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USING THE IJG JPEG LIBRARY
This file is part of the Independent JPEG Group's software.
For conditions of distribution and use, see the accompanying README file.
This file describes how to use the IJG JPEG library within an application
program. Read it if you want to write a program that uses the library.
The file example.c provides heavily commented skeleton code for calling the
JPEG library. Also see jpeglib.h (the include file to be used by application
programs) for full details about data structures and function parameter lists.
The library source code, of course, is the ultimate reference.
Note that there have been *major* changes from the application interface
presented by IJG version 4 and earlier versions. The old design had several
inherent limitations, and it had accumulated a lot of cruft as we added
features while trying to minimize application-interface changes. We have
sacrificed backward compatibility in the version 5 rewrite, but we think the
improvements justify this.
TABLE OF CONTENTS
-----------------
Overview:
Functions provided by the library
Outline of typical usage
Basic library usage:
Data formats
Compression details
Decompression details
Mechanics of usage: include files, linking, etc
Advanced features:
Compression parameter selection
Decompression parameter selection
Special color spaces
Error handling
Compressed data handling (source and destination managers)
I/O suspension
Progressive JPEG support
Buffered-image mode
Abbreviated datastreams and multiple images
Special markers
Raw (downsampled) image data
Really raw data: DCT coefficients
Progress monitoring
Memory management
Library compile-time options
Portability considerations
Notes for MS-DOS implementors
You should read at least the overview and basic usage sections before trying
to program with the library. The sections on advanced features can be read
if and when you need them.
OVERVIEW
========
Functions provided by the library
---------------------------------
The IJG JPEG library provides C code to read and write JPEG-compressed image
files. The surrounding application program receives or supplies image data a
scanline at a time, using a straightforward uncompressed image format. All
details of color conversion and other preprocessing/postprocessing can be
handled by the library.
The library includes a substantial amount of code that is not covered by the
JPEG standard but is necessary for typical applications of JPEG. These
functions preprocess the image before JPEG compression or postprocess it after
decompression. They include colorspace conversion, downsampling/upsampling,
and color quantization. The application indirectly selects use of this code
by specifying the format in which it wishes to supply or receive image data.
For example, if colormapped output is requested, then the decompression
library automatically invokes color quantization.
A wide range of quality vs. speed tradeoffs are possible in JPEG processing,
and even more so in decompression postprocessing. The decompression library
provides multiple implementations that cover most of the useful tradeoffs,
ranging from very-high-quality down to fast-preview operation. On the
compression side we have generally not provided low-quality choices, since
compression is normally less time-critical. It should be understood that the
low-quality modes may not meet the JPEG standard's accuracy requirements;
nonetheless, they are useful for viewers.
A word about functions *not* provided by the library. We handle a subset of
the ISO JPEG standard; most baseline, extended-sequential, and progressive
JPEG processes are supported. (Our subset includes all features now in common
use.) Unsupported ISO options include:
* Hierarchical storage
* Lossless JPEG
* Arithmetic entropy coding (unsupported for legal reasons)
* DNL marker
* Nonintegral subsampling ratios
We support both 8- and 12-bit data precision, but this is a compile-time
choice rather than a run-time choice; hence it is difficult to use both
precisions in a single application.
By itself, the library handles only interchange JPEG datastreams --- in
particular the widely used JFIF file format. The library can be used by
surrounding code to process interchange or abbreviated JPEG datastreams that
are embedded in more complex file formats. (For example, this library is
used by the free LIBTIFF library to support JPEG compression in TIFF.)
Outline of typical usage
------------------------
The rough outline of a JPEG compression operation is:
Allocate and initialize a JPEG compression object
Specify the destination for the compressed data (eg, a file)
Set parameters for compression, including image size & colorspace
jpeg_start_compress(...);
while (scan lines remain to be written)
jpeg_write_scanlines(...);
jpeg_finish_compress(...);
Release the JPEG compression object
A JPEG compression object holds parameters and working state for the JPEG
library. We make creation/destruction of the object separate from starting
or finishing compression of an image; the same object can be re-used for a
series of image compression operations. This makes it easy to re-use the
same parameter settings for a sequence of images. Re-use of a JPEG object
also has important implications for processing abbreviated JPEG datastreams,
as discussed later.
The image data to be compressed is supplied to jpeg_write_scanlines() from
in-memory buffers. If the application is doing file-to-file compression,
reading image data from the source file is the application's responsibility.
The library emits compressed data by calling a "data destination manager",
which typically will write the data into a file; but the application can
provide its own destination manager to do something else.
Similarly, the rough outline of a JPEG decompression operation is:
Allocate and initialize a JPEG decompression object
Specify the source of the compressed data (eg, a file)
Call jpeg_read_header() to obtain image info
Set parameters for decompression
jpeg_start_decompress(...);
while (scan lines remain to be read)
jpeg_read_scanlines(...);
jpeg_finish_decompress(...);
Release the JPEG decompression object
This is comparable to the compression outline except that reading the
datastream header is a separate step. This is helpful because information
about the image's size, colorspace, etc is available when the application
selects decompression parameters. For example, the application can choose an
output scaling ratio that will fit the image into the available screen size.
The decompression library obtains compressed data by calling a data source
manager, which typically will read the data from a file; but other behaviors
can be obtained with a custom source manager. Decompressed data is delivered
into in-memory buffers passed to jpeg_read_scanlines().
It is possible to abort an incomplete compression or decompression operation
by calling jpeg_abort(); or, if you do not need to retain the JPEG object,
simply release it by calling jpeg_destroy().
JPEG compression and decompression objects are two separate struct types.
However, they share some common fields, and certain routines such as
jpeg_destroy() can work on either type of object.
The JPEG library has no static variables: all state is in the compression
or decompression object. Therefore it is possible to process multiple
compression and decompression operations concurrently, using multiple JPEG
objects.
Both compression and decompression can be done in an incremental memory-to-
memory fashion, if suitable source/destination managers are used. See the
section on "I/O suspension" for more details.
BASIC LIBRARY USAGE
===================
Data formats
------------
Before diving into procedural details, it is helpful to understand the
image data format that the JPEG library expects or returns.
The standard input image format is a rectangular array of pixels, with each
pixel having the same number of "component" or "sample" values (color
channels). You must specify how many components there are and the colorspace
interpretation of the components. Most applications will use RGB data
(three components per pixel) or grayscale data (one component per pixel).
PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE.
A remarkable number of people manage to miss this, only to find that their
programs don't work with grayscale JPEG files.
There is no provision for colormapped input. JPEG files are always full-color
or full grayscale (or sometimes another colorspace such as CMYK). You can
feed in a colormapped image by expanding it to full-color format. However
JPEG often doesn't work very well with source data that has been colormapped,
because of dithering noise. This is discussed in more detail in the JPEG FAQ
and the other references mentioned in the README file.
Pixels are stored by scanlines, with each scanline running from left to
right. The component values for each pixel are adjacent in the row; for
example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color. Each scanline is an
array of data type JSAMPLE --- which is typically "unsigned char", unless
you've changed jmorecfg.h. (You can also change the RGB pixel layout, say
to B,G,R order, by modifying jmorecfg.h. But see the restrictions listed in
that file before doing so.)
A 2-D array of pixels is formed by making a list of pointers to the starts of
scanlines; so the scanlines need not be physically adjacent in memory. Even
if you process just one scanline at a time, you must make a one-element
pointer array to conform to this structure. Pointers to JSAMPLE rows are of
type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY.
The library accepts or supplies one or more complete scanlines per call.
It is not possible to process part of a row at a time. Scanlines are always
processed top-to-bottom. You can process an entire image in one call if you
have it all in memory, but usually it's simplest to process one scanline at
a time.
For best results, source data values should have the precision specified by
BITS_IN_JSAMPLE (normally 8 bits). For instance, if you choose to compress
data that's only 6 bits/channel, you should left-justify each value in a
byte before passing it to the compressor. If you need to compress data
that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 12.
(See "Library compile-time options", later.)
The data format returned by the decompressor is the same in all details,
except that colormapped output is supported. (Again, a JPEG file is never
colormapped. But you can ask the decompressor to perform on-the-fly color
quantization to deliver colormapped output.) If you request colormapped
output then the returned data array contains a single JSAMPLE per pixel;
its value is an index into a color map. The color map is represented as
a 2-D JSAMPARRAY in which each row holds the values of one color component,
that is, colormap[i][j] is the value of the i'th color component for pixel
value (map index) j. Note that since the colormap indexes are stored in
JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE
(ie, at most 256 colors for an 8-bit JPEG library).
Compression details
-------------------
Here we revisit the JPEG compression outline given in the overview.
1. Allocate and initialize a JPEG compression object.
A JPEG compression object is a "struct jpeg_compress_struct". (It also has
a bunch of subsidiary structures which are allocated via malloc(), but the
application doesn't control those directly.) This struct can be just a local
variable in the calling routine, if a single routine is going to execute the
whole JPEG compression sequence. Otherwise it can be static or allocated
from malloc().
You will also need a structure representing a JPEG error handler. The part
of this that the library cares about is a "struct jpeg_error_mgr". If you
are providing your own error handler, you'll typically want to embed the
jpeg_error_mgr struct in a larger structure; this is discussed later under
"Error handling". For now we'll assume you are just using the default error
handler. The default error handler will print JPEG error/warning messages
on stderr, and it will call exit() if a fatal error occurs.
You must initialize the error handler structure, store a pointer to it into
the JPEG object's "err" field, and then call jpeg_create_compress() to
initialize the rest of the JPEG object.
Typical code for this step, if you are using the default error handler, is
struct jpeg_compress_struct cinfo;
struct jpeg_error_mgr jerr;
...
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_compress(&cinfo);
jpeg_create_compress allocates a small amount of memory, so it could fail
if you are out of memory. In that case it will exit via the error handler;
that's why the error handler must be initialized first.
2. Specify the destination for the compressed data (eg, a file).
As previously mentioned, the JPEG library delivers compressed data to a
"data destination" module. The library includes one data destination
module which knows how to write to a stdio stream. You can use your own
destination module if you want to do something else, as discussed later.
If you use the standard destination module, you must open the target stdio
stream beforehand. Typical code for this step looks like:
FILE * outfile;
...
if ((outfile = fopen(filename, "wb")) == NULL) {
fprintf(stderr, "can't open %sn", filename);
exit(1);
}
jpeg_stdio_dest(&cinfo, outfile);
where the last line invokes the standard destination module.
WARNING: it is critical that the binary compressed data be delivered to the
output file unchanged. On non-Unix systems the stdio library may perform
newline translation or otherwise corrupt binary data. To suppress this
behavior, you may need to use a "b" option to fopen (as shown above), or use
setmode() or another routine to put the stdio stream in binary mode. See
cjpeg.c and djpeg.c for code that has been found to work on many systems.
You can select the data destination after setting other parameters (step 3),
if that's more convenient. You may not change the destination between
calling jpeg_start_compress() and jpeg_finish_compress().
3. Set parameters for compression, including image size & colorspace.
You must supply information about the source image by setting the following
fields in the JPEG object (cinfo structure):
image_width Width of image, in pixels
image_height Height of image, in pixels
input_components Number of color channels (samples per pixel)
in_color_space Color space of source image
The image dimensions are, hopefully, obvious. JPEG supports image dimensions
of 1 to 64K pixels in either direction. The input color space is typically
RGB or grayscale, and input_components is 3 or 1 accordingly. (See "Special
color spaces", later, for more info.) The in_color_space field must be
assigned one of the J_COLOR_SPACE enum constants, typically JCS_RGB or
JCS_GRAYSCALE.
JPEG has a large number of compression parameters that determine how the
image is encoded. Most applications don't need or want to know about all
these parameters. You can set all the parameters to reasonable defaults by
calling jpeg_set_defaults(); then, if there are particular values you want
to change, you can do so after that. The "Compression parameter selection"
section tells about all the parameters.
You must set in_color_space correctly before calling jpeg_set_defaults(),
because the defaults depend on the source image colorspace. However the
other three source image parameters need not be valid until you call
jpeg_start_compress(). There's no harm in calling jpeg_set_defaults() more
than once, if that happens to be convenient.
Typical code for a 24-bit RGB source image is
cinfo.image_width = Width; /* image width and height, in pixels */
cinfo.image_height = Height;
cinfo.input_components = 3; /* # of color components per pixel */
cinfo.in_color_space = JCS_RGB; /* colorspace of input image */
jpeg_set_defaults(&cinfo);
/* Make optional parameter settings here */
4. jpeg_start_compress(...);
After you have established the data destination and set all the necessary
source image info and other parameters, call jpeg_start_compress() to begin
a compression cycle. This will initialize internal state, allocate working
storage, and emit the first few bytes of the JPEG datastream header.
Typical code:
jpeg_start_compress(&cinfo, TRUE);
The "TRUE" parameter ensures that a complete JPEG interchange datastream
will be written. This is appropriate in most cases. If you think you might
want to use an abbreviated datastream, read the section on abbreviated
datastreams, below.
Once you have called jpeg_start_compress(), you may not alter any JPEG
parameters or other fields of the JPEG object until you have completed
the compression cycle.
5. while (scan lines remain to be written)
jpeg_write_scanlines(...);
Now write all the required image data by calling jpeg_write_scanlines()
one or more times. You can pass one or more scanlines in each call, up
to the total image height. In most applications it is convenient to pass
just one or a few scanlines at a time. The expected format for the passed
data is discussed under "Data formats", above.
Image data should be written in top-to-bottom scanline order. The JPEG spec
contains some weasel wording about how top and bottom are application-defined
terms (a curious interpretation of the English language...) but if you want
your files to be compatible with everyone else's, you WILL use top-to-bottom
order. If the source data must be read in bottom-to-top order, you can use
the JPEG library's virtual array mechanism to invert the data efficiently.
Examples of this can be found in the sample application cjpeg.
The library maintains a count of the number of scanlines written so far
in the next_scanline field of the JPEG object. Usually you can just use
this variable as the loop counter, so that the loop test looks like
"while (cinfo.next_scanline < cinfo.image_height)".
Code for this step depends heavily on the way that you store the source data.
example.c shows the following code for the case of a full-size 2-D source
array containing 3-byte RGB pixels:
JSAMPROW row_pointer[1]; /* pointer to a single row */
int row_stride; /* physical row width in buffer */
row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */
while (cinfo.next_scanline < cinfo.image_height) {
row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
jpeg_write_scanlines(&cinfo, row_pointer, 1);
}
jpeg_write_scanlines() returns the number of scanlines actually written.
This will normally be equal to the number passed in, so you can usually
ignore the return value. It is different in just two cases:
* If you try to write more scanlines than the declared image height,
the additional scanlines are ignored.
* If you use a suspending data destination manager, output buffer overrun
will cause the compressor to return before accepting all the passed lines.
This feature is discussed under "I/O suspension", below. The normal
stdio destination manager will NOT cause this to happen.
In any case, the return value is the same as the change in the value of
next_scanline.
6. jpeg_finish_compress(...);
After all the image data has been written, call jpeg_finish_compress() to
complete the compression cycle. This step is ESSENTIAL to ensure that the
last bufferload of data is written to the data destination.
jpeg_finish_compress() also releases working memory associated with the JPEG
object.
Typical code:
jpeg_finish_compress(&cinfo);
If using the stdio destination manager, don't forget to close the output
stdio stream if necessary.
If you have requested a multi-pass operating mode, such as Huffman code
optimization, jpeg_finish_compress() will perform the additional passes using
data buffered by the first pass. In this case jpeg_finish_compress() may take
quite a while to complete. With the default compression parameters, this will
not happen.
It is an error to call jpeg_finish_compress() before writing the necessary
total number of scanlines. If you wish to abort compression, call
jpeg_abort() as discussed below.
After completing a compression cycle, you may dispose of the JPEG object
as discussed next, or you may use it to compress another image. In that case
return to step 2, 3, or 4 as appropriate. If you do not change the
destination manager, the new datastream will be written to the same target.
If you do not change any JPEG parameters, the new datastream will be written
with the same parameters as before. Note that you can change the input image
dimensions freely between cycles, but if you change the input colorspace, you
should call jpeg_set_defaults() to adjust for the new colorspace; and then
you'll need to repeat all of step 3.
7. Release the JPEG compression object.
When you are done with a JPEG compression object, destroy it by calling
jpeg_destroy_compress(). This will free all subsidiary memory. Or you can
call jpeg_destroy() which works for either compression or decompression
objects --- this may be more convenient if you are sharing code between
compression and decompression cases. (Actually, these routines are equivalent
except for the declared type of the passed pointer. To avoid gripes from
ANSI C compilers, 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
jpeg_abort() which works on both compression and decompression objects.
This will return the object to an idle state, releasing any working memory.
jpeg_abort() is allowed at any time after successful object creation.
Note that cleaning up the data destination, if required, is your
responsibility.
Decompression details
---------------------
Here we revisit the JPEG decompression outline given in the overview.
1. Allocate and initialize a JPEG decompression object.
This is just like initialization for compression, as discussed above,
except that the object is a "struct jpeg_decompress_struct" and you
call jpeg_create_decompress(). Error handling is exactly the same.
Typical code:
struct jpeg_decompress_struct cinfo;
struct jpeg_error_mgr jerr;
...
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_decompress(&cinfo);
(Both here and in the IJG code, we usually use variable name "cinfo" for
both compression and decompression objects.)
2. Specify the source of the compressed data (eg, a file).
As previously mentioned, the JPEG library reads compressed data from a "data
source" module. The library includes one data source module which knows how
to read from a stdio stream. You can use your own source module if you want
to do something else, as discussed later.
If you use the standard source module, you must open the source stdio stream
beforehand. Typical code for this step looks like:
FILE * infile;
...
if ((infile = fopen(filename, "rb")) == NULL) {
fprintf(stderr, "can't open %sn", 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 won't return
more than cinfo.rec_outbuf_height scanlines per call, no matter how large
a buffer you pass.) So you must always provide a loop that calls
jpeg_read_scanlines() repeatedly until the whole image has been read.
7. jpeg_finish_decompress(...);
After all the image data has been read, call jpeg_finish_decompress() to
complete the decompression cycle. This causes working memory associated
with the JPEG object to be released.
Typical code:
jpeg_finish_decompress(&cinfo);
If using the stdio source manager, don't forget to close the source stdio
stream if necessary.
It is an error to call jpeg_finish_decompress() before reading the correct
total number of scanlines. If you wish to abort compression, call
jpeg_abort() as discussed below.
After completing a decompression cycle, you may dispose of the JPEG object as
discussed next, or you may use it to decompress another image. In that case
return to step 2 or 3 as appropriate. If you do not change the source
manager, the next image will be read from the same source.
8. Release the JPEG decompression object.
When you are done with a JPEG decompression object, destroy it by calling
jpeg_destroy_decompress() or jpeg_destroy(). The previous discussion of
destroying compression objects applies here too.
Typical code:
jpeg_destroy_decompress(&cinfo);
9. Aborting.
You can abort a decompression cycle by calling jpeg_destroy_decompress() or
jpeg_destroy() if you don't need the JPEG object any more, or
jpeg_abort_decompress() or jpeg_abort() if you want to reuse the object.
The previous discussion of aborting compression cycles applies here too.
Mechanics of usage: include files, linking, etc
-----------------------------------------------
Applications using the JPEG library should include the header file jpeglib.h
to obtain declarations of data types and routines. Before including
jpeglib.h, include system headers that define at least the typedefs FILE and
size_t. On ANSI-conforming systems, including <stdio.h> is sufficient; on
older Unix systems, you may need <sys/types.h> to define size_t.
If the application needs to refer to individual JPEG library error codes, also
include jerror.h to define those symbols.
jpeglib.h indirectly includes the files jconfig.h and jmorecfg.h. If you are
installing the JPEG header files in a system directory, you will want to
install all four files: jpeglib.h, jerror.h, jconfig.h, jmorecfg.h.
The most convenient way to include the JPEG code into your executable program
is to prepare a library file ("libjpeg.a", or a corresponding name on non-Unix
machines) and reference it at your link step. If you use only half of the
library (only compression or only decompression), only that much code will be
included from the library, unless your linker is hopelessly brain-damaged.
The supplied makefiles build libjpeg.a automatically (see install.doc).
On some systems your application may need to set up a signal handler to ensure
that temporary files are deleted if the program is interrupted. This is most
critical if you are on MS-DOS and use the jmemdos.c memory manager back end;
it will try to grab extended memory for temp files, and that space will NOT be
freed automatically. See cjpeg.c or djpeg.c for an example signal handler.
It may be worth pointing out that the core JPEG library does not actually
require the stdio library: only the default source/destination managers and
error handler need it. You can use the library in a stdio-less environment
if you replace those modules and use jmemnobs.c (or another memory manager of
your own devising). More info about the minimum system library requirements
may be found in jinclude.h.
ADVANCED FEATURES
=================
Compression parameter selection
-------------------------------
This section describes all the optional parameters you can set for JPEG
compression, as well as the "helper" routines provided to assist in this
task. Proper setting of some parameters requires detailed understanding
of the JPEG standard; if you don't know what a parameter is for, it's best
not to mess with it! See REFERENCES in the README file for pointers to
more info about JPEG.
It's a good idea to call jpeg_set_defaults() first, even if you plan to set
all the parameters; that way your code is more likely to work with future JPEG
libraries that have additional parameters. For the same reason, we recommend
you use a helper routine where one is provided, in preference to twiddling
cinfo fields directly.
The helper routines are:
jpeg_set_defaults (j_compress_ptr cinfo)
This routine sets all JPEG parameters to reasonable defaults, using
only the input image's color space (field in_color_space, which must
already be set in cinfo). Many applications will only need to use
this routine and perhaps jpeg_set_quality().
jpeg_set_colorspace (j_compress_ptr cinfo, J_COLOR_SPACE colorspace)
Sets the JPEG file's colorspace (field jpeg_color_space) as specified,
and sets other color-space-dependent parameters appropriately. See
"Special color spaces", below, before using this. A large number of
parameters, including all per-component parameters, are set by this
routine; if you want to twiddle individual parameters you should call
jpeg_set_colorspace() before rather than after.
jpeg_default_colorspace (j_compress_ptr cinfo)
Selects an appropriate JPEG colorspace based on cinfo->in_color_space,
and calls jpeg_set_colorspace(). This is actually a subroutine of
jpeg_set_defaults(). It's broken out in case you want to change
just the colorspace-dependent JPEG parameters.
jpeg_set_quality (j_compress_ptr cinfo, int quality, boolean force_baseline)
Constructs JPEG quantization tables appropriate for the indicated
quality setting. The quality value is expressed on the 0..100 scale
recommended by IJG (cjpeg's "-quality" switch uses this routine).
Note that the exact mapping from quality values to tables may change
in future IJG releases as more is learned about DCT quantization.
If the force_baseline parameter is TRUE, then the quantization table
entries are constrained to the range 1..255 for full JPEG baseline
compatibility. In the current implementation, this only makes a
difference for quality settings below 25, and it effectively prevents
very small/low quality files from being generated. The IJG decoder
is capable of reading the non-baseline files generated at low quality
settings when force_baseline is FALSE, but other decoders may not be.
jpeg_set_linear_quality (j_compress_ptr cinfo, int scale_factor,
boolean force_baseline)
Same as jpeg_set_quality() except that the generated tables are the
sample tables given in the JPEC spec section K.1, multiplied by the
specified scale factor (which is expressed as a percentage; thus
scale_factor = 100 reproduces the spec's tables). Note that larger
scale factors give lower quality. This entry point is useful for
conforming to the Adobe PostScript DCT conventions, but we do not
recommend linear scaling as a user-visible quality scale otherwise.
force_baseline again constrains the computed table entries to 1..255.
int jpeg_quality_scaling (int quality)
Converts a value on the IJG-recommended quality scale to a linear
scaling percentage. Note that this routine may change or go away
in future releases --- IJG may choose to adopt a scaling method that
can't be expressed as a simple scalar multiplier, in which case the
premise of this routine collapses. Caveat user.
jpeg_add_quant_table (j_compress_ptr cinfo, int which_tbl,
const unsigned int *basic_table,
int scale_factor, boolean force_baseline)
Allows an arbitrary quantization table to be created. which_tbl
indicates which table slot to fill. basic_table points to an array
of 64 unsigned ints given in normal array order. These values are
multiplied by scale_factor/100 and then clamped to the range 1..65535
(or to 1..255 if force_baseline is TRUE).
CAUTION: prior to library version 6a, jpeg_add_quant_table expected
the basic table to be given in JPEG zigzag order. If you need to
write code that works with either older or newer versions of this
routine, you must check the library version number. Something like
"#if JPEG_LIB_VERSION >= 61" is the right test.
jpeg_simple_progression (j_compress_ptr cinfo)
Generates a default scan script for writing a progressive-JPEG file.
This is the recommended method of creating a progressive file,
unless you want to make a custom scan sequence. You must ensure that
the JPEG color space is set correctly before calling this routine.
Compression parameters (cinfo fields) include:
J_DCT_METHOD dct_method
Selects the algorithm used for the DCT step. Choices are:
JDCT_ISLOW: slow but accurate integer algorithm
JDCT_IFAST: faster, less accurate integer method
JDCT_FLOAT: floating-point method
JDCT_DEFAULT: default method (normally JDCT_ISLOW)
JDCT_FASTEST: fastest method (normally JDCT_IFAST)
The FLOAT method is very slightly more accurate than the ISLOW method,
but may give different results on different machines due to varying
roundoff behavior. The integer methods should give the same results
on all machines. On machines with sufficiently fast FP hardware, the
floating-point method may also be the fastest. The IFAST method is
considerably less accurate than the other two; its use is not
recommended if high quality is a concern. JDCT_DEFAULT and
JDCT_FASTEST are macros configurable by each installation.
J_COLOR_SPACE jpeg_color_space
int num_components
The JPEG color space and corresponding number of components; see
"Special color spaces", below, for more info. We recommend using
jpeg_set_color_space() if you want to change these.
boolean optimize_coding
TRUE causes the compressor to compute optimal Huffman coding tables
for the image. This requires an extra pass over the data and
therefore costs a good deal of space and time. The default is
FALSE, which tells the compressor to use the supplied or default
Huffman tables. In most cases optimal tables save only a few percent
of file size compared to the default tables. Note that when this is
TRUE, you need not supply Huffman tables at all, and any you do
supply will be overwritten.
unsigned int restart_interval
int restart_in_rows
To emit restart markers in the JPEG file, set one of these nonzero.
Set restart_interval to specify the exact interval in MCU blocks.
Set restart_in_rows to specify the interval in MCU rows. (If
restart_in_rows is not 0, then restart_interval is set after the
image width in MCUs is computed.) Defaults are zero (no restarts).
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 density_unit
UINT16 X_density
UINT16 Y_density
The resolution information to be written into the JFIF marker;
not used otherwise. density_unit may be 0 for unknown,
1 for dots/inch, or 2 for dots/cm. The default values are 0,1,1
indicating square pixels of unknown size.
boolean write_Adobe_marker
If TRUE, an Adobe APP14 marker is emitted. jpeg_set_defaults() and
jpeg_set_colorspace() set this TRUE if JPEG color space RGB, CMYK,
or YCCK is selected, otherwise FALSE. It is generally a bad idea
to set both write_JFIF_header and write_Adobe_marker. In fact,
you probably shouldn't change the default settings at all --- the
default behavior ensures that the JPEG file's color space can be
recognized by the decoder.
JQUANT_TBL * quant_tbl_ptrs[NUM_QUANT_TBLS]
Pointers to coefficient quantization tables, one per table slot,
or NULL if no table is defined for a slot. Usually these should
be set via one of the above helper routines; jpeg_add_quant_table()
is general enough to define any quantization table. The other
routines will set up table slot 0 for luminance quality and table
slot 1 for chrominance.
JHUFF_TBL * dc_huff_tbl_ptrs[NUM_HUFF_TBLS]
JHUFF_TBL * ac_huff_tbl_ptrs[NUM_HUFF_TBLS]
Pointers to Huffman coding tables, one per table slot, or NULL if
no table is defined for a slot. Slots 0 and 1 are filled with the
JPEG sample tables by jpeg_set_defaults(). If you need to allocate
more table structures, jpeg_alloc_huff_table() may be used.
Note that optimal Huffman tables can be computed for an image
by setting optimize_coding, as discussed above; there's seldom
any need to mess with providing your own Huffman tables.
There are some additional cinfo fields which are not documented here
because you currently can't change them; for example, you can't set
arith_code TRUE because arithmetic coding is unsupported.
Per-component parameters are stored in the struct cinfo.comp_info[i] for
component number i. Note that components here refer to components of the
JPEG color space, *not* the source image color space. A suitably large
comp_info[] array is allocated by jpeg_set_defaults(); if you choose not
to use that routine, it's up to you to allocate the array.
int component_id
The one-byte identifier code to be recorded in the JPEG file for
this component. For the standard color spaces, we recommend you
leave the default values alone.
int h_samp_factor
int v_samp_factor
Horizontal and vertical sampling factors for the component; must
be 1..4 according to the JPEG standard. Note that larger sampling
factors indicate a higher-resolution component; many people find
this behavior quite unintuitive. The default values are 2,2 for
luminance components and 1,1 for chrominance components, except
for grayscale where 1,1 is used.
int quant_tbl_no
Quantization table number for component. The default value is
0 for luminance components and 1 for chrominance components.
int dc_tbl_no
int ac_tbl_no
DC and AC entropy coding table numbers. The default values are
0 for luminance components and 1 for chrominance components.
int component_index
Must equal the component's index in comp_info[]. (Beginning in
release v6, the compressor library will fill this in automatically;
you don't have to.)
Decompression parameter selection
---------------------------------
Decompression parameter selection is somewhat simpler than compression
parameter selection, since all of the JPEG internal parameters are
recorded in the source file and need not be supplied by the application.
(Unless you are working with abbreviated files, in which case see
"Abbreviated datastreams", below.) Decompression parameters control
the postprocessing done on the image to deliver it in a format suitable
for the application's use. Many of the parameters control speed/quality
tradeoffs, in which faster decompression may be obtained at the price of
a poorer-quality image. The defaults select the highest quality (slowest)
processing.
The following fields in the JPEG object are set by jpeg_read_header() and
may be useful to the application in choosing decompression parameters:
JDIMENSION image_width Width and height of image
JDIMENSION image_height
int num_components Number of color components
J_COLOR_SPACE jpeg_color_space Colorspace of image
boolean saw_JFIF_marker TRUE if a JFIF APP0 marker was seen
UINT8 density_unit Resolution data from JFIF marker
UINT16 X_density
UINT16 Y_density
boolean saw_Adobe_marker TRUE if an Adobe APP14 marker was seen
UINT8 Adobe_transform Color transform code from Adobe marker
The JPEG color space, unfortunately, is something of a guess since the JPEG
standard proper does not provide a way to record it. In practice most files
adhere to the JFIF or Adobe conventions, and the decoder will recognize these
correctly. See "Special color spaces", below, for more info.
The decompression parameters that determine the basic properties of the
returned image are:
J_COLOR_SPACE out_color_space
Output color space. jpeg_read_header() sets an appropriate default
based on jpeg_color_space; typically it will be RGB or grayscale.
The application can change this field to request output in a different
colorspace. For example, set it to JCS_GRAYSCALE to get grayscale
output from a color file. (This is useful for previewing: grayscale
output is faster than full color since the color components need not
be processed.) Note that not all possible color space transforms are
currently implemented; you may need to extend jdcolor.c if you want an
unusual conversion.
unsigned int scale_num, scale_denom
Scale the image by the fraction scale_num/scale_denom. Default is
1/1, or no scaling. Currently, the only supported scaling ratios
are 1/1, 1/2, 1/4, and 1/8. (The library design allows for arbitrary
scaling ratios but this is not likely to be implemented any time soon.)
Smaller scaling ratios permit significantly faster decoding since
fewer pixels need be processed and a simpler IDCT method can be used.
boolean quantize_colors
If set TRUE, colormapped output will be delivered. Default is FALSE,
meaning that full-color output will be delivered.
The next three parameters are relevant only if quantize_colors is TRUE.
int desired_number_of_colors
Maximum number of colors to use in generating a library-supplied color
map (the actual number of colors is returned in a different field).
Default 256. Ignored when the application supplies its own color map.
boolean two_pass_quantize
If TRUE, an extra pass over the image is made to select a custom color
map for the image. This usually looks a lot better than the one-size-
fits-all colormap that is used otherwise. Default is TRUE. Ignored
when the application supplies its own color map.
J_DITHER_MODE dither_mode
Selects color dithering method. Supported values are:
JDITHER_NONE no dithering: fast, very low quality
JDITHER_ORDERED ordered dither: moderate speed and quality
JDITHER_FS Floyd-Steinberg dither: slow, high quality
Default is JDITHER_FS. (At present, ordered dither is implemented
only in the single-pass, standard-colormap case. If you ask for
ordered dither when two_pass_quantize is TRUE or when you supply
an external color map, you'll get F-S dithering.)
When quantize_colors is TRUE, the target color map is described by the next
two fields. colormap is set to NULL by jpeg_read_header(). The application
can supply a color map by setting colormap non-NULL and setting
actual_number_of_colors to the map size. Otherwise, jpeg_start_decompress()
selects a suitable color map and sets these two fields itself.
[Implementation restriction: at present, an externally supplied colormap is
only accepted for 3-component output color spaces.]
JSAMPARRAY colormap
The color map, represented as a 2-D pixel array of out_color_components
rows and actual_number_of_colors columns. Ignored if not quantizing.
CAUTION: if the JPEG library creates its own colormap, the storage
pointed to by this field is released by jpeg_finish_decompress().
Copy the colormap somewhere else first, if you want to save it.
int actual_number_of_colors
The number of colors in the color map.
Additional decompression parameters that the application may set include:
J_DCT_METHOD dct_method
Selects the algorithm used for the DCT step. Choices are the same
as described above for compression.
boolean do_fancy_upsampling
If TRUE, do careful upsampling of chroma components. If FALSE,
a faster but sloppier method is used. Default is TRUE. The visual
impact of the sloppier method is often very small.
boolean do_block_smoothing
If TRUE, interblock smoothing is applied in early stages of decoding
progressive JPEG files; if FALSE, not. Default is TRUE. Early
progression stages look "fuzzy" with smoothing, "blocky" without.
In any case, block smoothing ceases to be applied after the first few
AC coefficients are known to full accuracy, so it is relevant only
when using buffered-image mode for progressive images.
boolean enable_1pass_quant
boolean enable_external_quant
boolean enable_2pass_quant
These are significant only in buffered-image mode, which is
described in its own section below.
The output image dimensions are given by the following fields. These are
computed from the source image dimensions and the decompression parameters
by jpeg_start_decompress(). You can also call jpeg_calc_output_dimensions()
to obtain the values that will result from the current parameter settings.
This can be useful if you are trying to pick a scaling ratio that will get
close to a desired target size. It's also important if you are using the
JPEG library's memory manager to allocate output buffer space, because you
are supposed to request such buffers *before* jpeg_start_decompress().
JDIMENSION output_width Actual dimensions of output image.
JDIMENSION output_height
int out_color_components Number of color components in out_color_space.
int output_components Number of color components returned.
int rec_outbuf_height Recommended height of scanline buffer.
When quantizing colors, output_components is 1, indicating a single color map
index per pixel. Otherwise it equals out_color_components. The output arrays
are required to be output_width * output_components JSAMPLEs wide.
rec_outbuf_height is the recommended minimum height (in scanlines) of the
buffer passed to jpeg_read_scanlines(). If the buffer is smaller, the
library will still work, but time will be wasted due to unnecessary data
copying. In high-quality modes, rec_outbuf_height is always 1, but some
faster, lower-quality modes set it to larger values (typically 2 to 4).
If you are going to ask for a high-speed processing mode, you may as well
go to the trouble of honoring rec_outbuf_height so as to avoid data copying.
Special color spaces
--------------------
The JPEG standard itself is "color blind" and doesn't specify any particular
color space. It is customary to convert color data to a luminance/chrominance
color space before compressing, since this permits greater compression. The
existing de-facto JPEG file format standards specify YCbCr or grayscale data
(JFIF), or grayscale, RGB, YCbCr, CMYK, or YCCK (Adobe). For special
applications such as multispectral images, other color spaces can be used,
but it must be understood that such files will be unportable.
The JPEG library can handle the most common colorspace conversions (namely
RGB <=> YCbCr and CMYK <=> YCCK). It can also deal with data of an unknown
color space, passing it through without conversion. If you deal extensively
with an unusual color space, you can easily extend the library to understand
additional color spaces and perform appropriate conversions.
For compression, the source data's color space is specified by field
in_color_space. This is transformed to the JPEG file's color space given
by jpeg_color_space. jpeg_set_defaults() chooses a reasonable JPEG color
space depending on in_color_space, but you can override this by calling
jpeg_set_colorspace(). Of course you must select a supported transformation.
jccolor.c currently supports the following transformations:
RGB => YCbCr
RGB => GRAYSCALE
YCbCr => GRAYSCALE
CMYK => YCCK
plus the null transforms: GRAYSCALE => GRAYSCALE, RGB => RGB,
YCbCr => YCbCr, CMYK => CMYK, YCCK => YCCK, and UNKNOWN => UNKNOWN.
The de-facto file format standards (JFIF and Adobe) specify APPn markers that
indicate the color space of the JPEG file. It is important to ensure that
these are written correctly, or omitted if the JPEG file's color space is not
one of the ones supported by the de-facto standards. jpeg_set_colorspace()
will set the compression parameters to include or omit the APPn markers
properly, so long as it is told the truth about the JPEG color space.
For example, if you are writing some random 3-component color space without
conversion, don't try to fake out the library by setting in_color_space and
jpeg_color_space to JCS_YCbCr; use JCS_UNKNOWN. You may want to write an
APPn marker of your own devising to identify the colorspace --- see "Special
markers", below.
When told that the color space is UNKNOWN, the library will default to using
luminance-quality compression parameters for all color components. You may
well want to change these parameters. See the source code for
jpeg_set_colorspace(), in jcparam.c, for details.
For decompression, the JPEG file's color space is given in jpeg_color_space,
and this is transformed to the output color space out_color_space.
jpeg_read_header's setting of jpeg_color_space can be relied on if the file
conforms to JFIF or Adobe conventions, but otherwise it is no better than a
guess. If you know the JPEG file's color space for certain, you can override
jpeg_read_header's guess by setting jpeg_color_space. jpeg_read_header also
selects a default output color space based on (its guess of) jpeg_color_space;
set out_color_space to override this. Again, you must select a supported
transformation. jdcolor.c currently supports
YCbCr => GRAYSCALE
YCbCr => RGB
YCCK => CMYK
as well as the null transforms.
The two-pass color quantizer, jquant2.c, is specialized to handle RGB data
(it weights distances appropriately for RGB colors). You'll need to modify
the code if you want to use it for non-RGB output color spaces. Note that
jquant2.c is used to map to an application-supplied colormap as well as for
the normal two-pass colormap selection process.
CAUTION: it appears that Adobe Photoshop writes inverted data in CMYK JPEG
files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect.
This is arguably a bug in Photoshop, but if you need to work with Photoshop
CMYK files, you will have to deal with it in your application. We cannot
"fix" this in the library by inverting the data during the CMYK<=>YCCK
transform, because that would break other applications, notably Ghostscript.
Photoshop versions prior to 3.0 write EPS files containing JPEG-encoded CMYK
data in the same inverted-YCCK representation used in bare JPEG files, but
the surrounding PostScript code performs an inversion using the PS image
operator. I am told that Photoshop 3.0 will write uninverted YCCK in
EPS/JPEG files, and will omit the PS-level inversion. (But the data
polarity used in bare JPEG files will not change in 3.0.) In either case,
the JPEG library must not invert the data itself, or else Ghostscript would
read these EPS files incorrectly.
Error handling
--------------
When the default error handler is used, any error detected inside the JPEG
routines will cause a message to be printed on stderr, followed by exit().
You can supply your own error handling routines to override this behavior
and to control the treatment of nonfatal warnings and trace/debug messages.
The file example.c illustrates the most common case, which is to have the
application regain control after an error rather than exiting.
The JPEG library never writes any message directly; it always goes through
the error handling routines. Three classes of messages are recognized:
* Fatal errors: the library cannot continue.
* Warnings: the library can continue, but the data is corrupt, and a
damaged output image is likely to result.
* Trace/informational messages. These come with a trace level indicating
the importance of the message; you can control the verbosity of the
program by adjusting the maximum trace level that will be displayed.
You may, if you wish, simply replace the entire JPEG error handling module
(jerror.c) with your own code. However, you can avoid code duplication by
only replacing some of the routines depending on the behavior you need.
This is accomplished by calling jpeg_std_error() as usual, but then overriding
some of the method pointers in the jpeg_error_mgr struct, as illustrated by
example.c.
All of the error handling routines will receive a pointer to the JPEG object
(a j_common_ptr which points to either a jpeg_compress_struct or a
jpeg_decompress_struct; if you need to tell which, test the is_decompressor
field). This struct includes a pointer to the error manager struct in its
"err" field. Frequently, custom error handler routines will need to access
additional data which is not known to the JPEG library or the standard error
handler. The most convenient way to do this is to embed either the JPEG
object or the jpeg_error_mgr struct in a larger structure that contains
additional fields; then casting the passed pointer provides access to the
additional fields. Again, see example.c for one way to do it.
The individual methods that you might wish to override are:
error_exit (j_common_ptr cinfo)
Receives control for a fatal error. Information sufficient to
generate the error message has been stored in cinfo->err; call
output_message to display it. Control must NOT return to the caller;
generally this routine will exit() or longjmp() somewhere.
Typically you would override this routine to get rid of the exit()
default behavior. Note that if you continue processing, you should
clean up the JPEG object with jpeg_abort() or jpeg_destroy().
output_message (j_common_ptr cinfo)
Actual output of any JPEG message. Override this to send messages
somewhere other than stderr. Note that this method does not know
how to generate a message, only where to send it.
format_message (j_common_ptr cinfo, char * buffer)
Constructs a readable error message string based on the error info
stored in cinfo->err. This method is called by output_message. Few
applications should need to override this method. One possible
reason for doing so is to implement dynamic switching of error message
language.
emit_message (j_common_ptr cinfo, int msg_level)
Decide whether or not to emit a warning or trace message; if so,
calls output_message. The main reason for overriding this method
would be to abort on warnings. msg_level is -1 for warnings,
0 and up for trace messages.
Only error_exit() and emit_message() are called from the rest of the JPEG
library; the other two are internal to the error handler.
The actual message texts are stored in an array of strings which is pointed to
by the field err->jpeg_message_table. The messages are numbered from 0 to
err->last_jpeg_message, and it is these code numbers that are used in the
JPEG library code. You could replace the message texts (for instance, with
messages in French or German) by changing the message table pointer. See
jerror.h for the default texts. CAUTION: this table will almost certainly
change or grow from one library version to the next.
It may be useful for an application to add its own message texts that are
handled by the same mechanism. The error handler supports a second "add-on"
message table for this purpose. To define an addon table, set the pointer
err->addon_message_table and the message numbers err->first_addon_message and
err->last_addon_message. If you number the addon messages beginning at 1000
or so, you won't have to worry about conflicts with the library's built-in
messages. See the sample applications cjpeg/djpeg for an example of using
addon messages (the addon messages are defined in cderror.h).
Actual invocation of the error handler is done via macros defined in jerror.h:
ERREXITn(...) for fatal errors
WARNMSn(...) for corrupt-data warnings
TRACEMSn(...) for trace and informational messages.
These macros store the message code and any additional parameters into the
error handler struct, then invoke the error_exit() or emit_message() method.
The variants of each macro are for varying numbers of additional parameters.
The additional parameters are inserted into the generated message using
standard printf() format codes.
See jerror.h and jerror.c for further details.
Compressed data handling (source and destination managers)
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The JPEG compression library sends its compressed data to a "destination
manager" module. The default destination manager just writes the data to a
stdio stream, but you can provide your own manager to do something else.
Similarly, the decompression library calls a "source manager" to obtain the
compressed data; you can provide your own source manager if you want the data
to come from somewhere other than a stdio stream.
In both cases, compressed data is processed a bufferload at a time: the
destination or source manager provides a work buffer, and the library invokes
the manager only when the buffer is filled or emptied. (You could define a
one-character buffer to force the manager to be invoked for each byte, but
that would be rather inefficient.) The buffer's size and location are
controlled by the manager, not by the library. For example, if you desired to
decompress a JPEG datastream that was all in memory, you could just make the
buffer pointer and length point to the original data in memory. Then the
buffer-reload procedure would be invoked only if the decompressor ran off the
end of the datastream, which would indicate an erroneous datastream.
The work buffer is defined as an array of datatype JOCTET, which is generally
"char" or "unsigned char". On a machine where char is not exactly 8 bits
wide, you must define JOCTET as a wider data type and then modify the data
source and destination modules to transcribe the work arrays into 8-bit units
on external storage.
A data destination manager struct contains a pointer and count defining the
next byte to write in the work buffer and the remaining free space:
JOCTET * next_output_byte; /* => next byte to write in buffer */
size_t free_in_buffer; /* # of byte spaces remaining in buffer */
The library increments the pointer and decrements the count until the buffer
is filled. The manager's empty_output_buffer method must reset the pointer
and count. The manager is expected to remember the buffer's starting address
and total size in private fields not visible to the library.
A data destination manager provides three methods:
init_destination (j_compress_ptr cinfo)
Initialize destination. This is called by jpeg_start_compress()
before any data is actually written. It must initialize
next_output_byte and free_in_buffer. free_in_buffer must be
initialized to a positive value.
empty_output_buffer (j_compress_ptr cinfo)
This is called whenever the buffer has filled (free_in_buffer
reaches zero). In typical applications, it should write out the
*entire* buffer (use the saved start address and buffer length;
ignore the current state of next_output_byte and free_in_buffer).
Then reset the pointer & count to the start of the buffer, and
return TRUE indicating that the buffer has been dumped.
free_in_buffer must be set to a positive value when TRUE is
returned. A FALSE return should only be used when I/O suspension is
desired (this operating mode is discussed in the next section).