
"data destination" module.  The library includes one data destination
module which knows how to write to a stdio stream.  You can use your own
destination module if you want to do something else, as discussed later.

If you use the standard destination module, you must open the target stdio
stream beforehand.  Typical code for this step looks like:

FILE *outfile;
        ...
        if ((outfile = fopen(filename, "wb")) == NULL) {
            fprintf(stderr, "can't open %s\n", filename);
            exit(1);
        }
        jpeg_stdio_dest(&cinfo, outfile);

where the last line invokes the standard destination module.

WARNING: it is critical that the binary compressed data be delivered to the
output file unchanged.  On non-Unix systems the stdio library may perform
newline translation or otherwise corrupt binary data.  To suppress this
behavior, you may need to use a "b" option to fopen (as shown above), or use
setmode() or another routine to put the stdio stream in binary mode.  See
cjpeg.c and djpeg.c for code that has been found to work on many systems.

You can select the data destination after setting other parameters (step 3),
if that's more convenient.  You may not change the destination between
calling jpeg_start_compress() and jpeg_finish_compress().

3. Set parameters for compression, including image size & colorspace.

You must supply information about the source image by setting the following
fields in the JPEG object (cinfo structure):

image_width             Width of image, in pixels
        image_height            Height of image, in pixels
        input_components        Number of color channels (samples per pixel)
        in_color_space          Color space of source image

The image dimensions are, hopefully, obvious.  JPEG supports image dimensions
of 1 to 64K pixels in either direction.  The input color space is typically
RGB or grayscale, and input_components is 3 or 1 accordingly.  (See "Special
color spaces", later, for more info.)  The in_color_space field must be
assigned one of the J_COLOR_SPACE enum constants, typically JCS_RGB or
JCS_GRAYSCALE.

JPEG has a large number of compression parameters that determine how the
image is encoded.  Most applications don't need or want to know about all
these parameters.  You can set all the parameters to reasonable defaults by
calling jpeg_set_defaults(); then, if there are particular values you want
to change, you can do so after that.  The "Compression parameter selection"
section tells about all the parameters.

You must set in_color_space correctly before calling jpeg_set_defaults(),
because the defaults depend on the source image colorspace.  However the
other three source image parameters need not be valid until you call
jpeg_start_compress().  There's no harm in calling jpeg_set_defaults() more
than once, if that happens to be convenient.

Typical code for a 24-bit RGB source image is

cinfo.image_width = Width;      /* image width and height, in pixels */
        cinfo.image_height = Height;
        cinfo.input_components = 3;     /* # of color components per pixel */
        cinfo.in_color_space = JCS_RGB; /* colorspace of input image */

jpeg_set_defaults(&cinfo);
        /* Make optional parameter settings here */

4. jpeg_start_compress(...);

After you have established the data destination and set all the necessary
source image info and other parameters, call jpeg_start_compress() to begin
a compression cycle.  This will initialize internal state, allocate working
storage, and emit the first few bytes of the JPEG datastream header.

Typical code:

jpeg_start_compress(&cinfo, TRUE);

The "TRUE" parameter ensures that a complete JPEG interchange datastream
will be written.  This is appropriate in most cases.  If you think you might
want to use an abbreviated datastream, read the section on abbreviated
datastreams, below.

Once you have called jpeg_start_compress(), you may not alter any JPEG
parameters or other fields of the JPEG object until you have completed
the compression cycle.

5. while (scan lines remain to be written)
        jpeg_write_scanlines(...);

Now write all the required image data by calling jpeg_write_scanlines()
one or more times.  You can pass one or more scanlines in each call, up
to the total image height.  In most applications it is convenient to pass
just one or a few scanlines at a time.  The expected format for the passed
data is discussed under "Data formats", above.

Image data should be written in top-to-bottom scanline order.
Rec. ITU-T T.81 | ISO/IEC 10918-1 says, "Applications determine which edges of
a source image are defined as top, bottom, left, and right."  However, if you
want your files to be compatible with everyone else's, then top-to-bottom order
must be used.  If the source data must be read in bottom-to-top order, then 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.txt shows the following code for the case of a full-size 2-D source
array containing 3-byte RGB pixels:

JSAMPROW row_pointer[1];        /* pointer to a single row */
        int row_stride;                 /* physical row width in buffer */

row_stride = image_width * 3;   /* JSAMPLEs per row in image_buffer */

while (cinfo.next_scanline < cinfo.image_height) {
            row_pointer[0] = &image_buffer[cinfo.next_scanline * row_stride];
            jpeg_write_scanlines(&cinfo, row_pointer, 1);
        }

jpeg_write_scanlines() returns the number of scanlines actually written.
This will normally be equal to the number passed in, so you can usually
ignore the return value.  It is different in just two cases:
  * If you try to write more scanlines than the declared image height,
    the additional scanlines are ignored.
  * If you use a suspending data destination manager, output buffer overrun
    will cause the compressor to return before accepting all the passed lines.
    This feature is discussed under "I/O suspension", below.  The normal
    stdio destination manager will NOT cause this to happen.
In any case, the return value is the same as the change in the value of
next_scanline.

6. jpeg_finish_compress(...);

After all the image data has been written, call jpeg_finish_compress() to
complete the compression cycle.  This step is ESSENTIAL to ensure that the
last bufferload of data is written to the data destination.
jpeg_finish_compress() also releases working memory associated with the JPEG
object.

Typical code:

jpeg_finish_compress(&cinfo);

If using the stdio destination manager, don't forget to close the output
stdio stream (if necessary) afterwards.

If you have requested a multi-pass operating mode, such as Huffman code
optimization, jpeg_finish_compress() will perform the additional passes using
data buffered by the first pass.  In this case jpeg_finish_compress() may take
quite a while to complete.  With the default compression parameters, this will
not happen.

It is an error to call jpeg_finish_compress() before writing the necessary
total number of scanlines.  If you wish to abort compression, call
jpeg_abort() as discussed below.

After completing a compression cycle, you may dispose of the JPEG object
as discussed next, or you may use it to compress another image.  In that case
return to step 2, 3, or 4 as appropriate.  If you do not change the
destination manager, the new datastream will be written to the same target.
If you do not change any JPEG parameters, the new datastream will be written
with the same parameters as before.  Note that you can change the input image
dimensions freely between cycles, but if you change the input colorspace, you
should call jpeg_set_defaults() to adjust for the new colorspace; and then
you'll need to repeat all of step 3.

7. Release the JPEG compression object.

When you are done with a JPEG compression object, destroy it by calling
jpeg_destroy_compress().  This will free all subsidiary memory (regardless of
the previous state of the object).  Or you can call jpeg_destroy(), which
works for either compression or decompression objects --- this may be more
convenient if you are sharing code between compression and decompression
cases.  (Actually, these routines are equivalent except for the declared type
of the passed pointer.  To avoid gripes from ANSI C compilers, jpeg_destroy()
should be passed a j_common_ptr.)

If you allocated the jpeg_compress_struct structure from malloc(), freeing
it is your responsibility --- jpeg_destroy() won't.  Ditto for the error
handler structure.

Typical code:

jpeg_destroy_compress(&cinfo);

8. Aborting.

If you decide to abort a compression cycle before finishing, you can clean up
in either of two ways:

* If you don't need the JPEG object any more, just call
  jpeg_destroy_compress() or jpeg_destroy() to release memory.  This is
  legitimate at any point after calling jpeg_create_compress() --- in fact,
  it's safe even if jpeg_create_compress() fails.

* If you want to re-use the JPEG object, call jpeg_abort_compress(), or call
  jpeg_abort() which works on both compression and decompression objects.
  This will return the object to an idle state, releasing any working memory.
  jpeg_abort() is allowed at any time after successful object creation.

Note that cleaning up the data destination, if required, is your
responsibility; neither of these routines will call term_destination().
(See "Compressed data handling", below, for more about that.)

jpeg_destroy() and jpeg_abort() are the only safe calls to make on a JPEG
object that has reported an error by calling error_exit (see "Error handling"
for more info).  The internal state of such an object is likely to be out of
whack.  Either of these two routines will return the object to a known state.

Decompression details
---------------------

Here we revisit the JPEG decompression outline given in the overview.

1. Allocate and initialize a JPEG decompression object.

This is just like initialization for compression, as discussed above,
except that the object is a "struct jpeg_decompress_struct" and you
call jpeg_create_decompress().  Error handling is exactly the same.

Typical code:

struct jpeg_decompress_struct cinfo;
        struct jpeg_error_mgr jerr;
        ...
        cinfo.err = jpeg_std_error(&jerr);
        jpeg_create_decompress(&cinfo);

(Both here and in the IJG code, we usually use variable name "cinfo" for
both compression and decompression objects.)

2. Specify the source of the compressed data (eg, a file).

As previously mentioned, the JPEG library reads compressed data from a "data
source" module.  The library includes one data source module which knows how
to read from a stdio stream.  You can use your own source module if you want
to do something else, as discussed later.

If you use the standard source module, you must open the source stdio stream
beforehand.  Typical code for this step looks like:

FILE *infile;
        ...
        if ((infile = fopen(filename, "rb")) == NULL) {
            fprintf(stderr, "can't open %s\n", filename);
            exit(1);
        }
        jpeg_stdio_src(&cinfo, infile);

where the last line invokes the standard source module.

WARNING: it is critical that the binary compressed data be read unchanged.
On non-Unix systems the stdio library may perform newline translation or
otherwise corrupt binary data.  To suppress this behavior, you may need to use
a "b" option to fopen (as shown above), or use setmode() or another routine to
put the stdio stream in binary mode.  See cjpeg.c and djpeg.c for code that
has been found to work on many systems.

You may not change the data source between calling jpeg_read_header() and
jpeg_finish_decompress().  If you wish to read a series of JPEG images from
a single source file, you should repeat the jpeg_read_header() to
jpeg_finish_decompress() sequence without reinitializing either the JPEG
object or the data source module; this prevents buffered input data from
being discarded.

3. Call jpeg_read_header() to obtain image info.

Typical code for this step is just

jpeg_read_header(&cinfo, TRUE);

This will read the source datastream header markers, up to the beginning
of the compressed data proper.  On return, the image dimensions and other
info have been stored in the JPEG object.  The application may wish to
consult this information before selecting decompression parameters.

More complex code is necessary if
  * A suspending data source is used --- in that case jpeg_read_header()
    may return before it has read all the header data.  See "I/O suspension",
    below.  The normal stdio source manager will NOT cause this to happen.
  * Abbreviated JPEG files are to be processed --- see the section on
    abbreviated datastreams.  Standard applications that deal only in
    interchange JPEG files need not be concerned with this case either.

It is permissible to stop at this point if you just wanted to find out the
image dimensions and other header info for a JPEG file.  In that case,
call jpeg_destroy() when you are done with the JPEG object, or call
jpeg_abort() to return it to an idle state before selecting a new data
source and reading another header.

4. Set parameters for decompression.

jpeg_read_header() sets appropriate default decompression parameters based on
the properties of the image (in particular, its colorspace).  However, you
may well want to alter these defaults before beginning the decompression.
For example, the default is to produce full color output from a color file.
If you want colormapped output you must ask for it.  Other options allow the
returned image to be scaled and allow various speed/quality tradeoffs to be
selected.  "Decompression parameter selection", below, gives details.

If the defaults are appropriate, nothing need be done at this step.

Note that all default values are set by each call to jpeg_read_header().
If you reuse a decompression object, you cannot expect your parameter
settings to be preserved across cycles, as you can for compression.
You must set desired parameter values each time.

5. jpeg_start_decompress(...);

Once the parameter values are satisfactory, call jpeg_start_decompress() to
begin decompression.  This will initialize internal state, allocate working
memory, and prepare for returning data.

Typical code is just

jpeg_start_decompress(&cinfo);

If you have requested a multi-pass operating mode, such as 2-pass color
quantization, jpeg_start_decompress() will do everything needed before data
output can begin.  In this case jpeg_start_decompress() may take quite a while
to complete.  With a single-scan (non progressive) JPEG file and default
decompression parameters, this will not happen; jpeg_start_decompress() will
return quickly.

After this call, the final output image dimensions, including any requested
scaling, are available in the JPEG object; so is the selected colormap, if
colormapped output has been requested.  Useful fields include

output_width            image width and height, as scaled
        output_height
        out_color_components    # of color components in out_color_space
        output_components       # of color components returned per pixel
        colormap                the selected colormap, if any
        actual_number_of_colors         number of entries in colormap

output_components is 1 (a colormap index) when quantizing colors; otherwise it
equals out_color_components.  It is the number of JSAMPLE values that will be
emitted per pixel in the output arrays.

Typically you will need to allocate data buffers to hold the incoming image.
You will need output_width * output_components JSAMPLEs per scanline in your
output buffer, and a total of output_height scanlines will be returned.

Note: if you are using the JPEG library's internal memory manager to allocate
data buffers (as djpeg does), then the manager's protocol requires that you
request large buffers *before* calling jpeg_start_decompress().  This is a
little tricky since the output_XXX fields are not normally valid then.  You
can make them valid by calling jpeg_calc_output_dimensions() after setting the
relevant parameters (scaling, output color space, and quantization flag).

6. while (scan lines remain to be read)
        jpeg_read_scanlines(...);

Now you can read the decompressed image data by calling jpeg_read_scanlines()
one or more times.  At each call, you pass in the maximum number of scanlines
to be read (ie, the height of your working buffer); jpeg_read_scanlines()
will return up to that many lines.  The return value is the number of lines
actually read.  The format of the returned data is discussed under "Data
formats", above.  Don't forget that grayscale and color JPEGs will return
different data formats!

Image data is returned in top-to-bottom scanline order.  If you must write
out the image in bottom-to-top order, you can use the JPEG library's virtual
array mechanism to invert the data efficiently.  Examples of this can be
found in the sample application djpeg.

The library maintains a count of the number of scanlines returned so far
in the output_scanline field of the JPEG object.  Usually you can just use
this variable as the loop counter, so that the loop test looks like
"while (cinfo.output_scanline < cinfo.output_height)".  (Note that the test
should NOT be against image_height, unless you never use scaling.  The
image_height field is the height of the original unscaled image.)
The return value always equals the change in the value of output_scanline.

If you don't use a suspending data source, it is safe to assume that
jpeg_read_scanlines() reads at least one scanline per call, until the
bottom of the image has been reached.

If you use a buffer larger than one scanline, it is NOT safe to assume that
jpeg_read_scanlines() fills it.  (The current implementation returns only a
few scanlines per call, no matter how large a buffer you pass.)  So you must
always provide a loop that calls jpeg_read_scanlines() repeatedly until the
whole image has been read.

7. jpeg_finish_decompress(...);

After all the image data has been read, call jpeg_finish_decompress() to
complete the decompression cycle.  This causes working memory associated
with the JPEG object to be released.

Typical code:

jpeg_finish_decompress(&cinfo);

If using the stdio source manager, don't forget to close the source stdio
stream if necessary.

It is an error to call jpeg_finish_decompress() before reading the correct
total number of scanlines.  If you wish to abort decompression, call
jpeg_abort() as discussed below.

After completing a decompression cycle, you may dispose of the JPEG object as
discussed next, or you may use it to decompress another image.  In that case
return to step 2 or 3 as appropriate.  If you do not change the source
manager, the next image will be read from the same source.

8. Release the JPEG decompression object.

When you are done with a JPEG decompression object, destroy it by calling
jpeg_destroy_decompress() or jpeg_destroy().  The previous discussion of
destroying compression objects applies here too.

Typical code:

jpeg_destroy_decompress(&cinfo);

9. Aborting.

You can abort a decompression cycle by calling jpeg_destroy_decompress() or
jpeg_destroy() if you don't need the JPEG object any more, or
jpeg_abort_decompress() or jpeg_abort() if you want to reuse the object.
The previous discussion of aborting compression cycles applies here too.

Partial image decompression
---------------------------

Partial image decompression is convenient for performance-critical applications
that wish to view only a portion of a large JPEG image without decompressing
the whole thing.  It it also useful in memory-constrained environments (such as
on mobile devices.)  This library provides the following functions to support
partial image decompression:

1. Skipping rows when decompressing

jpeg_skip_scanlines(j_decompress_ptr cinfo, JDIMENSION num_lines);

This function provides application programmers with the ability to skip over
multiple rows in the JPEG image.

Suspending data sources are not supported by this function.  Calling
jpeg_skip_scanlines() with a suspending data source will result in undefined
behavior.  Two-pass color quantization is also not supported by this function.
Calling jpeg_skip_scanlines() with two-pass color quantization enabled will
result in an error.

jpeg_skip_scanlines() will not allow skipping past the bottom of the image.  If
the value of num_lines is large enough to skip past the bottom of the image,
then the function will skip to the end of the image instead.

If the value of num_lines is valid, then jpeg_skip_scanlines() will always
skip all of the input rows requested.  There is no need to inspect the return
value of the function in that case.

Best results will be achieved by calling jpeg_skip_scanlines() for large chunks
of rows.  The function should be viewed as a way to quickly jump to a
particular vertical offset in the JPEG image in order to decode a subset of the
image.  Used in this manner, it will provide significant performance
improvements.

Calling jpeg_skip_scanlines() for small values of num_lines has several
potential drawbacks:
    1) JPEG decompression occurs in blocks, so if jpeg_skip_scanlines() is
       called from the middle of a decompression block, then it is likely that
       much of the decompression work has already been done for the first
       couple of rows that need to be skipped.
    2) When this function returns, it must leave the decompressor in a state
       such that it is ready to read the next line.  This may involve
       decompressing a block that must be partially skipped.
These issues are especially tricky for cases in which upsampling requires
context rows.  In the worst case, jpeg_skip_scanlines() will perform similarly
to jpeg_read_scanlines() (since it will actually call jpeg_read_scanlines().)

2. Decompressing partial scanlines

jpeg_crop_scanline (j_decompress_ptr cinfo, JDIMENSION *xoffset,
                            JDIMENSION *width)

This function provides application programmers with the ability to decompress
only a portion of each row in the JPEG image.  It must be called after
jpeg_start_decompress() and before any calls to jpeg_read_scanlines() or
jpeg_skip_scanlines().

If xoffset and width do not form a valid subset of the image row, then this
function will generate an error.  Note that if the output image is scaled, then
xoffset and width are relative to the scaled image dimensions.

xoffset and width are passed by reference because xoffset must fall on an iMCU
boundary.  If it doesn't, then it will be moved left to the nearest iMCU
boundary, and width will be increased accordingly.  If the calling program does
not like the adjusted values of xoffset and width, then it can call
jpeg_crop_scanline() again with new values (for instance, if it wants to move
xoffset to the nearest iMCU boundary to the right instead of to the left.)

After calling this function, cinfo->output_width will be set to the adjusted
width.  This value should be used when allocating an output buffer to pass to
jpeg_read_scanlines().

The output image from a partial-width decompression will be identical to the
corresponding image region from a full decode, with one exception:  The "fancy"
(smooth) h2v2 (4:2:0) and h2v1 (4:2:2) upsampling algorithms fill in the
missing chroma components by averaging the chroma components from neighboring
pixels, except on the right and left edges of the image (where there are no
neighboring pixels.)  When performing a partial-width decompression, these
"fancy" upsampling algorithms may treat the left and right edges of the partial
image region as if they are the left and right edges of the image, meaning that
the upsampling algorithm may be simplified.  The result is that the pixels on
the left or right edge of the partial image may not be exactly identical to the
corresponding pixels in the original image.

Mechanics of usage: include files, linking, etc
-----------------------------------------------

Applications using the JPEG library should include the header file jpeglib.h
to obtain declarations of data types and routines.  Before including
jpeglib.h, include system headers that define at least the typedefs FILE and
size_t.  On ANSI-conforming systems, including <stdio.h> is sufficient; on
older Unix systems, you may need <sys/types.h> to define size_t.

If the application needs to refer to individual JPEG library error codes, also
include jerror.h to define those symbols.

jpeglib.h indirectly includes the files jconfig.h and jmorecfg.h.  If you are
installing the JPEG header files in a system directory, you will want to
install all four files: jpeglib.h, jerror.h, jconfig.h, jmorecfg.h.

The most convenient way to include the JPEG code into your executable program
is to prepare a library file ("libjpeg.a", or a corresponding name on non-Unix
machines) and reference it at your link step.  If you use only half of the
library (only compression or only decompression), only that much code will be
included from the library, unless your linker is hopelessly brain-damaged.
The supplied makefiles build libjpeg.a automatically (see install.txt).

While you can build the JPEG library as a shared library if the whim strikes
you, we don't really recommend it.  The trouble with shared libraries is that
at some point you'll probably try to substitute a new version of the library
without recompiling the calling applications.  That generally doesn't work
because the parameter struct declarations usually change with each new
version.  In other words, the library's API is *not* guaranteed binary
compatible across versions; we only try to ensure source-code compatibility.
(In hindsight, it might have been smarter to hide the parameter structs from
applications and introduce a ton of access functions instead.  Too late now,
however.)

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.ijg 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 Annex K (Clause K.1) of
        Rec. ITU-T T.81 (1992) | ISO/IEC 10918-1:1994, multiplied by the
        specified scale factor (which is expressed as a percentage; thus
        scale_factor = 100 reproduces the spec's tables).  Note that larger
        scale factors give lower quality.  This entry point is useful for
        conforming to the Adobe PostScript DCT conventions, but we do not
        recommend linear scaling as a user-visible quality scale otherwise.
        force_baseline again constrains the computed table entries to 1..255.

int jpeg_quality_scaling (int quality)
        Converts a value on the IJG-recommended quality scale to a linear
        scaling percentage.  Note that this routine may change or go away
        in future releases --- IJG may choose to adopt a scaling method that
        can't be expressed as a simple scalar multiplier, in which case the
        premise of this routine collapses.  Caveat user.

jpeg_default_qtables (j_compress_ptr cinfo, boolean force_baseline)
        [libjpeg v7+ API/ABI emulation only]
        Set default quantization tables with linear q_scale_factor[] values
        (see below).

jpeg_add_quant_table (j_compress_ptr cinfo, int which_tbl,
                      const unsigned int *basic_table,
                      int scale_factor, boolean force_baseline)
        Allows an arbitrary quantization table to be created.  which_tbl
        indicates which table slot to fill.  basic_table points to an array
        of 64 unsigned ints given in normal array order.  These values are
        multiplied by scale_factor/100 and then clamped to the range 1..65535
        (or to 1..255 if force_baseline is TRUE).
        CAUTION: prior to library version 6a, jpeg_add_quant_table expected
        the basic table to be given in JPEG zigzag order.  If you need to
        write code that works with either older or newer versions of this
        routine, you must check the library version number.  Something like
        "#if JPEG_LIB_VERSION >= 61" is the right test.

jpeg_simple_progression (j_compress_ptr cinfo)
        Generates a default scan script for writing a progressive-JPEG file.
        This is the recommended method of creating a progressive file,
        unless you want to make a custom scan sequence.  You must ensure that
        the JPEG color space is set correctly before calling this routine.

Compression parameters (cinfo fields) include:

boolean arith_code
        If TRUE, use arithmetic coding.
        If FALSE, use Huffman coding.

J_DCT_METHOD dct_method
        Selects the algorithm used for the DCT step.  Choices are:
                JDCT_ISLOW: accurate integer method
                JDCT_IFAST: less accurate integer method [legacy feature]
                JDCT_FLOAT: floating-point method [legacy feature]
                JDCT_DEFAULT: default method (normally JDCT_ISLOW)
                JDCT_FASTEST: fastest method (normally JDCT_IFAST)
        When the Independent JPEG Group's software was first released in 1991,
        the compression time for a 1-megapixel JPEG image on a mainstream PC
        was measured in minutes.  Thus, JDCT_IFAST provided noticeable
        performance benefits.  On modern CPUs running libjpeg-turbo, however,
        the compression time for a 1-megapixel JPEG image is measured in
        milliseconds, and thus the performance benefits of JDCT_IFAST are much
        less noticeable.  On modern x86/x86-64 CPUs that support AVX2
        instructions, JDCT_IFAST and JDCT_ISLOW have similar performance.  On
        other types of CPUs, JDCT_IFAST is generally about 5-15% faster than
        JDCT_ISLOW.

For quality levels of 90 and below, there should be little or no
        perceptible quality difference between the two algorithms.  For quality
        levels above 90, however, the difference between JDCT_IFAST and
        JDCT_ISLOW becomes more pronounced.  With quality=97, for instance,
        JDCT_IFAST incurs generally about a 1-3 dB loss in PSNR relative to
        JDCT_ISLOW, but this can be larger for some images.  Do not use
        JDCT_IFAST with quality levels above 97.  The algorithm often
        degenerates at quality=98 and above and can actually produce a more
        lossy image than if lower quality levels had been used.  Also, in
        libjpeg-turbo, JDCT_IFAST is not fully accelerated for quality levels
        above 97, so it will be slower than JDCT_ISLOW.

JDCT_FLOAT does not produce significantly more accurate results than
        JDCT_ISLOW, and it is much slower.  JDCT_FLOAT may also give different
        results on different machines due to varying roundoff behavior, whereas
        the integer methods should give the same results on all machines.

J_COLOR_SPACE jpeg_color_space
int num_components
        The JPEG color space and corresponding number of components; see
        "Special color spaces", below, for more info.  We recommend using
        jpeg_set_color_space() if you want to change these.

boolean optimize_coding
        TRUE causes the compressor to compute optimal Huffman coding tables
        for the image.  This requires an extra pass over the data and
        therefore costs a good deal of space and time.  The default is
        FALSE, which tells the compressor to use the supplied or default
        Huffman tables.  In most cases optimal tables save only a few percent
        of file size compared to the default tables.  Note that when this is
        TRUE, you need not supply Huffman tables at all, and any you do
        supply will be overwritten.

unsigned int restart_interval
int restart_in_rows
        To emit restart markers in the JPEG file, set one of these nonzero.
        Set restart_interval to specify the exact interval in MCU blocks.
        Set restart_in_rows to specify the interval in MCU rows.  (If
        restart_in_rows is not 0, then restart_interval is set after the
        image width in MCUs is computed.)  Defaults are zero (no restarts).
        One restart marker per MCU row is often a good choice.
        NOTE: the overhead of restart markers is higher in grayscale JPEG
        files than in color files, and MUCH higher in progressive JPEGs.
        If you use restarts, you may want to use larger intervals in those
        cases.

const jpeg_scan_info *scan_info
int num_scans
        By default, scan_info is NULL; this causes the compressor to write a
        single-scan sequential JPEG file.  If not NULL, scan_info points to
        an array of scan definition records of length num_scans.  The
        compressor will then write a JPEG file having one scan for each scan
        definition record.  This is used to generate noninterleaved or
        progressive JPEG files.  The library checks that the scan array
        defines a valid JPEG scan sequence.  (jpeg_simple_progression creates
        a suitable scan definition array for progressive JPEG.)  This is
        discussed further under "Progressive JPEG support".

int smoothing_factor
        If non-zero, the input image is smoothed; the value should be 1 for
        minimal smoothing to 100 for maximum smoothing.  Consult jcsample.c
        for details of the smoothing algorithm.  The default is zero.

boolean write_JFIF_header
        If TRUE, a JFIF APP0 marker is emitted.  jpeg_set_defaults() and
        jpeg_set_colorspace() set this TRUE if a JFIF-legal JPEG color space
        (ie, YCbCr or grayscale) is selected, otherwise FALSE.

UINT8 JFIF_major_version
UINT8 JFIF_minor_version
        The version number to be written into the JFIF marker.
        jpeg_set_defaults() initializes the version to 1.01 (major=minor=1).
        You should set it to 1.02 (major=1, minor=2) if you plan to write
        any JFIF 1.02 extension markers.

UINT8 density_unit
UINT16 X_density
UINT16 Y_density
        The resolution information to be written into the JFIF marker;
        not used otherwise.  density_unit may be 0 for unknown,
        1 for dots/inch, or 2 for dots/cm.  The default values are 0,1,1
        indicating square pixels of unknown size.

boolean write_Adobe_marker
        If TRUE, an Adobe APP14 marker is emitted.  jpeg_set_defaults() and
        jpeg_set_colorspace() set this TRUE if JPEG color space RGB, CMYK,
        or YCCK is selected, otherwise FALSE.  It is generally a bad idea
        to set both write_JFIF_header and write_Adobe_marker.  In fact,
        you probably shouldn't change the default settings at all --- the
        default behavior ensures that the JPEG file's color space can be
        recognized by the decoder.

JQUANT_TBL *quant_tbl_ptrs[NUM_QUANT_TBLS]
        Pointers to coefficient quantization tables, one per table slot,
        or NULL if no table is defined for a slot.  Usually these should
        be set via one of the above helper routines; jpeg_add_quant_table()
        is general enough to define any quantization table.  The other
        routines will set up table slot 0 for luminance quality and table
        slot 1 for chrominance.

int q_scale_factor[NUM_QUANT_TBLS]
        [libjpeg v7+ API/ABI emulation only]
        Linear quantization scaling factors (0-100, default 100)
        for use with jpeg_default_qtables().
        See rdswitch.c and cjpeg.c for an example of usage.
        Note that the q_scale_factor[] values use "linear" scales, so JPEG
        quality levels chosen by the user must be converted to these scales
        using jpeg_quality_scaling().  Here is an example that corresponds to
        cjpeg -quality 90,70:

jpeg_set_defaults(cinfo);

/* Set luminance quality 90. */
                cinfo->q_scale_factor[0] = jpeg_quality_scaling(90);
                /* Set chrominance quality 70. */
                cinfo->q_scale_factor[1] = jpeg_quality_scaling(70);

jpeg_default_qtables(cinfo, force_baseline);

CAUTION: Setting separate quality levels for chrominance and luminance
        is mainly only useful if chrominance subsampling is disabled.  2x2
        chrominance subsampling (AKA "4:2:0") is the default, but you can
        explicitly disable subsampling as follows:

cinfo->comp_info[0].v_samp_factor = 1;
                cinfo->comp_info[0].h_samp_factor = 1;

JHUFF_TBL *dc_huff_tbl_ptrs[NUM_HUFF_TBLS]
JHUFF_TBL *ac_huff_tbl_ptrs[NUM_HUFF_TBLS]
        Pointers to Huffman coding tables, one per table slot, or NULL if
        no table is defined for a slot.  Slots 0 and 1 are filled with the
        JPEG sample tables by jpeg_set_defaults().  If you need to allocate
        more table structures, jpeg_alloc_huff_table() may be used.
        Note that optimal Huffman tables can be computed for an image
        by setting optimize_coding, as discussed above; there's seldom
        any need to mess with providing your own Huffman tables.

[libjpeg v7+ API/ABI emulation only]
The actual dimensions of the JPEG image that will be written to the file are
given by the following fields.  These are computed from the input image
dimensions and the compression parameters by jpeg_start_compress().  You can
also call jpeg_calc_jpeg_dimensions() to obtain the values that will result
from the current parameter settings.  This can be useful if you are trying
to pick a scaling ratio that will get close to a desired target size.

JDIMENSION jpeg_width           Actual dimensions of output image.
JDIMENSION jpeg_height

Per-component parameters are stored in the struct cinfo.comp_info[i] for
component number i.  Note that components here refer to components of the
JPEG color space, *not* the source image color space.  A suitably large
comp_info[] array is allocated by jpeg_set_defaults(); if you choose not
to use that routine, it's up to you to allocate the array.

int component_id
        The one-byte identifier code to be recorded in the JPEG file for
        this component.  For the standard color spaces, we recommend you
        leave the default values alone.

int h_samp_factor
int v_samp_factor
        Horizontal and vertical sampling factors for the component; must
        be 1..4 according to the JPEG standard.  Note that larger sampling
        factors indicate a higher-resolution component; many people find
        this behavior quite unintuitive.  The default values are 2,2 for
        luminance components and 1,1 for chrominance components, except
        for grayscale where 1,1 is used.

int quant_tbl_no
        Quantization table number for component.  The default value is
        0 for luminance components and 1 for chrominance components.

int dc_tbl_no
int ac_tbl_no
        DC and AC entropy coding table numbers.  The default values are
        0 for luminance components and 1 for chrominance components.

int component_index
        Must equal the component's index in comp_info[].  (Beginning in
        release v6, the compressor library will fill this in automatically;
        you don't have to.)

Decompression parameter selection
---------------------------------

Decompression parameter selection is somewhat simpler than compression
parameter selection, since all of the JPEG internal parameters are
recorded in the source file and need not be supplied by the application.
(Unless you are working with abbreviated files, in which case see
"Abbreviated datastreams", below.)  Decompression parameters control
the postprocessing done on the image to deliver it in a format suitable
for the application's use.  Many of the parameters control speed/quality
tradeoffs, in which faster decompression may be obtained at the price of
a poorer-quality image.  The defaults select the highest quality (slowest)
processing.

The following fields in the JPEG object are set by jpeg_read_header() and
may be useful to the application in choosing decompression parameters:

JDIMENSION image_width                  Width and height of image
JDIMENSION image_height
int num_components                      Number of color components
J_COLOR_SPACE jpeg_color_space          Colorspace of image
boolean saw_JFIF_marker                 TRUE if a JFIF APP0 marker was seen
  UINT8 JFIF_major_version              Version information from JFIF marker
  UINT8 JFIF_minor_version
  UINT8 density_unit                    Resolution data from JFIF marker
  UINT16 X_density
  UINT16 Y_density
boolean saw_Adobe_marker                TRUE if an Adobe APP14 marker was seen
  UINT8 Adobe_transform                 Color transform code from Adobe marker

The JPEG color space, unfortunately, is something of a guess since the JPEG
standard proper does not provide a way to record it.  In practice most files
adhere to the JFIF or Adobe conventions, and the decoder will recognize these
correctly.  See "Special color spaces", below, for more info.

The decompression parameters that determine the basic properties of the
returned image are:

J_COLOR_SPACE out_color_space
        Output color space.  jpeg_read_header() sets an appropriate default
        based on jpeg_color_space; typically it will be RGB or grayscale.
        The application can change this field to request output in a different
        colorspace.  For example, set it to JCS_GRAYSCALE to get grayscale
        output from a color file.  (This is useful for previewing: grayscale
        output is faster than full color since the color components need not
        be processed.)  Note that not all possible color space transforms are
        currently implemented; you may need to extend jdcolor.c if you want an
        unusual conversion.

unsigned int scale_num, scale_denom
        Scale the image by the fraction scale_num/scale_denom.  Default is
        1/1, or no scaling.  Currently, the only supported scaling ratios
        are M/8 with all M from 1 to 16, or any reduced fraction thereof (such
        as 1/2, 3/4, etc.)  (The library design allows for arbitrary
        scaling ratios but this is not likely to be implemented any time soon.)
        Smaller scaling ratios permit significantly faster decoding since
        fewer pixels need be processed and a simpler IDCT method can be used.

boolean quantize_colors
        If set TRUE, colormapped output will be delivered.  Default is FALSE,
        meaning that full-color output will be delivered.

The next three parameters are relevant only if quantize_colors is TRUE.

int desired_number_of_colors
        Maximum number of colors to use in generating a library-supplied color
        map (the actual number of colors is returned in a different field).
        Default 256.  Ignored when the application supplies its own color map.

boolean two_pass_quantize
        If TRUE, an extra pass over the image is made to select a custom color
        map for the image.  This usually looks a lot better than the one-size-
        fits-all colormap that is used otherwise.  Default is TRUE.  Ignored
        when the application supplies its own color map.

J_DITHER_MODE dither_mode
        Selects color dithering method.  Supported values are:
                JDITHER_NONE    no dithering: fast, very low quality
                JDITHER_ORDERED ordered dither: moderate speed and quality
                JDITHER_FS      Floyd-Steinberg dither: slow, high quality
        Default is JDITHER_FS.  (At present, ordered dither is implemented
        only in the single-pass, standard-colormap case.  If you ask for
        ordered dither when two_pass_quantize is TRUE or when you supply
        an external color map, you'll get F-S dithering.)

When quantize_colors is TRUE, the target color map is described by the next
two fields.  colormap is set to NULL by jpeg_read_header().  The application
can supply a color map by setting colormap non-NULL and setting
actual_number_of_colors to the map size.  Otherwise, jpeg_start_decompress()
selects a suitable color map and sets these two fields itself.
[Implementation restriction: at present, an externally supplied colormap is
only accepted for 3-component output color spaces.]

JSAMPARRAY colormap
        The color map, represented as a 2-D pixel array of out_color_components
        rows and actual_number_of_colors columns.  Ignored if not quantizing.
        CAUTION: if the JPEG library creates its own colormap, the storage
        pointed to by this field is released by jpeg_finish_decompress().
        Copy the colormap somewhere else first, if you want to save it.

int actual_number_of_colors
        The number of colors in the color map.

Additional decompression parameters that the application may set include:

J_DCT_METHOD dct_method
        Selects the algorithm used for the DCT step.  Choices are:
                JDCT_ISLOW: accurate integer method
                JDCT_IFAST: less accurate integer method [legacy feature]
                JDCT_FLOAT: floating-point method [legacy feature]
                JDCT_DEFAULT: default method (normally JDCT_ISLOW)
                JDCT_FASTEST: fastest method (normally JDCT_IFAST)
        When the Independent JPEG Group's software was first released in 1991,
        the decompression time for a 1-megapixel JPEG image on a mainstream PC
        was measured in minutes.  Thus, JDCT_IFAST provided noticeable
        performance benefits.  On modern CPUs running libjpeg-turbo, however,
        the decompression time for a 1-megapixel JPEG image is measured in
        milliseconds, and thus the performance benefits of JDCT_IFAST are much
        less noticeable.  On modern x86/x86-64 CPUs that support AVX2
        instructions, JDCT_IFAST and JDCT_ISLOW have similar performance.  On
        other types of CPUs, JDCT_IFAST is generally about 5-15% faster than
        JDCT_ISLOW.

If the JPEG image was compressed using a quality level of 85 or below,
        then there should be little or no perceptible quality difference
        between the two algorithms.  When decompressing images that were
        compressed using quality levels above 85, however, the difference
        between JDCT_IFAST and JDCT_ISLOW becomes more pronounced.  With images
        compressed using quality=97, for instance, JDCT_IFAST incurs generally
        about a 4-6 dB loss in PSNR relative to JDCT_ISLOW, but this can be
        larger for some images.  If you can avoid it, do not use JDCT_IFAST
        when decompressing images that were compressed using quality levels
        above 97.  The algorithm often degenerates for such images and can
        actually produce a more lossy output image than if the JPEG image had
        been compressed using lower quality levels.

boolean do_fancy_upsampling
        If TRUE, do careful upsampling of chroma components.  If FALSE,
        a faster but sloppier method is used.  Default is TRUE.  The visual
        impact of the sloppier method is often very small.

boolean do_block_smoothing
        If TRUE, interblock smoothing is applied in early stages of decoding
        progressive JPEG files; if FALSE, not.  Default is TRUE.  Early
        progression stages look "fuzzy" with smoothing, "blocky" without.
        In any case, block smoothing ceases to be applied after the first few
        AC coefficients are known to full accuracy, so it is relevant only
        when using buffered-image mode for progressive images.

boolean enable_1pass_quant
boolean enable_external_quant
boolean enable_2pass_quant
        These are significant only in buffered-image mode, which is
        described in its own section below.

The output image dimensions are given by the following fields.  These are
computed from the source image dimensions and the decompression parameters
by jpeg_start_decompress().  You can also call jpeg_calc_output_dimensions()
to obtain the values that will result from the current parameter settings.
This can be useful if you are trying to pick a scaling ratio that will get
close to a desired target size.  It's also important if you are using the
JPEG library's memory manager to allocate output buffer space, because you
are supposed to request such buffers *before* jpeg_start_decompress().

JDIMENSION output_width         Actual dimensions of output image.
JDIMENSION output_height
int out_color_components        Number of color components in out_color_space.
int output_components           Number of color components returned.
int rec_outbuf_height           Recommended height of scanline buffer.

When quantizing colors, output_components is 1, indicating a single color map
index per pixel.  Otherwise it equals out_color_components.  The output arrays
are required to be output_width * output_components JSAMPLEs wide.

rec_outbuf_height is the recommended minimum height (in scanlines) of the
buffer passed to jpeg_read_scanlines().  If the buffer is smaller, the
library will still work, but time will be wasted due to unnecessary data
copying.  In high-quality modes, rec_outbuf_height is always 1, but some
faster, lower-quality modes set it to larger values (typically 2 to 4).
If you are going to ask for a high-speed processing mode, you may as well
go to the trouble of honoring rec_outbuf_height so as to avoid data copying.
(An output buffer larger than rec_outbuf_height lines is OK, but won't
provide any material speed improvement over that height.)

Special color spaces
--------------------

The JPEG standard itself is "color blind" and doesn't specify any particular
color space.  It is customary to convert color data to a luminance/chrominance
color space before compressing, since this permits greater compression.  The
existing de-facto JPEG file format standards specify YCbCr or grayscale data
(JFIF), or grayscale, RGB, YCbCr, CMYK, or YCCK (Adobe).  For special
applications such as multispectral images, other color spaces can be used,
but it must be understood that such files will be unportable.

The JPEG library can handle the most common colorspace conversions (namely
RGB <=> YCbCr and CMYK <=> YCCK).  It can also deal with data of an unknown
color space, passing it through without conversion.  If you deal extensively
with an unusual color space, you can easily extend the library to understand
additional color spaces and perform appropriate conversions.

For compression, the source data's color space is specified by field
in_color_space.  This is transformed to the JPEG file's color space given
by jpeg_color_space.  jpeg_set_defaults() chooses a reasonable JPEG color
space depending on in_color_space, but you can override this by calling
jpeg_set_colorspace().  Of course you must select a supported transformation.
jccolor.c currently supports the following transformations:
        RGB => YCbCr
        RGB => GRAYSCALE
        YCbCr => GRAYSCALE
        CMYK => YCCK
plus the null transforms: GRAYSCALE => GRAYSCALE, RGB => RGB,
YCbCr => YCbCr, CMYK => CMYK, YCCK => YCCK, and UNKNOWN => UNKNOWN.

The de-facto file format standards (JFIF and Adobe) specify APPn markers that
indicate the color space of the JPEG file.  It is important to ensure that
these are written correctly, or omitted if the JPEG file's color space is not
one of the ones supported by the de-facto standards.  jpeg_set_colorspace()
will set the compression parameters to include or omit the APPn markers
properly, so long as it is told the truth about the JPEG color space.
For example, if you are writing some random 3-component color space without
conversion, don't try to fake out the library by setting in_color_space and
jpeg_color_space to JCS_YCbCr; use JCS_UNKNOWN.  You may want to write an
APPn marker of your own devising to identify the colorspace --- see "Special
markers", below.

When told that the color space is UNKNOWN, the library will default to using
luminance-quality compression parameters for all color components.  You may
well want to change these parameters.  See the source code for
jpeg_set_colorspace(), in jcparam.c, for details.

For decompression, the JPEG file's color space is given in jpeg_color_space,
and this is transformed to the output color space out_color_space.
jpeg_read_header's setting of jpeg_color_space can be relied on if the file
conforms to JFIF or Adobe conventions, but otherwise it is no better than a
guess.  If you know the JPEG file's color space for certain, you can override
jpeg_read_header's guess by setting jpeg_color_space.  jpeg_read_header also
selects a default output color space based on (its guess of) jpeg_color_space;
set out_color_space to override this.  Again, you must select a supported
transformation.  jdcolor.c currently supports
        YCbCr => RGB
        YCbCr => GRAYSCALE
        RGB => GRAYSCALE
        GRAYSCALE => RGB
        YCCK => CMYK
as well as the null transforms.  (Since GRAYSCALE=>RGB is provided, an
application can force grayscale JPEGs to look like color JPEGs if it only
wants to handle one case.)

The two-pass color quantizer, jquant2.c, is specialized to handle RGB data
(it weights distances appropriately for RGB colors).  You'll need to modify
the code if you want to use it for non-RGB output color spaces.  Note that
jquant2.c is used to map to an application-supplied colormap as well as for
the normal two-pass colormap selection process.

CAUTION: it appears that Adobe Photoshop writes inverted data in CMYK JPEG
files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect.
This is arguably a bug in Photoshop, but if you need to work with Photoshop
CMYK files, you will have to deal with it in your application.  We cannot
"fix" this in the library by inverting the data during the CMYK<=>YCCK
transform, because that would break other applications, notably Ghostscript.
Photoshop versions prior to 3.0 write EPS files containing JPEG-encoded CMYK
data in the same inverted-YCCK representation used in bare JPEG files, but
the surrounding PostScript code performs an inversion using the PS image
operator.  I am told that Photoshop 3.0 will write uninverted YCCK in
EPS/JPEG files, and will omit the PS-level inversion.  (But the data
polarity used in bare JPEG files will not change in 3.0.)  In either case,
the JPEG library must not invert the data itself, or else Ghostscript would
read these EPS files incorrectly.

Error handling
--------------

When the default error handler is used, any error detected inside the JPEG
routines will cause a message to be printed on stderr, followed by exit().
You can supply your own error handling routines to override this behavior
and to control the treatment of nonfatal warnings and trace/debug messages.
The file example.txt 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.txt.

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.txt for one way to do it.  (Beginning
with IJG version 6b, there is also a void pointer "client_data" in each
JPEG object, which the application can also use to find related data.
The library does not touch client_data at all.)

The individual methods that you might wish to override are:

error_exit (j_common_ptr cinfo)
        Receives control for a fatal error.  Information sufficient to
        generate the error message has been stored in cinfo->err; call
        output_message to display it.  Control must NOT return to the caller;
        generally this routine will exit() or longjmp() somewhere.
        Typically you would override this routine to get rid of the exit()
        default behavior.  Note that if you continue processing, you should
        clean up the JPEG object with jpeg_abort() or jpeg_destroy().

output_message (j_common_ptr cinfo)
        Actual output of any JPEG message.  Override this to send messages
        somewhere other than stderr.  Note that this method does not know
        how to generate a message, only where to send it.

format_message (j_common_ptr cinfo, char *buffer)
        Constructs a readable error message string based on the error info
        stored in cinfo->err.  This method is called by output_message.  Few
        applications should need to override this method.  One possible
        reason for doing so is to implement dynamic switching of error message
        language.

emit_message (j_common_ptr cinfo, int msg_level)
        Decide whether or not to emit a warning or trace message; if so,
        calls output_message.  The main reason for overriding this method
        would be to abort on warnings.  msg_level is -1 for warnings,
        0 and up for trace messages.

Only error_exit() and emit_message() are called from the rest of the JPEG
library; the other two are internal to the error handler.

The actual message texts are stored in an array of strings which is pointed to
by the field err->jpeg_message_table.  The messages are numbered from 0 to
err->last_jpeg_message, and it is these code numbers that are used in the
JPEG library code.  You could replace the message texts (for instance, with
messages in French or German) by changing the message table pointer.  See
jerror.h for the default texts.  CAUTION: this table will almost certainly
change or grow from one library version to the next.

It may be useful for an application to add its own message texts that are
handled by the same mechanism.  The error handler supports a second "add-on"
message table for this purpose.  To define an addon table, set the pointer
err->addon_message_table and the message numbers err->first_addon_message and
err->last_addon_message.  If you number the addon messages beginning at 1000
or so, you won't have to worry about conflicts with the library's built-in
messages.  See the sample applications cjpeg/djpeg for an example of using
addon messages (the addon messages are defined in cderror.h).

Actual invocation of the error handler is done via macros defined in jerror.h:
        ERREXITn(...)   for fatal errors
        WARNMSn(...)    for corrupt-data warnings
        TRACEMSn(...)   for trace and informational messages.
These macros store the message code and any additional parameters into the
error handler struct, then invoke the error_exit() or emit_message() method.
The variants of each macro are for varying numbers of additional parameters.
The additional parameters are inserted into the generated message using
standard printf() format codes.

See jerror.h and jerror.c for further details.

Compressed data handling (source and destination managers)
----------------------------------------------------------

The JPEG compression library sends its compressed data to a "destination
manager" module.  The default destination manager just writes the data to a
memory buffer or to a stdio stream, but you can provide your own manager to
do something else.  Similarly, the decompression library calls a "source
manager" to obtain the compressed data; you can provide your own source
manager if you want the data to come from somewhere other than a memory
buffer or a stdio stream.

In both cases, compressed data is processed a bufferload at a time: the
destination or source manager provides a work buffer, and the library invokes
the manager only when the buffer is filled or emptied.  (You could define a
one-character buffer to force the manager to be invoked for each byte, but
that would be rather inefficient.)  The buffer's size and location are
controlled by the manager, not by the library.  For example, the memory
source manager just makes the buffer pointer and length point to the original
data in memory.  In this case the buffer-reload procedure will be invoked
only if the decompressor ran off the end of the datastream, which would
indicate an erroneous datastream.

The work buffer is defined as an array of datatype JOCTET, which is generally
"char" or "unsigned char".  On a machine where char is not exactly 8 bits
wide, you must define JOCTET as a wider data type and then modify the data
source and destination modules to transcribe the work arrays into 8-bit units
on external storage.

A data destination manager struct contains a pointer and count defining the
next byte to write in the work buffer and the remaining free space:

JOCTET *next_output_byte;   /* => next byte to write in buffer */
        size_t free_in_buffer;      /* # of byte spaces remaining in buffer */

The library increments the pointer and decrements the count until the buffer
is filled.  The manager's empty_output_buffer method must reset the pointer
and count.  The manager is expected to remember the buffer's starting address
and total size in private fields not visible to the library.

A data destination manager provides three methods:

init_destination (j_compress_ptr cinfo)
        Initialize destination.  This is called by jpeg_start_compress()
        before any data is actually written.  It must initialize
        next_output_byte and free_in_buffer.  free_in_buffer must be
        initialized to a positive value.

empty_output_buffer (j_compress_ptr cinfo)
        This is called whenever the buffer has filled (free_in_buffer
        reaches zero).  In typical applications, it should write out the
        *entire* buffer (use the saved start address and buffer length;
        ignore the current state of next_output_byte and free_in_buffer).
        Then reset the pointer & count to the start of the buffer, and
        return TRUE indicating that the buffer has been dumped.
        free_in_buffer must be set to a positive value when TRUE is
        returned.  A FALSE return should only be used when I/O suspension is
        desired (this operating mode is discussed in the next section).

term_destination (j_compress_ptr cinfo)
        Terminate destination --- called by jpeg_finish_compress() after all
        data has been written.  In most applications, this must flush any
        data remaining in the buffer.  Use either next_output_byte or
        free_in_buffer to determine how much data is in the buffer.

term_destination() is NOT called by jpeg_abort() or jpeg_destroy().  If you
want the destination manager to be cleaned up during an abort, you must do it
yourself.

You will also need code to create a jpeg_destination_mgr struct, fill in its
method pointers, and insert a pointer to the struct into the "dest" field of
the JPEG compression object.  This can be done in-line in your setup code if
you like, but it's probably cleaner to provide a separate routine similar to
the jpeg_stdio_dest() or jpeg_mem_dest() routines of the supplied destination
managers.

Decompression source managers follow a parallel design, but with some
additional frammishes.  The source manager struct contains a pointer and count
defining the next byte to read from the work buffer and the number of bytes
remaining:

const JOCTET *next_input_byte;  /* => next byte to read from buffer */
        size_t bytes_in_buffer;         /* # of bytes remaining in buffer */

The library increments the pointer and decrements the count until the buffer
is emptied.  The manager's fill_input_buffer method must reset the pointer and
count.  In most applications, the manager must remember the buffer's starting
address and total size in private fields not visible to the library.

A data source manager provides five methods:

init_source (j_decompress_ptr cinfo)
        Initialize source.  This is called by jpeg_read_header() before any
        data is actually read.  Unlike init_destination(), it may leave
        bytes_in_buffer set to 0 (in which case a fill_input_buffer() call
        will occur immediately).

fill_input_buffer (j_decompress_ptr cinfo)
        This is called whenever bytes_in_buffer has reached zero and more
        data is wanted.  In typical applications, it should read fresh data
        into the buffer (ignoring the current state of next_input_byte and
        bytes_in_buffer), reset the pointer & count to the start of the
        buffer, and return TRUE indicating that the buffer has been reloaded.
        It is not necessary to fill the buffer entirely, only to obtain at
        least one more byte.  bytes_in_buffer MUST be set to a positive value
        if TRUE is returned.  A FALSE return should only be used when I/O
        suspension is desired (this mode is discussed in the next section).

skip_input_data (j_decompress_ptr cinfo, long num_bytes)
        Skip num_bytes worth of data.  The buffer pointer and count should
        be advanced over num_bytes input bytes, refilling the buffer as
        needed.  This is used to skip over a potentially large amount of
        uninteresting data (such as an APPn marker).  In some applications
        it may be possible to optimize away the reading of the skipped data,
        but it's not clear that being smart is worth much trouble; large
        skips are uncommon.  bytes_in_buffer may be zero on return.
        A zero or negative skip count should be treated as a no-op.

resync_to_restart (j_decompress_ptr cinfo, int desired)
        This routine is called only when the decompressor has failed to find
        a restart (RSTn) marker where one is expected.  Its mission is to
        find a suitable point for resuming decompression.  For most
        applications, we recommend that you just use the default resync
        procedure, jpeg_resync_to_restart().  However, if you are able to back
        up in the input data stream, or if you have a-priori knowledge about
        the likely location of restart markers, you may be able to do better.
        Read the read_restart_marker() and jpeg_resync_to_restart() routines
        in jdmarker.c if you think you'd like to implement your own resync
        procedure.

term_source (j_decompress_ptr cinfo)
        Terminate source --- called by jpeg_finish_decompress() after all
        data has been read.  Often a no-op.

For both fill_input_buffer() and skip_input_data(), there is no such thing
as an EOF return.  If the end of the file has been reached, the routine has
a choice of exiting via ERREXIT() or inserting fake data into the buffer.
In most cases, generating a warning message and inserting a fake EOI marker
is the best course of action --- this will allow the decompressor to output
however much of the image is there.  In pathological cases, the decompressor
may swallow the EOI and again demand data ... just keep feeding it fake EOIs.
jdatasrc.c illustrates the recommended error recovery behavior.

term_source() is NOT called by jpeg_abort() or jpeg_destroy().  If you want
the source manager to be cleaned up during an abort, you must do it yourself.

You will also need code to create a jpeg_source_mgr struct, fill in its method
pointers, and insert a pointer to the struct into the "src" field of the JPEG
decompression object.  This can be done in-line in your setup code if you
like, but it's probably cleaner to provide a separate routine similar to the
jpeg_stdio_src() or jpeg_mem_src() routines of the supplied source managers.

For more information, consult the memory and stdio source and destination
managers in jdatasrc.c and jdatadst.c.

I/O suspension
--------------

Some applications need to use the JPEG library as an incremental memory-to-
memory filter: when the compressed data buffer is filled or emptied, they want
control to return to the outer loop, rather than expecting that the buffer can
be emptied or reloaded within the data source/destination manager subroutine.
The library supports this need by providing an "I/O suspension" mode, which we
describe in this section.

The I/O suspension mode is not a panacea: nothing is guaranteed about the
maximum amount of time spent in any one call to the library, so it will not
eliminate response-time problems in single-threaded applications.  If you
need guaranteed response time, we suggest you "bite the bullet" and implement
a real multi-tasking capability.

To use I/O suspension, cooperation is needed between the calling application
and the data source or destination manager; you will always need a custom
source/destination manager.  (Please read the previous section if you haven't
already.)  The basic idea is that the empty_output_buffer() or
fill_input_buffer() routine is a no-op, merely returning FALSE to indicate
that it has done nothing.  Upon seeing this, the JPEG library suspends
operation and returns to its caller.  The surrounding application is
responsible for emptying or refilling the work buffer before calling the
JPEG library again.

Compression suspension:

For compression suspension, use an empty_output_buffer() routine that returns
FALSE; typically it will not do anything else.  This will cause the
compressor to return to the caller of jpeg_write_scanlines(), with the return
value indicating that not all the supplied scanlines have been accepted.
The application must make more room in the output buffer, adjust the output
buffer pointer/count appropriately, and then call jpeg_write_scanlines()
again, pointing to the first unconsumed scanline.

When forced to suspend, the compressor will backtrack to a convenient stopping
point (usually the start of the current MCU); it will regenerate some output
data when restarted.  Therefore, although empty_output_buffer() is only
called when the buffer is filled, you should NOT write out the entire buffer
after a suspension.  Write only the data up to the current position of
next_output_byte/free_in_buffer.  The data beyond that point will be
regenerated after resumption.

Because of the backtracking behavior, a good-size output buffer is essential
for efficiency; you don't want the compressor to suspend often.  (In fact, an
overly small buffer could lead to infinite looping, if a single MCU required
more data than would fit in the buffer.)  We recommend a buffer of at least
several Kbytes.  You may want to insert explicit code to ensure that you don't
call jpeg_write_scanlines() unless there is a reasonable amount of space in
the output buffer; in other words, flush the buffer before trying to compress
more data.

The compressor does not allow suspension while it is trying to write JPEG
markers at the beginning and end of the file.  This means that:
  * At the beginning of a compression operation, there must be enough free
    space in the output buffer to hold the header markers (typically 600 or
    so bytes).  The recommended buffer size is bigger than this anyway, so
    this is not a problem as long as you start with an empty buffer.  However,
    this restriction might catch you if you insert large special markers, such
    as a JFIF thumbnail image, without flushing the buffer afterwards.
  * When you call jpeg_finish_compress(), there must be enough space in the
    output buffer to emit any buffered data and the final EOI marker.  In the
    current implementation, half a dozen bytes should suffice for this, but
    for safety's sake we recommend ensuring that at least 100 bytes are free
    before calling jpeg_finish_compress().

A more significant restriction is that jpeg_finish_compress() cannot suspend.
This means you cannot use suspension with multi-pass operating modes, namely
Huffman code optimization and multiple-scan output.  Those modes write the
whole file during jpeg_finish_compress(), which will certainly result in
buffer overrun.  (Note that this restriction applies only to compression,
not decompression.  The decompressor supports input suspension in all of its
operating modes.)

Decompression suspension:

For decompression suspension, use a fill_input_buffer() routine that simply
returns FALSE (except perhaps during error recovery, as discussed below).
This will cause the decompressor to return to its caller with an indication
that suspension has occurred.  This can happen at four places:
  * jpeg_read_header(): will return JPEG_SUSPENDED.
  * jpeg_start_decompress(): will return FALSE, rather than its usual TRUE.
  * jpeg_read_scanlines(): will return the number of scanlines already
        completed (possibly 0).
  * jpeg_finish_decompress(): will return FALSE, rather than its usual TRUE.
The surrounding application must recognize these cases, load more data into
the input buffer, and repeat the call.  In the case of jpeg_read_scanlines(),
increment the passed pointers past any scanlines successfully read.

Just as with compression, the decompressor will typically backtrack to a
convenient restart point before suspending.  When fill_input_buffer() is
called, next_input_byte/bytes_in_buffer point to the current restart point,
which is where the decompressor will backtrack to if FALSE is returned.
The data beyond that position must NOT be discarded if you suspend; it needs
to be re-read upon resumption.  In most implementations, you'll need to shift
this data down to the start of your work buffer and then load more data after
it.  Again, this behavior means that a several-Kbyte work buffer is essential
for decent performance; furthermore, you should load a reasonable amount of
new data before resuming decompression.  (If you loaded, say, only one new
byte each time around, you could waste a LOT of cycles.)

The skip_input_data() source manager routine requires special care in a
suspension scenario.  This routine is NOT granted the ability to suspend the
decompressor; it can decrement bytes_in_buffer to zero, but no more.  If the
requested skip distance exceeds the amount of data currently in the input
buffer, then skip_input_data() must set bytes_in_buffer to zero and record the
additional skip distance somewhere else.  The decompressor will immediately
call fill_input_buffer(), which should return FALSE, which will cause a
suspension return.  The surrounding application must then arrange to discard
the recorded number of bytes before it resumes loading the input buffer.
(Yes, this design is rather baroque, but it avoids complexity in the far more
common case where a non-suspending source manager is used.)

If the input data has been exhausted, we recommend that you emit a warning
and insert dummy EOI markers just as a non-suspending data source manager
would do.  This can be handled either in the surrounding application logic or
within fill_input_buffer(); the latter is probably more efficient.  If
fill_input_buffer() knows that no more data is available, it can set the
pointer/count to point to a dummy EOI marker and then return TRUE just as
though it had read more data in a non-suspending situation.

The decompressor does not attempt to suspend within standard JPEG markers;
instead it will backtrack to the start of the marker and reprocess the whole
marker next time.  Hence the input buffer must be large enough to hold the
longest standard marker in the file.  Standard JPEG markers should normally
not exceed a few hundred bytes each (DHT tables are typically the longest).
We recommend at least a 2K buffer for performance reasons, which is much
larger than any correct marker is likely to be.  For robustness against
damaged marker length counts, you may wish to insert a test in your
application for the case that the input buffer is completely full and yet
the decoder has suspended without consuming any data --- otherwise, if this
situation did occur, it would lead to an endless loop.  (The library can't
provide this test since it has no idea whether "the buffer is full", or
even whether there is a fixed-size input buffer.)

The input buffer would need to be 64K to allow for arbitrary COM or APPn
markers, but these are handled specially: they are either saved into allocated
memory, or skipped over by calling skip_input_data().  In the former case,
suspension is handled correctly, and in the latter case, the problem of
buffer overrun is placed on skip_input_data's shoulders, as explained above.
Note that if you provide your own marker handling routine for large markers,
you should consider how to deal with buffer overflow.

Multiple-buffer management:

In some applications it is desirable to store the compressed data in a linked
list of buffer areas, so as to avoid data copying.  This can be handled by
having empty_output_buffer() or fill_input_buffer() set the pointer and count
to reference the next available buffer; FALSE is returned only if no more
buffers are available.  Although seemingly straightforward, there is a
pitfall in this approach: the backtrack that occurs when FALSE is returned
could back up into an earlier buffer.  For example, when fill_input_buffer()
is called, the current pointer & count indicate the backtrack restart point.
Since fill_input_buffer() will set the pointer and count to refer to a new
buffer, the restart position must be saved somewhere else.  Suppose a second
call to fill_input_buffer() occurs in the same library call, and no
additional input data is available, so fill_input_buffer must return FALSE.
If the JPEG library has not moved the pointer/count forward in the current
buffer, then *the correct restart point is the saved position in the prior
buffer*.  Prior buffers may be discarded only after the library establishes
a restart point within a later buffer.  Similar remarks apply for output into
a chain of buffers.

The library will never attempt to backtrack over a skip_input_data() call,
so any skipped data can be permanently discarded.  You still have to deal
with the case of skipping not-yet-received data, however.

It's much simpler to use only a single buffer; when fill_input_buffer() is
called, move any unconsumed data (beyond the current pointer/count) down to
the beginning of this buffer and then load new data into the remaining buffer
space.  This approach requires a little more data copying but is far easier
to get right.

Progressive JPEG support
------------------------

Progressive JPEG rearranges the stored data into a series of scans of
increasing quality.  In situations where a JPEG file is transmitted across a
slow communications link, a decoder can generate a low-quality image very
quickly from the first scan, then gradually improve the displayed quality as
more scans are received.  The final image after all scans are complete is
identical to that of a regular (sequential) JPEG file of the same quality
setting.  Progressive JPEG files are often slightly smaller than equivalent
sequential JPEG files, but the possibility of incremental display is the main
reason for using progressive JPEG.

The IJG encoder library generates progressive JPEG files when given a
suitable "scan script" defining how to divide the data into scans.
Creation of progressive JPEG files is otherwise transparent to the encoder.
Progressive JPEG files can also be read transparently by the decoder library.
If the decoding application simply uses the library as defined above, it
will receive a final decoded image without any indication that the file was
progressive.  Of course, this approach does not allow incremental display.
To perform incremental display, an application needs to use the decoder
library's "buffered-image" mode, in which it receives a decoded image
multiple times.

Each displayed scan requires about as much work to decode as a full JPEG
image of the same size, so the decoder must be fairly fast in relation to the
data transmission rate in order to make incremental display useful.  However,
it is possible to skip displaying the image and simply add the incoming bits
to the decoder's coefficient buffer.  This is fast because only Huffman
decoding need be done, not IDCT, upsampling, colorspace conversion, etc.
The IJG decoder library allows the application to switch dynamically between
displaying the image and simply absorbing the incoming bits.  A properly
coded application can automatically adapt the number of display passes to
suit the time available as the image is received.  Also, a final
higher-quality display cycle can be performed from the buffered data after
the end of the file is reached.

Progressive compression:

To create a progressive JPEG file (or a multiple-scan sequential JPEG file),
set the scan_info cinfo field to point to an array of scan descriptors, and
perform compression as usual.  Instead of constructing your own scan list,
you can call the jpeg_simple_progression() helper routine to create a
recommended progression sequence; this method should be used by all
applications that don't want to get involved in the nitty-gritty of
progressive scan sequence design.  (If you want to provide user control of
scan sequences, you may wish to borrow the scan script reading code found
in rdswitch.c, so that you can read scan script files just like cjpeg's.)
When scan_info is not NULL, the compression library will store DCT'd data
into a buffer array as jpeg_write_scanlines() is called, and will emit all
the requested scans during jpeg_finish_compress().  This implies that
multiple-scan output cannot be created with a suspending data destination
manager, since jpeg_finish_compress() does not support suspension.  We
should also note that the compressor currently forces Huffman optimization
mode when creating a progressive JPEG file, because the default Huffman
tables are unsuitable for progressive files.

Progressive decompression:

When buffered-image mode is not used, the decoder library will read all of
a multi-scan file during jpeg_start_decompress(), so that it can provide a
final decoded image.  (Here "multi-scan" means either progressive or
multi-scan sequential.)  This makes multi-scan files transparent to the
decoding application.  However, existing applications that used suspending
input with version 5 of the IJG library will need to be modified to check
for a suspension return from jpeg_start_decompress().

To perform incremental display, an application must use the library's
buffered-image mode.  This is described in the next section.

Buffered-image mode
-------------------

In buffered-image mode, the library stores the partially decoded image in a
coefficient buffer, from which it can be read out as many times as desired.
This mode is typically used for incremental display of progressive JPEG files,
but it can be used with any JPEG file.  Each scan of a progressive JPEG file
adds more data (more detail) to the buffered image.  The application can
display in lockstep with the source file (one display pass per input scan),
or it can allow input processing to outrun display processing.  By making
input and display processing run independently, it is possible for the
application to adapt progressive display to a wide range of data transmission
rates.

The basic control flow for buffered-image decoding is

jpeg_create_decompress()
        set data source
        jpeg_read_header()
        set overall decompression parameters
        cinfo.buffered_image = TRUE;    /* select buffered-image mode */
        jpeg_start_decompress()
        for (each output pass) {
            adjust output decompression parameters if required
            jpeg_start_output()         /* start a new output pass */
            for (all scanlines in image) {
                jpeg_read_scanlines()
                display scanlines
            }
            jpeg_finish_output()        /* terminate output pass */
        }
        jpeg_finish_decompress()
        jpeg_destroy_decompress()

This differs from ordinary unbuffered decoding in that there is an additional
level of looping.  The application can choose how many output passes to make
and how to display each pass.

The simplest approach to displaying progressive images is to do one display
pass for each scan appearing in the input file.  In this case the outer loop
condition is typically
        while (!jpeg_input_complete(&cinfo))
and the start-output call should read
        jpeg_start_output(&cinfo, cinfo.input_scan_number);
The second parameter to jpeg_start_output() indicates which scan of the input
file is to be displayed; the scans are numbered starting at 1 for this
purpose.  (You can use a loop counter starting at 1 if you like, but using
the library's input scan counter is easier.)  The library automatically reads
data as necessary to complete each requested scan, and jpeg_finish_output()
advances to the next scan or end-of-image marker (hence input_scan_number
will be incremented by the time control arrives back at jpeg_start_output()).
With this technique, data is read from the input file only as needed, and
input and output processing run in lockstep.

After reading the final scan and reaching the end of the input file, the
buffered image remains available; it can be read additional times by
repeating the jpeg_start_output()/jpeg_read_scanlines()/jpeg_finish_output()
sequence.  For example, a useful technique is to use fast one-pass color
quantization for display passes made while the image is arriving, followed by
a final display pass using two-pass quantization for highest quality.  This
is done by changing the library parameters before the final output pass.
Changing parameters between passes is discussed in detail below.

In general the last scan of a progressive file cannot be recognized as such
until after it is read, so a post-input display pass is the best approach if
you want special processing in the final pass.

When done with the image, be sure to call jpeg_finish_decompress() to release
the buffered image (or just use jpeg_destroy_decompress()).

If input data arrives faster than it can be displayed, the application can
cause the library to decode input data in advance of what's needed to produce
output.  This is done by calling the routine jpeg_consume_input().
The return value is one of the following:
        JPEG_REACHED_SOS:    reached an SOS marker (the start of a new scan)
        JPEG_REACHED_EOI:    reached the EOI marker (end of image)
        JPEG_ROW_COMPLETED:  completed reading one MCU row of compressed data
        JPEG_SCAN_COMPLETED: completed reading last MCU row of current scan
        JPEG_SUSPENDED:      suspended before completing any of the above
(JPEG_SUSPENDED can occur only if a suspending data source is used.)  This
routine can be called at any time after initializing the JPEG object.  It
reads some additional data and returns when one of the indicated significant
events occurs.  (If called after the EOI marker is reached, it will
immediately return JPEG_REACHED_EOI without attempting to read more data.)

The library's output processing will automatically call jpeg_consume_input()
whenever the output processing overtakes the input; thus, simple lockstep
display requires no direct calls to jpeg_consume_input().  But by adding
calls to jpeg_consume_input(), you can absorb data in advance of what is
being displayed.  This has two benefits:
  * You can limit buildup of unprocessed data in your input buffer.
  * You can eliminate extra display passes by paying attention to the
    state of the library's input processing.

The first of these benefits only requires interspersing calls to
jpeg_consume_input() with your display operations and any other processing
you may be doing.  To avoid wasting cycles due to backtracking, it's best to
call jpeg_consume_input() only after a hundred or so new bytes have arrived.
This is discussed further under "I/O suspension", above.  (Note: the JPEG
library currently is not thread-safe.  You must not call jpeg_consume_input()
from one thread of control if a different library routine is working on the
same JPEG object in another thread.)

When input arrives fast enough that more than one new scan is available
before you start a new output pass, you may as well skip the output pass
corresponding to the completed scan.  This occurs for free if you pass
cinfo.input_scan_number as the target scan number to jpeg_start_output().
The input_scan_number field is simply the index of the scan currently being
consumed by the input processor.  You can ensure that this is up-to-date by
emptying the input buffer just before calling jpeg_start_output(): call
jpeg_consume_input() repeatedly until it returns JPEG_SUSPENDED or
JPEG_REACHED_EOI.

The target scan number passed to jpeg_start_output() is saved in the
cinfo.output_scan_number field.  The library's output processing calls
jpeg_consume_input() whenever the current input scan number and row within
that scan is less than or equal to the current output scan number and row.
Thus, input processing can "get ahead" of the output processing but is not
allowed to "fall behind".  You can achieve several different effects by
manipulating this interlock rule.  For example, if you pass a target scan
number greater than the current input scan number, the output processor will
wait until that scan starts to arrive before producing any output.  (To avoid
an infinite loop, the target scan number is automatically reset to the last
scan number when the end of image is reached.  Thus, if you specify a large
target scan number, the library will just absorb the entire input file and
then perform an output pass.  This is effectively the same as what
jpeg_start_decompress() does when you don't select buffered-image mode.)
When you pass a target scan number equal to the current input scan number,
the image is displayed no faster than the current input scan arrives.  The
final possibility is to pass a target scan number less than the current input
scan number; this disables the input/output interlock and causes the output
processor to simply display whatever it finds in the image buffer, without
waiting for input.  (However, the library will not accept a target scan
number less than one, so you can't avoid waiting for the first scan.)

When data is arriving faster than the output display processing can advance
through the image, jpeg_consume_input() will store data into the buffered
image beyond the point at which the output processing is reading data out
again.  If the input arrives fast enough, it may "wrap around" the buffer to
the point where the input is more than one whole scan ahead of the output.
If the output processing simply proceeds through its display pass without
paying attention to the input, the effect seen on-screen is that the lower
part of the image is one or more scans better in quality than the upper part.
Then, when the next output scan is started, you have a choice of what target
scan number to use.  The recommended choice is to use the current input scan
number at that time, which implies that you've skipped the output scans
corresponding to the input scans that were completed while you processed the
previous output scan.  In this way, the decoder automatically adapts its
speed to the arriving data, by skipping output scans as necessary to keep up
with the arriving data.

When using this strategy, you'll want to be sure that you perform a final
output pass after receiving all the data; otherwise your last display may not
be full quality across the whole screen.  So the right outer loop logic is
something like this:
        do {
            absorb any waiting input by calling jpeg_consume_input()
            final_pass = jpeg_input_complete(&cinfo);
            adjust output decompression parameters if required
            jpeg_start_output(&cinfo, cinfo.input_scan_number);
            ...
            jpeg_finish_output()
        } while (!final_pass);
rather than quitting as soon as jpeg_input_complete() returns TRUE.  This
arrangement makes it simple to use higher-quality decoding parameters
for the final pass.  But if you don't want to use special parameters for
the final pass, the right loop logic is like this:
        for (;;) {
            absorb any waiting input by calling jpeg_consume_input()
            jpeg_start_output(&cinfo, cinfo.input_scan_number);
            ...
            jpeg_finish_output()
            if (jpeg_input_complete(&cinfo) &&
                cinfo.input_scan_number == cinfo.output_scan_number)
              break;
        }
In this case you don't need to know in advance whether an output pass is to
be the last one, so it's not necessary to have reached EOF before starting
the final output pass; rather, what you want to test is whether the output
pass was performed in sync with the final input scan.  This form of the loop
will avoid an extra output pass whenever the decoder is able (or nearly able)
to keep up with the incoming data.

When the data transmission speed is high, you might begin a display pass,
then find that much or all of the file has arrived before you can complete
the pass.  (You can detect this by noting the JPEG_REACHED_EOI return code
from jpeg_consume_input(), or equivalently by testing jpeg_input_complete().)
In this situation you may wish to abort the current display pass and start a
new one using the newly arrived information.  To do so, just call
jpeg_finish_output() and then start a new pass with jpeg_start_output().

A variant strategy is to abort and restart display if more than one complete
scan arrives during an output pass; this can be detected by noting
JPEG_REACHED_SOS returns and/or examining cinfo.input_scan_number.  This
idea should be employed with caution, however, since the display process
might never get to the bottom of the image before being aborted, resulting
in the lower part of the screen being several passes worse than the upper.
In most cases it's probably best to abort an output pass only if the whole
file has arrived and you want to begin the final output pass immediately.

When receiving data across a communication link, we recommend always using
the current input scan number for the output target scan number; if a
higher-quality final pass is to be done, it should be started (aborting any
incomplete output pass) as soon as the end of file is received.  However,
many other strategies are possible.  For example, the application can examine
the parameters of the current input scan and decide whether to display it or
not.  If the scan contains only chroma data, one might choose not to use it
as the target scan, expecting that the scan will be small and will arrive
quickly.  To skip to the next scan, call jpeg_consume_input() until it
returns JPEG_REACHED_SOS or JPEG_REACHED_EOI.  Or just use the next higher
number as the target scan for jpeg_start_output(); but that method doesn't
let you inspect the next scan's parameters before deciding to display it.

In buffered-image mode, jpeg_start_decompress() never performs input and
thus never suspends.  An application that uses input suspension with
buffered-image mode must be prepared for suspension returns from these
routines:
* jpeg_start_output() performs input only if you request 2-pass quantization
  and the target scan isn't fully read yet.  (This is discussed below.)
* jpeg_read_scanlines(), as always, returns the number of scanlines that it
  was able to produce before suspending.
* jpeg_finish_output() will read any markers following the target scan,
  up to the end of the file or the SOS marker that begins another scan.
  (But it reads no input if jpeg_consume_input() has already reached the
  end of the file or a SOS marker beyond the target output scan.)
* jpeg_finish_decompress() will read until the end of file, and thus can
  suspend if the end hasn't already been reached (as can be tested by
  calling jpeg_input_complete()).
jpeg_start_output(), jpeg_finish_output(), and jpeg_finish_decompress()
all return TRUE if they completed their tasks, FALSE if they had to suspend.
In the event of a FALSE return, the application must load more input data
and repeat the call.  Applications that use non-suspending data sources need
not check the return values of these three routines.

It is possible to change decoding parameters between output passes in the
buffered-image mode.  The decoder library currently supports only very
limited changes of parameters.  ONLY THE FOLLOWING parameter changes are
allowed after jpeg_start_decompress() is called:
* dct_method can be changed before each call to jpeg_start_output().
  For example, one could use a fast DCT method for early scans, changing
  to a higher quality method for the final scan.
* dither_mode can be changed before each call to jpeg_start_output();
  of course this has no impact if not using color quantization.  Typically
  one would use ordered dither for initial passes, then switch to
  Floyd-Steinberg dither for the final pass.  Caution: changing dither mode
  can cause more memory to be allocated by the library.  Although the amount
  of memory involved is not large (a scanline or so), it may cause the
  initial max_memory_to_use specification to be exceeded, which in the worst
  case would result in an out-of-memory failure.
* do_block_smoothing can be changed before each call to jpeg_start_output().
  This setting is relevant only when decoding a progressive JPEG image.
  During the first DC-only scan, block smoothing provides a very "fuzzy" look
  instead of the very "blocky" look seen without it; which is better seems a
  matter of personal taste.  But block smoothing is nearly always a win
  during later stages, especially when decoding a successive-approximation
  image: smoothing helps to hide the slight blockiness that otherwise shows
  up on smooth gradients until the lowest coefficient bits are sent.
* Color quantization mode can be changed under the rules described below.
  You *cannot* change between full-color and quantized output (because that
  would alter the required I/O buffer sizes), but you can change which
  quantization method is used.

When generating color-quantized output, changing quantization method is a
very useful way of switching between high-speed and high-quality display.
The library allows you to change among its three quantization methods:
1. Single-pass quantization to a fixed color cube.
   Selected by cinfo.two_pass_quantize = FALSE and cinfo.colormap = NULL.
2. Single-pass quantization to an application-supplied colormap.
   Selected by setting cinfo.colormap to point to the colormap (the value of
   two_pass_quantize is ignored); also set cinfo.actual_number_of_colors.
3. Two-pass quantization to a colormap chosen specifically for the image.
   Selected by cinfo.two_pass_quantize = TRUE and cinfo.colormap = NULL.
   (This is the default setting selected by jpeg_read_header, but it is
   probably NOT what you want for the first pass of progressive display!)
These methods offer successively better quality and lesser speed.  However,
only the first method is available for quantizing in non-RGB color spaces.

IMPORTANT: because the different quantizer methods have very different
working-storage requirements, the library requires you to indicate which
one(s) you intend to use before you call jpeg_start_decompress().  (If we did
not require this, the max_memory_to_use setting would be a complete fiction.)
You do this by setting one or more of these three cinfo fields to TRUE:
        enable_1pass_quant              Fixed color cube colormap
        enable_external_quant           Externally-supplied colormap
        enable_2pass_quant              Two-pass custom colormap
All three are initialized FALSE by jpeg_read_header().  But
jpeg_start_decompress() automatically sets TRUE the one selected by the
current two_pass_quantize and colormap settings, so you only need to set the
enable flags for any other quantization methods you plan to change to later.

After setting the enable flags correctly at jpeg_start_decompress() time, you
can change to any enabled quantization method by setting two_pass_quantize
and colormap properly just before calling jpeg_start_output().  The following
special rules apply:
1. You must explicitly set cinfo.colormap to NULL when switching to 1-pass
   or 2-pass mode from a different mode, or when you want the 2-pass
   quantizer to be re-run to generate a new colormap.
2. To switch to an external colormap, or to change to a different external
   colormap than was used on the prior pass, you must call
   jpeg_new_colormap() after setting cinfo.colormap.
NOTE: if you want to use the same colormap as was used in the prior pass,
you should not do either of these things.  This will save some nontrivial
switchover costs.
(These requirements exist because cinfo.colormap will always be non-NULL
after completing a prior output pass, since both the 1-pass and 2-pass
quantizers set it to point to their output colormaps.  Thus you have to
do one of these two things to notify the library that something has changed.
Yup, it's a bit klugy, but it's necessary to do it this way for backwards
compatibility.)

Note that in buffered-image mode, the library generates any requested colormap
during jpeg_start_output(), not during jpeg_start_decompress().

When using two-pass quantization, jpeg_start_output() makes a pass over the
buffered image to determine the optimum color map; it therefore may take a
significant amount of time, whereas ordinarily it does little work.  The
progress monitor hook is called during this pass, if defined.  It is also
important to realize that if the specified target scan number is greater than
or equal to the current input scan number, jpeg_start_output() will attempt
to consume input as it makes this pass.  If you use a suspending data source,
you need to check for a FALSE return from jpeg_start_output() under these
conditions.  The combination of 2-pass quantization and a not-yet-fully-read
target scan is the only case in which jpeg_start_output() will consume input.

Application authors who support buffered-image mode may be tempted to use it
for all JPEG images, even single-scan ones.  This will work, but it is
inefficient: there is no need to create an image-sized coefficient buffer for
single-scan images.  Requesting buffered-image mode for such an image wastes
memory.  Worse, it can cost time on large images, since the buffered data has
to be swapped out or written to a temporary file.  If you are concerned about
maximum performance on baseline JPEG files, you should use buffered-image
mode only when the incoming file actually has multiple scans.  This can be
tested by calling jpeg_has_multiple_scans(), which will return a correct
result at any time after jpeg_read_header() completes.

It is also worth noting that when you use jpeg_consume_input() to let input
processing get ahead of output processing, the resulting pattern of access to
the coefficient buffer is quite nonsequential.  It's best to use the memory
manager jmemnobs.c if you can (ie, if you have enough real or virtual main
memory).  If not, at least make sure that max_memory_to_use is set as high as
possible.  If the JPEG memory manager has to use a temporary file, you will
probably see a lot of disk traffic and poor performance.  (This could be
improved with additional work on the memory manager, but we haven't gotten
around to it yet.)

In some applications it may be convenient to use jpeg_consume_input() for all
input processing, including reading the initial markers; that is, you may
wish to call jpeg_consume_input() instead of jpeg_read_header() during
startup.  This works, but note that you must check for JPEG_REACHED_SOS and
JPEG_REACHED_EOI return codes as the equivalent of jpeg_read_header's codes.
Once the first SOS marker has been reached, you must call
jpeg_start_decompress() before jpeg_consume_input() will consume more input;
it'll just keep returning JPEG_REACHED_SOS until you do.  If you read a
tables-only file this way, jpeg_consume_input() will return JPEG_REACHED_EOI
without ever returning JPEG_REACHED_SOS; be sure to check for this case.
If this happens, the decompressor will not read any more input until you call
jpeg_abort() to reset it.  It is OK to call jpeg_consume_input() even when not
using buffered-image mode, but in that case it's basically a no-op after the
initial markers have been read: it will just return JPEG_SUSPENDED.

Abbreviated datastreams and multiple images
-------------------------------------------

A JPEG compression or decompression object can be reused to process multiple
images.  This saves a small amount of time per image by eliminating the
"create" and "destroy" operations, but that isn't the real purpose of the
feature.  Rather, reuse of an object provides support for abbreviated JPEG
datastreams.  Object reuse can also simplify processing a series of images in
a single input or output file.  This section explains these features.

A JPEG file normally contains several hundred bytes worth of quantization
and Huffman tables.  In a situation where many images will be stored or
transmitted with identical tables, this may represent an annoying overhead.
The JPEG standard therefore permits tables to be omitted.  The standard
defines three classes of JPEG datastreams:
  * "Interchange" datastreams contain an image and all tables needed to decode
     the image.  These are the usual kind of JPEG file.
  * "Abbreviated image" datastreams contain an image, but are missing some or
    all of the tables needed to decode that image.
  * "Abbreviated table specification" (henceforth "tables-only") datastreams
    contain only table specifications.
To decode an abbreviated image, it is necessary to load the missing table(s)
into the decoder beforehand.  This can be accomplished by reading a separate
tables-only file.  A variant scheme uses a series of images in which the first
image is an interchange (complete) datastream, while subsequent ones are
abbreviated and rely on the tables loaded by the first image.  It is assumed
that once the decoder has read a table, it will remember that table until a
new definition for the same table number is encountered.

It is the application designer's responsibility to figure out how to associate
the correct tables with an abbreviated image.  While abbreviated datastreams
can be useful in a closed environment, their use is strongly discouraged in
any situation where data exchange with other applications might be needed.
Caveat designer.

The JPEG library provides support for reading and writing any combination of
tables-only datastreams and abbreviated images.  In both compression and
decompression objects, a quantization or Huffman table will be retained for
the lifetime of the object, unless it is overwritten by a new table definition.

To create abbreviated image datastreams, it is only necessary to tell the
compressor not to emit some or all of the tables it is using.  Each
quantization and Huffman table struct contains a boolean field "sent_table",
which normally is initialized to FALSE.  For each table used by the image, the
header-writing process emits the table and sets sent_table = TRUE unless it is
already TRUE.  (In normal usage, this prevents outputting the same table
definition multiple times, as would otherwise occur because the chroma
components typically share tables.)  Thus, setting this field to TRUE before
calling jpeg_start_compress() will prevent the table from being written at
all.

If you want to create a "pure" abbreviated image file containing no tables,
just call "jpeg_suppress_tables(&cinfo, TRUE)" after constructing all the
tables.  If you want to emit some but not all tables, you'll need to set the
individual sent_table fields directly.

To create an abbreviated image, you must also call jpeg_start_compress()
with a second parameter of FALSE, not TRUE.  Otherwise jpeg_start_compress()
will force all the sent_table fields to FALSE.  (This is a safety feature to
prevent abbreviated images from being created accidentally.)

To create a tables-only file, perform the same parameter setup that you
normally would, but instead of calling jpeg_start_compress() and so on, call
jpeg_write_tables(&cinfo).  This will write an abbreviated datastream
containing only SOI, DQT and/or DHT markers, and EOI.  All the quantization
and Huffman tables that are currently defined in the compression object will
be emitted unless their sent_tables flag is already TRUE, and then all the
sent_tables flags will be set TRUE.

A sure-fire way to create matching tables-only and abbreviated image files
is to proceed as follows:

create JPEG compression object
        set JPEG parameters
        set destination to tables-only file
        jpeg_write_tables(&cinfo);
        set destination to image file
        jpeg_start_compress(&cinfo, FALSE);
        write data...
        jpeg_finish_compress(&cinfo);

Since the JPEG parameters are not altered between writing the table file and
the abbreviated image file, the same tables are sure to be used.  Of course,
you can repeat the jpeg_start_compress() ... jpeg_finish_compress() sequence
many times to produce many abbreviated image files matching the table file.

You cannot suppress output of the computed Huffman tables when Huffman
optimization is selected.  (If you could, there'd be no way to decode the
image...)  Generally, you don't want to set optimize_coding = TRUE when
you are trying to produce abbreviated files.

In some cases you might want to compress an image using tables which are
not stored in the application, but are defined in an interchange or
tables-only file readable by the application.  This can be done by setting up
a JPEG decompression object to read the specification file, then copying the
tables into your compression object.  See jpeg_copy_critical_parameters()
for an example of copying quantization tables.

To read abbreviated image files, you simply need to load the proper tables
into the decompression object before trying to read the abbreviated image.
If the proper tables are stored in the application program, you can just
allocate the table structs and fill in their contents directly.  For example,
to load a fixed quantization table into table slot "n":

if (cinfo.quant_tbl_ptrs[n] == NULL)
      cinfo.quant_tbl_ptrs[n] = jpeg_alloc_quant_table((j_common_ptr) &cinfo);
    quant_ptr = cinfo.quant_tbl_ptrs[n];        /* quant_ptr is JQUANT_TBL* */
    for (i = 0; i < 64; i++) {
      /* Qtable[] is desired quantization table, in natural array order */
      quant_ptr->quantval[i] = Qtable[i];
    }

Code to load a fixed Huffman table is typically (for AC table "n"):

if (cinfo.ac_huff_tbl_ptrs[n] == NULL)
      cinfo.ac_huff_tbl_ptrs[n] = jpeg_alloc_huff_table((j_common_ptr) &cinfo);
    huff_ptr = cinfo.ac_huff_tbl_ptrs[n];       /* huff_ptr is JHUFF_TBL* */
    for (i = 1; i <= 16; i++) {
      /* counts[i] is number of Huffman codes of length i bits, i=1..16 */
      huff_ptr->bits[i] = counts[i];
    }
    for (i = 0; i < 256; i++) {
      /* symbols[] is the list of Huffman symbols, in code-length order */
      huff_ptr->huffval[i] = symbols[i];
    }

(Note that trying to set cinfo.quant_tbl_ptrs[n] to point directly at a
constant JQUANT_TBL object is not safe.  If the incoming file happened to
contain a quantization table definition, your master table would get
overwritten!  Instead allocate a working table copy and copy the master table
into it, as illustrated above.  Ditto for Huffman tables, of course.)

You might want to read the tables from a tables-only file, rather than
hard-wiring them into your application.  The jpeg_read_header() call is
sufficient to read a tables-only file.  You must pass a second parameter of
FALSE to indicate that you do not require an image to be present.  Thus, the
typical scenario is

create JPEG decompression object
        set source to tables-only file
        jpeg_read_header(&cinfo, FALSE);
        set source to abbreviated image file
        jpeg_read_header(&cinfo, TRUE);
        set decompression parameters
        jpeg_start_decompress(&cinfo);
        read data...
        jpeg_finish_decompress(&cinfo);

In some cases, you may want to read a file without knowing whether it contains
an image or just tables.  In that case, pass FALSE and check the return value
from jpeg_read_header(): it will be JPEG_HEADER_OK if an image was found,
JPEG_HEADER_TABLES_ONLY if only tables were found.  (A third return value,
JPEG_SUSPENDED, is possible when using a suspending data source manager.)
Note that jpeg_read_header() will not complain if you read an abbreviated
image for which you haven't loaded the missing tables; the missing-table check
occurs later, in jpeg_start_decompress().

It is possible to read a series of images from a single source file by
repeating the jpeg_read_header() ... jpeg_finish_decompress() sequence,
without releasing/recreating the JPEG object or the data source module.
(If you did reinitialize, any partial bufferload left in the data source
buffer at the end of one image would be discarded, causing you to lose the
start of the next image.)  When you use this method, stored tables are
automatically carried forward, so some of the images can be abbreviated images
that depend on tables from earlier images.

If you intend to write a series of images into a single destination file,
you might want to make a specialized data destination module that doesn't
flush the output buffer at term_destination() time.  This would speed things
up by some trifling amount.  Of course, you'd need to remember to flush the
buffer after the last image.  You can make the later images be abbreviated
ones by passing FALSE to jpeg_start_compress().

Special markers
---------------

Some applications may need to insert or extract special data in the JPEG
datastream.  The JPEG standard provides marker types "COM" (comment) and
"APP0" through "APP15" (application) to hold application-specific data.
Unfortunately, the use of these markers is not specified by the standard.
COM markers are fairly widely used to hold user-supplied text.  The JFIF file
format spec uses APP0 markers with specified initial strings to hold certain
data.  Adobe applications use APP14 markers beginning with the string "Adobe"
for miscellaneous data.  Other APPn markers are rarely seen, but might
contain almost anything.

If you wish to store user-supplied text, we recommend you use COM markers
and place readable 7-bit ASCII text in them.  Newline conventions are not
standardized --- expect to find LF (Unix style), CR/LF (DOS style), or CR
(Mac style).  A robust COM reader should be able to cope with random binary
garbage, including nulls, since some applications generate COM markers
containing non-ASCII junk.  (But yours should not be one of them.)

For program-supplied data, use an APPn marker, and be sure to begin it with an
identifying string so that you can tell whether the marker is actually yours.
It's probably best to avoid using APP0 or APP14 for any private markers.
(NOTE: the upcoming SPIFF standard will use APP8 markers; we recommend you
not use APP8 markers for any private purposes, either.)

Keep in mind that at most 65533 bytes can be put into one marker, but you
can have as many markers as you like.

By default, the IJG compression library will write a JFIF APP0 marker if the
selected JPEG colorspace is grayscale or YCbCr, or an Adobe APP14 marker if
the selected colorspace is RGB, CMYK, or YCCK.  You can disable this, but
we don't recommend it.  The decompression library will recognize JFIF and
Adobe markers and will set the JPEG colorspace properly when one is found.

You can write special markers immediately following the datastream header by
calling jpeg_write_marker() after jpeg_start_compress() and before the first
call to jpeg_write_scanlines().  When you do this, the markers appear after
the SOI and the JFIF APP0 and Adobe APP14 markers (if written), but before
all else.  Specify the marker type parameter as "JPEG_COM" for COM or
"JPEG_APP0 + n" for APPn.  (Actually, jpeg_write_marker will let you write
any marker type, but we don't recommend writing any other kinds of marker.)
For example, to write a user comment string pointed to by comment_text:
        jpeg_write_marker(cinfo, JPEG_COM, comment_text, strlen(comment_text));

If it's not convenient to store all the marker data in memory at once,
you can instead call jpeg_write_m_header() followed by multiple calls to
jpeg_write_m_byte().  If you do it this way, it's your responsibility to
call jpeg_write_m_byte() exactly the number of times given in the length
parameter to jpeg_write_m_header().  (This method lets you empty the
output buffer partway through a marker, which might be important when
using a suspending data destination module.  In any case, if you are using
a suspending destination, you should flush its buffer after inserting
any special markers.  See "I/O suspension".)

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

If you are writing JFIF 1.02 extension markers (thumbnail images), don't
forget to set cinfo.JFIF_minor_version = 2 so that the encoder will write the
correct JFIF version number in the JFIF header marker.  The library's default
is to write version 1.01, but that's wrong if you insert any 1.02 extension
markers.  (We could probably get away with just defaulting to 1.02, but there
used to be broken decoders that would complain about unknown minor version
numbers.  To reduce compatibility risks it's safest not to write 1.02 unless
you are actually using 1.02 extensions.)

When reading, two methods of handling special markers are available:
1. You can ask the library to save the contents of COM and/or APPn markers
into memory, and then examine them at your leisure afterwards.
2. You can supply your own routine to process COM and/or APPn markers
on-the-fly as they are read.
The first method is simpler to use, especially if you are using a suspending
data source; writing a marker processor that copes with input suspension is
not easy (consider what happens if the marker is longer than your available
input buffer).  However, the second method conserves memory since the marker
data need not be kept around after it's been processed.

For either method, you'd normally set up marker handling after creating a
decompression object and before calling jpeg_read_header(), because the
markers of interest will typically be near the head of the file and so will
be scanned by jpeg_read_header.  Once you've established a marker handling
method, it will be used for the life of that decompression object
(potentially many datastreams), unless you change it.  Marker handling is
determined separately for COM markers and for each APPn marker code.

To save the contents of special markers in memory, call
        jpeg_save_markers(cinfo, marker_code, length_limit)
where marker_code is the marker type to save, JPEG_COM or JPEG_APP0+n.
(To arrange to save all the special marker types, you need to call this
routine 17 times, for COM and APP0-APP15.)  If the incoming marker is longer
than length_limit data bytes, only length_limit bytes will be saved; this
parameter allows you to avoid chewing up memory when you only need to see the
first few bytes of a potentially large marker.  If you want to save all the
data, set length_limit to 0xFFFF; that is enough since marker lengths are only
16 bits.  As a special case, setting length_limit to 0 prevents that marker
type from being saved at all.  (That is the default behavior, in fact.)

After jpeg_read_header() completes, you can examine the special markers by
following the cinfo->marker_list pointer chain.  All the special markers in
the file appear in this list, in order of their occurrence in the file (but
omitting any markers of types you didn't ask for).  Both the original data
length and the saved data length are recorded for each list entry; the latter
will not exceed length_limit for the particular marker type.  Note that these
lengths exclude the marker length word, whereas the stored representation
within the JPEG file includes it.  (Hence the maximum data length is really
only 65533.)

It is possible that additional special markers appear in the file beyond the
SOS marker at which jpeg_read_header stops; if so, the marker list will be
extended during reading of the rest of the file.  This is not expected to be
common, however.  If you are short on memory you may want to reset the length
limit to zero for all marker types after finishing jpeg_read_header, to
ensure that the max_memory_to_use setting cannot be exceeded due to addition
of later markers.

The marker list remains stored until you call jpeg_finish_decompress or
jpeg_abort, at which point the memory is freed and the list is set to empty.
(jpeg_destroy also releases the storage, of course.)

Note that the library is internally interested in APP0 and APP14 markers;
if you try to set a small nonzero length limit on these types, the library
will silently force the length up to the minimum it wants.  (But you can set
a zero length limit to prevent them from being saved at all.)  Also, in a
16-bit environment, the maximum length limit may be constrained to less than
65533 by malloc() limitations.  It is therefore best not to assume that the
effective length limit is exactly what you set it to be.

If you want to supply your own marker-reading routine, you do it by calling
jpeg_set_marker_processor().  A marker processor routine must have the
signature
        boolean jpeg_marker_parser_method (j_decompress_ptr cinfo)
Although the marker code is not explicitly passed, the routine can find it
in cinfo->unread_marker.  At the time of call, the marker proper has been
read from the data source module.  The processor routine is responsible for
reading the marker length word and the remaining parameter bytes, if any.
Return TRUE to indicate success.  (FALSE should be returned only if you are
using a suspending data source and it tells you to suspend.  See the standard
marker processors in jdmarker.c for appropriate coding methods if you need to
use a suspending data source.)

If you override the default APP0 or APP14 processors, it is up to you to
recognize JFIF and Adobe markers if you want colorspace recognition to occur
properly.  We recommend copying and extending the default processors if you
want to do that.  (A better idea is to save these marker types for later
examination by calling jpeg_save_markers(); that method doesn't interfere
with the library's own processing of these markers.)

jpeg_set_marker_processor() and jpeg_save_markers() are mutually exclusive
--- if you call one it overrides any previous call to the other, for the
particular marker type specified.

A simple example of an external COM processor can be found in djpeg.c.
Also, see jpegtran.c for an example of using jpeg_save_markers.

ICC profiles
------------

Two functions are provided for writing and reading International Color
Consortium (ICC) device profiles embedded in JFIF JPEG image files:

void jpeg_write_icc_profile (j_compress_ptr cinfo,
                                     const JOCTET *icc_data_ptr,
                                     unsigned int icc_data_len);
        boolean jpeg_read_icc_profile (j_decompress_ptr cinfo,
                                       JOCTET **icc_data_ptr,
                                       unsigned int *icc_data_len);

The ICC has defined a standard for including such data in JPEG "APP2" markers.
The aforementioned functions do not know anything about the internal structure
of the ICC profile data; they just know how to embed the profile data into a
JPEG file while writing it, or to extract the profile data from a JPEG file
while reading it.

jpeg_write_icc_profile() must be called after calling jpeg_start_compress() and
before the first call to jpeg_write_scanlines() or jpeg_write_raw_data().  This
ordering ensures that the APP2 marker(s) will appear after the SOI and JFIF or
Adobe markers, but before all other data.

jpeg_read_icc_profile() returns TRUE if an ICC profile was found and FALSE
otherwise.  If an ICC profile was found, then the function will allocate a
memory region containing the profile and will return a pointer to that memory
region in *icc_data_ptr, as well as the length of the region in *icc_data_len.
This memory region is allocated by the library using malloc() and must be freed
by the caller using free() when the memory region is no longer needed.  Callers
wishing to use jpeg_read_icc_profile() must call

jpeg_save_markers(cinfo, JPEG_APP0 + 2, 0xFFFF);

prior to calling jpeg_read_header().  jpeg_read_icc_profile() can be called at
any point between jpeg_read_header() and jpeg_finish_decompress().

Raw (downsampled) image data
----------------------------

Some applications need to supply already-downsampled image data to the JPEG
compressor, or to receive raw downsampled data from the decompressor.  The
library supports this requirement by allowing the application to write or
read raw data, bypassing the normal preprocessing or postprocessing steps.
The interface is different from the standard one and is somewhat harder to
use.  If your interest is merely in bypassing color conversion, we recommend
that you use the standard interface and simply set jpeg_color_space =
in_color_space (or jpeg_color_space = out_color_space for decompression).
The mechanism described in this section is necessary only to supply or
receive downsampled image data, in which not all components have the same
dimensions.

To compress raw data, you must supply the data in the colorspace to be used
in the JPEG file (please read the earlier section on Special color spaces)
and downsampled to the sampling factors specified in the JPEG parameters.
You must supply the data in the format used internally by the JPEG library,
namely a JSAMPIMAGE array.  This is an array of pointers to two-dimensional
arrays, each of type JSAMPARRAY.  Each 2-D array holds the values for one
color component.  This structure is necessary since the components are of
different sizes.  If the image dimensions are not a multiple of the MCU size,
you must also pad the data correctly (usually, this is done by replicating
the last column and/or row).  The data must be padded to a multiple of a DCT
block in each component: that is, each downsampled row must contain a
multiple of 8 valid samples, and there must be a multiple of 8 sample rows
for each component.  (For applications such as conversion of digital TV
images, the standard image size is usually a multiple of the DCT block size,
so that no padding need actually be done.)

The procedure for compression of raw data is basically the same as normal
compression, except that you call jpeg_write_raw_data() in place of
jpeg_write_scanlines().  Before calling jpeg_start_compress(), you must do
the following:
  * Set cinfo->raw_data_in to TRUE.  (It is set FALSE by jpeg_set_defaults().)
    This notifies the library that you will be supplying raw data.
  * Ensure jpeg_color_space is correct --- an explicit jpeg_set_colorspace()
    call is a good idea.  Note that since color conversion is bypassed,
    in_color_space is ignored, except that jpeg_set_defaults() uses it to
    choose the default jpeg_color_space setting.
  * Ensure the sampling factors, cinfo->comp_info[i].h_samp_factor and
    cinfo->comp_info[i].v_samp_factor, are correct.  Since these indicate the
    dimensions of the data you are supplying, it's wise to set them
    explicitly, rather than assuming the library's defaults are what you want.

To pass raw data to the library, call jpeg_write_raw_data() in place of
jpeg_write_scanlines().  The two routines work similarly except that
jpeg_write_raw_data takes a JSAMPIMAGE data array rather than JSAMPARRAY.
The scanlines count passed to and returned from jpeg_write_raw_data is
measured in terms of the component with the largest v_samp_factor.

jpeg_write_raw_data() processes one MCU row per call, which is to say
v_samp_factor*DCTSIZE sample rows of each component.  The passed num_lines
value must be at least max_v_samp_factor*DCTSIZE, and the return value will
be exactly that amount (or possibly some multiple of that amount, in future
library versions).  This is true even on the last call at the bottom of the
image; don't forget to pad your data as necessary.

The required dimensions of the supplied data can be computed for each
component as
        cinfo->comp_info[i].width_in_blocks*DCTSIZE  samples per row
        cinfo->comp_info[i].height_in_blocks*DCTSIZE rows in image
after jpeg_start_compress() has initialized those fields.  If the valid data
is smaller than this, it must be padded appropriately.  For some sampling
factors and image sizes, additional dummy DCT blocks are inserted to make
the image a multiple of the MCU dimensions.  The library creates such dummy
blocks itself; it does not read them from your supplied data.  Therefore you
need never pad by more than DCTSIZE samples.  An example may help here.
Assume 2h2v downsampling of YCbCr data, that is
        cinfo->comp_info[0].h_samp_factor = 2           for Y
        cinfo->comp_info[0].v_samp_factor = 2
        cinfo->comp_info[1].h_samp_factor = 1           for Cb
        cinfo->comp_info[1].v_samp_factor = 1
        cinfo->comp_info[2].h_samp_factor = 1           for Cr
        cinfo->comp_info[2].v_samp_factor = 1
and suppose that the nominal image dimensions (cinfo->image_width and
cinfo->image_height) are 101x101 pixels.  Then jpeg_start_compress() will
compute downsampled_width = 101 and width_in_blocks = 13 for Y,
downsampled_width = 51 and width_in_blocks = 7 for Cb and Cr (and the same
for the height fields).  You must pad the Y data to at least 13*8 = 104
columns and rows, the Cb/Cr data to at least 7*8 = 56 columns and rows.  The
MCU height is max_v_samp_factor = 2 DCT rows so you must pass at least 16
scanlines on each call to jpeg_write_raw_data(), which is to say 16 actual
sample rows of Y and 8 each of Cb and Cr.  A total of 7 MCU rows are needed,
so you must pass a total of 7*16 = 112 "scanlines".  The last DCT block row
of Y data is dummy, so it doesn't matter what you pass for it in the data
arrays, but the scanlines count must total up to 112 so that all of the Cb
and Cr data gets passed.

Output suspension is supported with raw-data compression: if the data
destination module suspends, jpeg_write_raw_data() will return 0.
In this case the same data rows must be passed again on the next call.

Decompression with raw data output implies bypassing all postprocessing:
you cannot ask for rescaling or color quantization, for instance.  More
seriously, you must deal with the color space and sampling factors present in
the incoming file.  If your application only handles, say, 2h1v YCbCr data,
you must check for and fail on other color spaces or other sampling factors.
The library will not convert to a different color space for you.

To obtain raw data output, set cinfo->raw_data_out = TRUE before
jpeg_start_decompress() (it is set FALSE by jpeg_read_header()).  Be sure to
verify that the color space and sampling factors are ones you can handle.
Then call jpeg_read_raw_data() in place of jpeg_read_scanlines().  The
decompression process is otherwise the same as usual.

jpeg_read_raw_data() returns one MCU row per call, and thus you must pass a
buffer of at least max_v_samp_factor*DCTSIZE scanlines (scanline counting is
the same as for raw-data compression).  The buffer you pass must be large
enough to hold the actual data plus padding to DCT-block boundaries.  As with
compression, any entirely dummy DCT blocks are not processed so you need not
allocate space for them, but the total scanline count includes them.  The
above example of computing buffer dimensions for raw-data compression is
equally valid for decompression.

Input suspension is supported with raw-data decompression: if the data source
module suspends, jpeg_read_raw_data() will return 0.  You can also use
buffered-image mode to read raw data in multiple passes.

Really raw data: DCT coefficients
---------------------------------

It is possible to read or write the contents of a JPEG file as raw DCT
coefficients.  This facility is mainly intended for use in lossless
transcoding between different JPEG file formats.  Other possible applications
include lossless cropping of a JPEG image, lossless reassembly of a
multi-strip or multi-tile TIFF/JPEG file into a single JPEG datastream, etc.

To read the contents of a JPEG file as DCT coefficients, open the file and do
jpeg_read_header() as usual.  But instead of calling jpeg_start_decompress()
and jpeg_read_scanlines(), call jpeg_read_coefficients().  This will read the
entire image into a set of virtual coefficient-block arrays, one array per
component.  The return value is a pointer to an array of virtual-array
descriptors.  Each virtual array can be accessed directly using the JPEG
memory manager's access_virt_barray method (see Memory management, below,
and also read structure.txt's discussion of virtual array handling).  Or,
for simple transcoding to a different JPEG file format, the array list can
just be handed directly to jpeg_write_coefficients().

Each block in the block arrays contains quantized coefficient values in
normal array order (not JPEG zigzag order).  The block arrays contain only
DCT blocks containing real data; any entirely-dummy blocks added to fill out
interleaved MCUs at the right or bottom edges of the image are discarded
during reading and are not stored in the block arrays.  (The size of each
block array can be determined from the width_in_blocks and height_in_blocks
fields of the component's comp_info entry.)  This is also the data format
expected by jpeg_write_coefficients().

When you are done using the virtual arrays, call jpeg_finish_decompress()
to release the array storage and return the decompression object to an idle
state; or just call jpeg_destroy() if you don't need to reuse the object.

If you use a suspending data source, jpeg_read_coefficients() will return
NULL if it is forced to suspend; a non-NULL return value indicates successful
completion.  You need not test for a NULL return value when using a
non-suspending data source.

It is also possible to call jpeg_read_coefficients() to obtain access to the
decoder's coefficient arrays during a normal decode cycle in buffered-image
mode.  This frammish might be useful for progressively displaying an incoming
image and then re-encoding it without loss.  To do this, decode in buffered-
image mode as discussed previously, then call jpeg_read_coefficients() after
the last jpeg_finish_output() call.  The arrays will be available for your use
until you call jpeg_finish_decompress().

To write the contents of a JPEG file as DCT coefficients, you must provide
the DCT coefficients stored in virtual block arrays.  You can either pass
block arrays read from an input JPEG file by jpeg_read_coefficients(), or
allocate virtual arrays from the JPEG compression object and fill them
yourself.  In either case, jpeg_write_coefficients() is substituted for
jpeg_start_compress() and jpeg_write_scanlines().  Thus the sequence is
  * Create compression object
  * Set all compression parameters as necessary
  * Request virtual arrays if needed
  * jpeg_write_coefficients()
  * jpeg_finish_compress()
  * Destroy or re-use compression object
jpeg_write_coefficients() is passed a pointer to an array of virtual block
array descriptors; the number of arrays is equal to cinfo.num_components.

The virtual arrays need only have been requested, not realized, before
jpeg_write_coefficients() is called.  A side-effect of
jpeg_write_coefficients() is to realize any virtual arrays that have been
requested from the compression object's memory manager.  Thus, when obtaining
the virtual arrays from the compression object, you should fill the arrays
after calling jpeg_write_coefficients().  The data is actually written out
when you call jpeg_finish_compress(); jpeg_write_coefficients() only writes
the file header.

When writing raw DCT coefficients, it is crucial that the JPEG quantization
tables and sampling factors match the way the data was encoded, or the
resulting file will be invalid.  For transcoding from an existing JPEG file,
we recommend using jpeg_copy_critical_parameters().  This routine initializes
all the compression parameters to default values (like jpeg_set_defaults()),
then copies the critical information from a source decompression object.
The decompression object should have just been used to read the entire
JPEG input file --- that is, it should be awaiting jpeg_finish_decompress().

jpeg_write_coefficients() marks all tables stored in the compression object
as needing to be written to the output file (thus, it acts like
jpeg_start_compress(cinfo, TRUE)).  This is for safety's sake, to avoid
emitting abbreviated JPEG files by accident.  If you really want to emit an
abbreviated JPEG file, call jpeg_suppress_tables(), or set the tables'
individual sent_table flags, between calling jpeg_write_coefficients() and
jpeg_finish_compress().

Progress monitoring
-------------------

Some applications may need to regain control from the JPEG library every so
often.  The typical use of this feature is to produce a percent-done bar or
other progress display.  (For a simple example, see cjpeg.c or djpeg.c.)
Although you do get control back frequently during the data-transferring pass
(the jpeg_read_scanlines or jpeg_write_scanlines loop), any additional passes
will occur inside jpeg_finish_compress or jpeg_start_decompress; those
routines may take a long time to execute, and you don't get control back
until they are done.

You can define a progress-monitor routine which will be called periodically
by the library.  No guarantees are made about how often this call will occur,
so we don't recommend you use it for mouse tracking or anything like that.
At present, a call will occur once per MCU row, scanline, or sample row
group, whichever unit is convenient for the current processing mode; so the
wider the image, the longer the time between calls.  During the data
transferring pass, only one call occurs per call of jpeg_read_scanlines or
jpeg_write_scanlines, so don't pass a large number of scanlines at once if
you want fine resolution in the progress count.  (If you really need to use
the callback mechanism for time-critical tasks like mouse tracking, you could
insert additional calls inside some of the library's inner loops.)

To establish a progress-monitor callback, create a struct jpeg_progress_mgr,
fill in its progress_monitor field with a pointer to your callback routine,
and set cinfo->progress to point to the struct.  The callback will be called
whenever cinfo->progress is non-NULL.  (This pointer is set to NULL by
jpeg_create_compress or jpeg_create_decompress; the library will not change
it thereafter.  So if you allocate dynamic storage for the progress struct,
make sure it will live as long as the JPEG object does.  Allocating from the
JPEG memory manager with lifetime JPOOL_PERMANENT will work nicely.)  You
can use the same callback routine for both compression and decompression.

The jpeg_progress_mgr struct contains four fields which are set by the library:
        long pass_counter;      /* work units completed in this pass */
        long pass_limit;        /* total number of work units in this pass */
        int completed_passes;   /* passes completed so far */
        int total_passes;       /* total number of passes expected */
During any one pass, pass_counter increases from 0 up to (not including)
pass_limit; the step size is usually but not necessarily 1.  The pass_limit
value may change from one pass to another.  The expected total number of
passes is in total_passes, and the number of passes already completed is in
completed_passes.  Thus the fraction of work completed may be estimated as
                completed_passes + (pass_counter/pass_limit)
                --------------------------------------------
                                total_passes
ignoring the fact that the passes may not be equal amounts of work.

When decompressing, pass_limit can even change within a pass, because it
depends on the number of scans in the JPEG file, which isn't always known in
advance.  The computed fraction-of-work-done may jump suddenly (if the library
discovers it has overestimated the number of scans) or even decrease (in the
opposite case).  It is not wise to put great faith in the work estimate.

When using the decompressor's buffered-image mode, the progress monitor work
estimate is likely to be completely unhelpful, because the library has no way
to know how many output passes will be demanded of it.  Currently, the library
sets total_passes based on the assumption that there will be one more output
pass if the input file end hasn't yet been read (jpeg_input_complete() isn't
TRUE), but no more output passes if the file end has been reached when the
output pass is started.  This means that total_passes will rise as additional
output passes are requested.  If you have a way of determining the input file
size, estimating progress based on the fraction of the file that's been read
will probably be more useful than using the library's value.

Memory management
-----------------

This section covers some key facts about the JPEG library's built-in memory
manager.  For more info, please read structure.txt's section about the memory
manager, and consult the source code if necessary.

All memory and temporary file allocation within the library is done via the
memory manager.  If necessary, you can replace the "back end" of the memory
manager to control allocation yourself (for example, if you don't want the
library to use malloc() and free() for some reason).

Some data is allocated "permanently" and will not be freed until the JPEG
object is destroyed.  Most data is allocated "per image" and is freed by
jpeg_finish_compress, jpeg_finish_decompress, or jpeg_abort.  You can call the
memory manager yourself to allocate structures that will automatically be
freed at these times.  Typical code for this is
  ptr = (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE, size);
Use JPOOL_PERMANENT to get storage that lasts as long as the JPEG object.
Use alloc_large instead of alloc_small for anything bigger than a few Kbytes.
There are also alloc_sarray and alloc_barray routines that automatically
build 2-D sample or block arrays.

The library's minimum space requirements to process an image depend on the
image's width, but not on its height, because the library ordinarily works
with "strip" buffers that are as wide as the image but just a few rows high.
Some operating modes (eg, two-pass color quantization) require full-image
buffers.  Such buffers are treated as "virtual arrays": only the current strip
need be in memory, and the rest can be swapped out to a temporary file.

When using temporary files, the library will make the in-memory buffers for
its virtual arrays just big enough to stay within a "maximum memory" setting.
Your application can set this limit by setting cinfo->mem->max_memory_to_use
after creating the JPEG object.  (Of course, there is still a minimum size for
the buffers, so the max-memory setting is effective only if it is bigger than
the minimum space needed.)  If you allocate any large structures yourself, you
must allocate them before jpeg_start_compress() or jpeg_start_decompress() in
order to have them counted against the max memory limit.  Also keep in mind
that space allocated with alloc_small() is ignored, on the assumption that
it's too small to be worth worrying about; so a reasonable safety margin
should be left when setting max_memory_to_use.

NOTE: Unless you develop your own memory manager back end, then temporary files
will never be used.  The back end provided in libjpeg-turbo (jmemnobs.c) simply
malloc()s and free()s virtual arrays, and an error occurs if the required
memory exceeds the limit specified in cinfo->mem->max_memory_to_use.

Memory usage
------------

Working memory requirements while performing compression or decompression
depend on image dimensions, image characteristics (such as colorspace and
JPEG process), and operating mode (application-selected options).

As of v6b, the decompressor requires:
 1. About 24K in more-or-less-fixed-size data.  This varies a bit depending
    on operating mode and image characteristics (particularly color vs.
    grayscale), but it doesn't depend on image dimensions.
 2. Strip buffers (of size proportional to the image width) for IDCT and
    upsampling results.  The worst case for commonly used sampling factors
    is about 34 bytes * width in pixels for a color image.  A grayscale image
    only needs about 8 bytes per pixel column.
 3. A full-image DCT coefficient buffer is needed to decode a multi-scan JPEG
    file (including progressive JPEGs), or whenever you select buffered-image
    mode.  This takes 2 bytes/coefficient.  At typical 2x2 sampling, that's
    3 bytes per pixel for a color image.  Worst case (1x1 sampling) requires
    6 bytes/pixel.  For grayscale, figure 2 bytes/pixel.
 4. To perform 2-pass color quantization, the decompressor also needs a
    128K color lookup table and a full-image pixel buffer (3 bytes/pixel).
This does not count any memory allocated by the application, such as a
buffer to hold the final output image.

The above figures are valid for 8-bit JPEG data precision and a machine with
32-bit ints.  For 12-bit JPEG data, double the size of the strip buffers and
quantization pixel buffer.  The "fixed-size" data will be somewhat smaller
with 16-bit ints, larger with 64-bit ints.  Also, CMYK or other unusual
color spaces will require different amounts of space.

The full-image coefficient and pixel buffers, if needed at all, do not
have to be fully RAM resident; you can have the library use temporary
files instead when the total memory usage would exceed a limit you set.
(But if your OS supports virtual memory, it's probably better to just use
jmemnobs and let the OS do the swapping.)

The compressor's memory requirements are similar, except that it has no need
for color quantization.  Also, it needs a full-image DCT coefficient buffer
if Huffman-table optimization is asked for, even if progressive mode is not
requested.

If you need more detailed information about memory usage in a particular
situation, you can enable the MEM_STATS code in jmemmgr.c.

Library compile-time options
----------------------------

A number of compile-time options are available by modifying jmorecfg.h.

The JPEG standard provides for both the baseline 8-bit DCT process and
a 12-bit DCT process.  The IJG code supports 12-bit lossy JPEG if you define
BITS_IN_JSAMPLE as 12 rather than 8.  Note that this causes JSAMPLE to be
larger than a char, so it affects the surrounding application's image data.
The sample applications cjpeg and djpeg can support 12-bit mode only for PPM
and GIF file formats; you must disable the other file formats to compile a
12-bit cjpeg or djpeg.  (install.txt has more information about that.)
At present, a 12-bit library can handle *only* 12-bit images, not both
precisions.

Note that a 12-bit library always compresses in Huffman optimization mode,
in order to generate valid Huffman tables.  This is necessary because our
default Huffman tables only cover 8-bit data.  If you need to output 12-bit
files in one pass, you'll have to supply suitable default Huffman tables.
You may also want to supply your own DCT quantization tables; the existing
quality-scaling code has been developed for 8-bit use, and probably doesn't
generate especially good tables for 12-bit.

The maximum number of components (color channels) in the image is determined
by MAX_COMPONENTS.  The JPEG standard allows up to 255 components, but we
expect that few applications will need more than four or so.

On machines with unusual data type sizes, you may be able to improve
performance or reduce memory space by tweaking the various typedefs in
jmorecfg.h.  In particular, on some RISC CPUs, access to arrays of "short"s
is quite slow; consider trading memory for speed by making JCOEF, INT16, and
UINT16 be "int" or "unsigned int".  UINT8 is also a candidate to become int.
You probably don't want to make JSAMPLE be int unless you have lots of memory
to burn.

You can reduce the size of the library by compiling out various optional
functions.  To do this, undefine xxx_SUPPORTED symbols as necessary.

You can also save a few K by not having text error messages in the library;
the standard error message table occupies about 5Kb.  This is particularly
reasonable for embedded applications where there's no good way to display
a message anyway.  To do this, remove the creation of the message table
(jpeg_std_message_table[]) from jerror.c, and alter format_message to do
something reasonable without it.  You could output the numeric value of the
message code number, for example.  If you do this, you can also save a couple
more K by modifying the TRACEMSn() macros in jerror.h to expand to nothing;
you don't need trace capability anyway, right?

Portability considerations
--------------------------

The JPEG library has been written to be extremely portable; the sample
applications cjpeg and djpeg are slightly less so.  This section summarizes
the design goals in this area.  (If you encounter any bugs that cause the
library to be less portable than is claimed here, we'd appreciate hearing
about them.)

The code works fine on ANSI C and C++ compilers, using any of the popular
system include file setups, and some not-so-popular ones too.

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.

More info about porting the code may be gleaned by reading jconfig.txt,
jmorecfg.h, and jinclude.h.