USING THE IJG JPEG LIBRARY

Copyright (C) 1994-1998, Thomas G. Lane.

This file is part of the Independent JPEG Group's software.

For conditions of distribution and use, see the accompanying README file.

This file describes how to use the IJG JPEG library within an application

program.  Read it if you want to write a program that uses the library.

The file example.c provides heavily commented skeleton code for calling the

JPEG library.  Also see jpeglib.h (the include file to be used by application

programs) for full details about data structures and function parameter lists.

The library source code, of course, is the ultimate reference.

Note that there have been *major* changes from the application interface

presented by IJG version 4 and earlier versions.  The old design had several

inherent limitations, and it had accumulated a lot of cruft as we added

features while trying to minimize application-interface changes.  We have

sacrificed backward compatibility in the version 5 rewrite, but we think the

improvements justify this.

TABLE OF CONTENTS

-----------------

Overview:

	Functions provided by the library

	Outline of typical usage

Basic library usage:

	Data formats

	Compression details

	Decompression details

	Mechanics of usage: include files, linking, etc

Advanced features:

	Compression parameter selection

	Decompression parameter selection

	Special color spaces

	Error handling

	Compressed data handling (source and destination managers)

	I/O suspension

	Progressive JPEG support

	Buffered-image mode

	Abbreviated datastreams and multiple images

	Special markers

	Raw (downsampled) image data

	Really raw data: DCT coefficients

	Progress monitoring

	Memory management

	Memory usage

	Library compile-time options

	Portability considerations

	Notes for MS-DOS implementors

You should read at least the overview and basic usage sections before trying

to program with the library.  The sections on advanced features can be read

if and when you need them.

OVERVIEW

========

Functions provided by the library

---------------------------------

The IJG JPEG library provides C code to read and write JPEG-compressed image

files.  The surrounding application program receives or supplies image data a

scanline at a time, using a straightforward uncompressed image format.  All

details of color conversion and other preprocessing/postprocessing can be

handled by the library.

The library includes a substantial amount of code that is not covered by the

JPEG standard but is necessary for typical applications of JPEG.  These

functions preprocess the image before JPEG compression or postprocess it after

decompression.  They include colorspace conversion, downsampling/upsampling,

and color quantization.  The application indirectly selects use of this code

by specifying the format in which it wishes to supply or receive image data.

For example, if colormapped output is requested, then the decompression

library automatically invokes color quantization.

A wide range of quality vs. speed tradeoffs are possible in JPEG processing,

and even more so in decompression postprocessing.  The decompression library

provides multiple implementations that cover most of the useful tradeoffs,

ranging from very-high-quality down to fast-preview operation.  On the

compression side we have generally not provided low-quality choices, since

compression is normally less time-critical.  It should be understood that the

low-quality modes may not meet the JPEG standard's accuracy requirements;

nonetheless, they are useful for viewers.

A word about functions *not* provided by the library.  We handle a subset of

the ISO JPEG standard; most baseline, extended-sequential, and progressive

JPEG processes are supported.  (Our subset includes all features now in common

use.)  Unsupported ISO options include:

	* Hierarchical storage

	* Lossless JPEG

	* Arithmetic entropy coding (unsupported for legal reasons)

	* DNL marker

	* Nonintegral subsampling ratios

We support both 8- and 12-bit data precision, but this is a compile-time

choice rather than a run-time choice; hence it is difficult to use both

precisions in a single application.

By itself, the library handles only interchange JPEG datastreams --- in

particular the widely used JFIF file format.  The library can be used by

surrounding code to process interchange or abbreviated JPEG datastreams that

are embedded in more complex file formats.  (For example, this library is

used by the free LIBTIFF library to support JPEG compression in TIFF.)

Outline of typical usage

------------------------

The rough outline of a JPEG compression operation is:

	Allocate and initialize a JPEG compression object

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

	Set parameters for compression, including image size & colorspace

	jpeg_start_compress(...);

	while (scan lines remain to be written)

		jpeg_write_scanlines(...);

	jpeg_finish_compress(...);

	Release the JPEG compression object

A JPEG compression object holds parameters and working state for the JPEG

library.  We make creation/destruction of the object separate from starting

or finishing compression of an image; the same object can be re-used for a

series of image compression operations.  This makes it easy to re-use the

same parameter settings for a sequence of images.  Re-use of a JPEG object

also has important implications for processing abbreviated JPEG datastreams,

as discussed later.

The image data to be compressed is supplied to jpeg_write_scanlines() from

in-memory buffers.  If the application is doing file-to-file compression,

reading image data from the source file is the application's responsibility.

The library emits compressed data by calling a "data destination manager",

which typically will write the data into a file; but the application can

provide its own destination manager to do something else.

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

	Allocate and initialize a JPEG decompression object

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

	Call jpeg_read_header() to obtain image info

	Set parameters for decompression

	jpeg_start_decompress(...);

	while (scan lines remain to be read)

		jpeg_read_scanlines(...);

	jpeg_finish_decompress(...);

	Release the JPEG decompression object

This is comparable to the compression outline except that reading the

datastream header is a separate step.  This is helpful because information

about the image's size, colorspace, etc is available when the application

selects decompression parameters.  For example, the application can choose an

output scaling ratio that will fit the image into the available screen size.

The decompression library obtains compressed data by calling a data source

manager, which typically will read the data from a file; but other behaviors

can be obtained with a custom source manager.  Decompressed data is delivered

into in-memory buffers passed to jpeg_read_scanlines().

It is possible to abort an incomplete compression or decompression operation

by calling jpeg_abort(); or, if you do not need to retain the JPEG object,

simply release it by calling jpeg_destroy().

JPEG compression and decompression objects are two separate struct types.

However, they share some common fields, and certain routines such as

jpeg_destroy() can work on either type of object.

The JPEG library has no static variables: all state is in the compression

or decompression object.  Therefore it is possible to process multiple

compression and decompression operations concurrently, using multiple JPEG

objects.

Both compression and decompression can be done in an incremental memory-to-

memory fashion, if suitable source/destination managers are used.  See the

section on "I/O suspension" for more details.

BASIC LIBRARY USAGE

===================

Data formats

------------

Before diving into procedural details, it is helpful to understand the

image data format that the JPEG library expects or returns.

The standard input image format is a rectangular array of pixels, with each

pixel having the same number of "component" or "sample" values (color

channels).  You must specify how many components there are and the colorspace

interpretation of the components.  Most applications will use RGB data

(three components per pixel) or grayscale data (one component per pixel).

PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE.

A remarkable number of people manage to miss this, only to find that their

programs don't work with grayscale JPEG files.

There is no provision for colormapped input.  JPEG files are always full-color

or full grayscale (or sometimes another colorspace such as CMYK).  You can

feed in a colormapped image by expanding it to full-color format.  However

JPEG often doesn't work very well with source data that has been colormapped,

because of dithering noise.  This is discussed in more detail in the JPEG FAQ

and the other references mentioned in the README file.

Pixels are stored by scanlines, with each scanline running from left to

right.  The component values for each pixel are adjacent in the row; for

example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color.  Each scanline is an

array of data type JSAMPLE --- which is typically "unsigned char", unless

you've changed jmorecfg.h.  (You can also change the RGB pixel layout, say

to B,G,R order, by modifying jmorecfg.h.  But see the restrictions listed in

that file before doing so.)

A 2-D array of pixels is formed by making a list of pointers to the starts of

scanlines; so the scanlines need not be physically adjacent in memory.  Even

if you process just one scanline at a time, you must make a one-element

pointer array to conform to this structure.  Pointers to JSAMPLE rows are of

type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY.

The library accepts or supplies one or more complete scanlines per call.

It is not possible to process part of a row at a time.  Scanlines are always

processed top-to-bottom.  You can process an entire image in one call if you

have it all in memory, but usually it's simplest to process one scanline at

a time.

For best results, source data values should have the precision specified by

BITS_IN_JSAMPLE (normally 8 bits).  For instance, if you choose to compress

data that's only 6 bits/channel, you should left-justify each value in a

byte before passing it to the compressor.  If you need to compress data

that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 12.

(See "Library compile-time options", later.)

The data format returned by the decompressor is the same in all details,

except that colormapped output is supported.  (Again, a JPEG file is never

colormapped.  But you can ask the decompressor to perform on-the-fly color

quantization to deliver colormapped output.)  If you request colormapped

output then the returned data array contains a single JSAMPLE per pixel;

its value is an index into a color map.  The color map is represented as

a 2-D JSAMPARRAY in which each row holds the values of one color component,

that is, colormap[i][j] is the value of the i'th color component for pixel

value (map index) j.  Note that since the colormap indexes are stored in

JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE

(ie, at most 256 colors for an 8-bit JPEG library).

Compression details

-------------------

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

1. Allocate and initialize a JPEG compression object.

A JPEG compression object is a "struct jpeg_compress_struct".  (It also has

a bunch of subsidiary structures which are allocated via malloc(), but the

application doesn't control those directly.)  This struct can be just a local

variable in the calling routine, if a single routine is going to execute the

whole JPEG compression sequence.  Otherwise it can be static or allocated

from malloc().

You will also need a structure representing a JPEG error handler.  The part

of this that the library cares about is a "struct jpeg_error_mgr".  If you

are providing your own error handler, you'll typically want to embed the

jpeg_error_mgr struct in a larger structure; this is discussed later under

"Error handling".  For now we'll assume you are just using the default error

handler.  The default error handler will print JPEG error/warning messages

on stderr, and it will call exit() if a fatal error occurs.

You must initialize the error handler structure, store a pointer to it into

the JPEG object's "err" field, and then call jpeg_create_compress() to

initialize the rest of the JPEG object.

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

	struct jpeg_compress_struct cinfo;

	struct jpeg_error_mgr jerr;

	...

	cinfo.err = jpeg_std_error(&jerr);

	jpeg_create_compress(&cinfo);

jpeg_create_compress allocates a small amount of memory, so it could fail

if you are out of memory.  In that case it will exit via the error handler;

that's why the error handler must be initialized first.

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

As previously mentioned, the JPEG library delivers compressed data to a

"data destination" module.  The library includes one data destination

module which knows how to write to a stdio stream.  You can use your own

destination module if you want to do something else, as discussed later.

If you use the standard destination module, you must open the target stdio

stream beforehand.  Typical code for this step looks like:

	FILE * outfile;

	...

	if ((outfile = fopen(filename, "wb")) == NULL) {

	    fprintf(stderr, "can't open %s\n", filename);

	    exit(1);

	}

	jpeg_stdio_dest(&cinfo, outfile);

where the last line invokes the standard destination module.

WARNING: it is critical that the binary compressed data be delivered to the

output file unchanged.  On non-Unix systems the stdio library may perform

newline translation or otherwise corrupt binary data.  To suppress this

behavior, you may need to use a "b" option to fopen (as shown above), or use

setmode() or another routine to put the stdio stream in binary mode.  See

cjpeg.c and djpeg.c for code that has been found to work on many systems.

You can select the data destination after setting other parameters (step 3),

if that's more convenient.  You may not change the destination between

calling jpeg_start_compress() and jpeg_finish_compress().

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

You must supply information about the source image by setting the following

fields in the JPEG object (cinfo structure):

	image_width		Width of image, in pixels

	image_height		Height of image, in pixels

	input_components	Number of color channels (samples per pixel)

	in_color_space		Color space of source image

The image dimensions are, hopefully, obvious.  JPEG supports image dimensions

of 1 to 64K pixels in either direction.  The input color space is typically

RGB or grayscale, and input_components is 3 or 1 accordingly.  (See "Special

color spaces", later, for more info.)  The in_color_space field must be

assigned one of the J_COLOR_SPACE enum constants, typically JCS_RGB or

JCS_GRAYSCALE.

JPEG has a large number of compression parameters that determine how the

image is encoded.  Most applications don't need or want to know about all

these parameters.  You can set all the parameters to reasonable defaults by

calling jpeg_set_defaults(); then, if there are particular values you want

to change, you can do so after that.  The "Compression parameter selection"

section tells about all the parameters.

You must set in_color_space correctly before calling jpeg_set_defaults(),

because the defaults depend on the source image colorspace.  However the

other three source image parameters need not be valid until you call

jpeg_start_compress().  There's no harm in calling jpeg_set_defaults() more

than once, if that happens to be convenient.

Typical code for a 24-bit RGB source image is

	cinfo.image_width = Width; 	/* image width and height, in pixels */

	cinfo.image_height = Height;

	cinfo.input_components = 3;	/* # of color components per pixel */

	cinfo.in_color_space = JCS_RGB; /* colorspace of input image */

	jpeg_set_defaults(&cinfo);

	/* Make optional parameter settings here */

4. jpeg_start_compress(...);

After you have established the data destination and set all the necessary

source image info and other parameters, call jpeg_start_compress() to begin

a compression cycle.  This will initialize internal state, allocate working

storage, and emit the first few bytes of the JPEG datastream header.

Typical code:

	jpeg_start_compress(&cinfo, TRUE);

The "TRUE" parameter ensures that a complete JPEG interchange datastream

will be written.  This is appropriate in most cases.  If you think you might

want to use an abbreviated datastream, read the section on abbreviated

datastreams, below.

Once you have called jpeg_start_compress(), you may not alter any JPEG

parameters or other fields of the JPEG object until you have completed

the compression cycle.

5. while (scan lines remain to be written)

	jpeg_write_scanlines(...);

Now write all the required image data by calling jpeg_write_scanlines()

one or more times.  You can pass one or more scanlines in each call, up

to the total image height.  In most applications it is convenient to pass

just one or a few scanlines at a time.  The expected format for the passed

data is discussed under "Data formats", above.

Image data should be written in top-to-bottom scanline order.  The JPEG spec

contains some weasel wording about how top and bottom are application-defined

terms (a curious interpretation of the English language...) but if you want

your files to be compatible with everyone else's, you WILL use top-to-bottom

order.  If the source data must be read in bottom-to-top order, you can use

the JPEG library's virtual array mechanism to invert the data efficiently.

Examples of this can be found in the sample application cjpeg.

The library maintains a count of the number of scanlines written so far

in the next_scanline field of the JPEG object.  Usually you can just use

this variable as the loop counter, so that the loop test looks like

"while (cinfo.next_scanline < cinfo.image_height)".

Code for this step depends heavily on the way that you store the source data.

example.c shows the following code for the case of a full-size 2-D source

array containing 3-byte RGB pixels:

	JSAMPROW row_pointer[1];	/* pointer to a single row */

	int row_stride;			/* physical row width in buffer */

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

	while (cinfo.next_scanline < cinfo.image_height) {

	    row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];

	    jpeg_write_scanlines(&cinfo, row_pointer, 1);

	}

jpeg_write_scanlines() returns the number of scanlines actually written.

This will normally be equal to the number passed in, so you can usually

ignore the return value.  It is different in just two cases:

  * If you try to write more scanlines than the declared image height,

    the additional scanlines are ignored.

  * If you use a suspending data destination manager, output buffer overrun

    will cause the compressor to return before accepting all the passed lines.

    This feature is discussed under "I/O suspension", below.  The normal

    stdio destination manager will NOT cause this to happen.

In any case, the return value is the same as the change in the value of

next_scanline.

6. jpeg_finish_compress(...);

After all the image data has been written, call jpeg_finish_compress() to

complete the compression cycle.  This step is ESSENTIAL to ensure that the

last bufferload of data is written to the data destination.

jpeg_finish_compress() also releases working memory associated with the JPEG

object.

Typical code:

	jpeg_finish_compress(&cinfo);

If using the stdio destination manager, don't forget to close the output

stdio stream (if necessary) afterwards.

If you have requested a multi-pass operating mode, such as Huffman code

optimization, jpeg_finish_compress() will perform the additional passes using

data buffered by the first pass.  In this case jpeg_finish_compress() may take

quite a while to complete.  With the default compression parameters, this will

not happen.

It is an error to call jpeg_finish_compress() before writing the necessary

total number of scanlines.  If you wish to abort compression, call

jpeg_abort() as discussed below.

After completing a compression cycle, you may dispose of the JPEG object

as discussed next, or you may use it to compress another image.  In that case

return to step 2, 3, or 4 as appropriate.  If you do not change the

destination manager, the new datastream will be written to the same target.

If you do not change any JPEG parameters, the new datastream will be written

with the same parameters as before.  Note that you can change the input image

dimensions freely between cycles, but if you change the input colorspace, you

should call jpeg_set_defaults() to adjust for the new colorspace; and then

you'll need to repeat all of step 3.

7. Release the JPEG compression object.

When you are done with a JPEG compression object, destroy it by calling

jpeg_destroy_compress().  This will free all subsidiary memory (regardless of

the previous state of the object).  Or you can call jpeg_destroy(), which

works for either compression or decompression objects --- this may be more

convenient if you are sharing code between compression and decompression

cases.  (Actually, these routines are equivalent except for the declared type

of the passed pointer.  To avoid gripes from ANSI C compilers, jpeg_destroy()

should be passed a j_common_ptr.)

If you allocated the jpeg_compress_struct structure from malloc(), freeing

it is your responsibility --- jpeg_destroy() won't.  Ditto for the error

handler structure.

Typical code:

	jpeg_destroy_compress(&cinfo);

8. Aborting.

If you decide to abort a compression cycle before finishing, you can clean up

in either of two ways:

* If you don't need the JPEG object any more, just call

  jpeg_destroy_compress() or jpeg_destroy() to release memory.  This is

  legitimate at any point after calling jpeg_create_compress() --- in fact,

  it's safe even if jpeg_create_compress() fails.

* If you want to re-use the JPEG object, call jpeg_abort_compress(), or call

  jpeg_abort() which works on both compression and decompression objects.

  This will return the object to an idle state, releasing any working memory.

  jpeg_abort() is allowed at any time after successful object creation.

Note that cleaning up the data destination, if required, is your

responsibility; neither of these routines will call term_destination().

(See "Compressed data handling", below, for more about that.)

jpeg_destroy() and jpeg_abort() are the only safe calls to make on a JPEG

object that has reported an error by calling error_exit (see "Error handling"

for more info).  The internal state of such an object is likely to be out of

whack.  Either of these two routines will return the object to a known state.

Decompression details

---------------------

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

1. Allocate and initialize a JPEG decompression object.

This is just like initialization for compression, as discussed above,

except that the object is a "struct jpeg_decompress_struct" and you

call jpeg_create_decompress().  Error handling is exactly the same.

Typical code:

	struct jpeg_decompress_struct cinfo;

	struct jpeg_error_mgr jerr;

	...

	cinfo.err = jpeg_std_error(&jerr);

	jpeg_create_decompress(&cinfo);

(Both here and in the IJG code, we usually use variable name "cinfo" for

both compression and decompression objects.)