install.doc

常好且全面的jpeg图像压缩算法

makefile to equal the corresponding object file name (for example, jmemansi.o
or jmemansi.obj for jmemansi.c).

If you have plenty of (real or virtual) main memory, just use jmemnobs.c.
"Plenty" means about ten bytes for every pixel in the largest images
you plan to process, so a lot of systems don't meet this criterion.
If yours doesn't, try jmemansi.c first.  If that doesn't compile, you'll have
to use jmemname.c; be sure to adjust select_file_name() for local conditions.
You may also need to change unlink() to remove() in close_backing_store().

Except with jmemnobs.c or jmemmac.c, you need to adjust the DEFAULT_MAX_MEM
setting to a reasonable value for your system (either by adding a #define for
DEFAULT_MAX_MEM to jconfig.h, or by adding a -D switch to the Makefile).
This value limits the amount of data space the program will attempt to
allocate.  Code and static data space isn't counted, so the actual memory
needs for cjpeg or djpeg are typically 100 to 150Kb more than the max-memory
setting.  Larger max-memory settings reduce the amount of I/O needed to
process a large image, but too large a value can result in "insufficient
memory" failures.  On most Unix machines (and other systems with virtual
memory), just set DEFAULT_MAX_MEM to several million and forget it.  At the
other end of the spectrum, for MS-DOS machines you probably can't go much
above 300K to 400K.  (On MS-DOS the value refers to conventional memory only.
Extended/expanded memory is handled separately by jmemdos.c.)


BUILDING THE SOFTWARE
=====================

Now you should be able to compile the software.  Just say "make" (or
whatever's necessary to start the compilation).  Have a cup of coffee.

Here are some things that could go wrong:

If your compiler complains about undefined structures, you should be able to
shut it up by putting "#define INCOMPLETE_TYPES_BROKEN" in jconfig.h.

If you have trouble with missing system include files or inclusion of the
wrong ones, read jinclude.h.  This shouldn't happen if you used configure
or ckconfig.c to set up jconfig.h.

There are a fair number of routines that do not use all of their parameters;
some compilers will issue warnings about this, which you can ignore.  There
are also a few configuration checks that may give "unreachable code" warnings.
Any other warning deserves investigation.

If you don't have a getenv() library routine, define NO_GETENV.

Also see the system-specific hints, below.


TESTING THE SOFTWARE
====================

As a quick test of functionality we've included a small sample image in
several forms:
	testorig.jpg	Starting point for the djpeg tests.
	testimg.ppm	The output of djpeg testorig.jpg
	testimg.bmp	The output of djpeg -bmp -colors 256 testorig.jpg
	testimg.jpg	The output of cjpeg testimg.ppm
	testprog.jpg	Progressive-mode equivalent of testorig.jpg.
	testimgp.jpg	The output of cjpeg -progressive -optimize testimg.ppm
(The first- and second-generation .jpg files aren't identical since JPEG is
lossy.)  If you can generate duplicates of the testimg* files then you
probably have working programs.

With most of the makefiles, "make test" will perform the necessary
comparisons.

If you're using a makefile that doesn't provide the test option, run djpeg
and cjpeg by hand and compare the output files to testimg* with whatever
binary file comparison tool you have.  The files should be bit-for-bit
identical.

If the programs complain "MAX_ALLOC_CHUNK is wrong, please fix", then you
need to reduce MAX_ALLOC_CHUNK to a value that fits in type size_t.
Try adding "#define MAX_ALLOC_CHUNK 65520L" to jconfig.h.  A less likely
configuration error is "ALIGN_TYPE is wrong, please fix": defining ALIGN_TYPE
as long should take care of that one.

If the cjpeg test run fails with "Missing Huffman code table entry", it's a
good bet that you needed to define RIGHT_SHIFT_IS_UNSIGNED.  Go back to the
configuration step and run ckconfig.c.  (This is a good plan for any other
test failure, too.)

If you are using Unix (one-file) command line style on a non-Unix system,
it's a good idea to check that binary I/O through stdin/stdout actually
works.  You should get the same results from "djpeg <testorig.jpg >out.ppm"
as from "djpeg -outfile out.ppm testorig.jpg".  Note that the makefiles all
use the latter style and therefore do not exercise stdin/stdout!  If this
check fails, try recompiling with USE_SETMODE or USE_FDOPEN defined.
If it still doesn't work, better use two-file style.

If you chose a memory manager other than jmemnobs.c, you should test that
temporary-file usage works.  Try "djpeg -bmp -colors 256 -max 0 testorig.jpg"
and make sure its output matches testimg.bmp.  If you have any really large
images handy, try compressing them with -optimize and/or decompressing with
-colors 256 to make sure your DEFAULT_MAX_MEM setting is not too large.

NOTE: this is far from an exhaustive test of the JPEG software; some modules,
such as 1-pass color quantization, are not exercised at all.  It's just a
quick test to give you some confidence that you haven't missed something
major.


INSTALLING THE SOFTWARE
=======================

Once you're done with the above steps, you can install the software by
copying the executable files (cjpeg, djpeg, jpegtran, rdjpgcom, and wrjpgcom)
to wherever you normally install programs.  On Unix systems, you'll also want
to put the man pages (cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1)
in the man-page directory.  The pre-fab makefiles don't support this step
since there's such a wide variety of installation procedures on different
systems.

If you generated a Makefile with the "configure" script, you can just say
	make install
to install the programs and their man pages into the standard places.
(You'll probably need to be root to do this.)  We recommend first saying
	make -n install
to see where configure thought the files should go.  You may need to edit
the Makefile, particularly if your system's conventions for man page
filenames don't match what configure expects.

If you want to install the IJG library itself, for use in compiling other
programs besides ours, then you need to put the four include files
	jpeglib.h jerror.h jconfig.h jmorecfg.h
into your include-file directory, and put the library file libjpeg.a
(extension may vary depending on system) wherever library files go.
If you generated a Makefile with "configure", it will do what it thinks
is the right thing if you say
	make install-lib


OPTIONAL STUFF
==============

Progress monitor:

If you like, you can #define PROGRESS_REPORT (in jconfig.h) to enable display
of percent-done progress reports.  The routine provided in cdjpeg.c merely
prints percentages to stderr, but you can customize it to do something
fancier.

Utah RLE file format support:

We distribute the software with support for RLE image files (Utah Raster
Toolkit format) disabled, because the RLE support won't compile without the
Utah library.  If you have URT version 3.1 or later, you can enable RLE
support as follows:
	1.  #define RLE_SUPPORTED in jconfig.h.
	2.  Add a -I option to CFLAGS in the Makefile for the directory
	    containing the URT .h files (typically the "include"
	    subdirectory of the URT distribution).
	3.  Add -L... -lrle to LDLIBS in the Makefile, where ... specifies
	    the directory containing the URT "librle.a" file (typically the
	    "lib" subdirectory of the URT distribution).

Support for 12-bit-deep pixel data:

The JPEG standard allows either 8-bit or 12-bit data precision.  (For color,
this means 8 or 12 bits per channel, of course.)  If you need to work with
deeper than 8-bit data, you can compile the IJG code for 12-bit operation.
To do so:
  1. In jmorecfg.h, define BITS_IN_JSAMPLE as 12 rather than 8.
  2. In jconfig.h, undefine BMP_SUPPORTED, RLE_SUPPORTED, and TARGA_SUPPORTED,
     because the code for those formats doesn't handle 12-bit data and won't
     even compile.  (The PPM code does work, as explained below.  The GIF
     code works too; it scales 8-bit GIF data to and from 12-bit depth
     automatically.)
  3. Compile.  Don't expect "make test" to pass, since the supplied test
     files are for 8-bit data.

Currently, 12-bit support does not work on 16-bit-int machines.

Note that a 12-bit version will not read 8-bit JPEG files, nor vice versa;
so you'll want to keep around a regular 8-bit compilation as well.
(Run-time selection of data depth, to allow a single copy that does both,
is possible but would probably slow things down considerably; it's very low
on our to-do list.)

The PPM reader (rdppm.c) can read 12-bit data from either text-format or
binary-format PPM and PGM files.  Binary-format PPM/PGM files which have a
maxval greater than 255 are assumed to use 2 bytes per sample, LSB first
(little-endian order).  As of early 1995, 2-byte binary format is not
officially supported by the PBMPLUS library, but it is expected that a
future release of PBMPLUS will support it.  Note that the PPM reader will
read files of any maxval regardless of the BITS_IN_JSAMPLE setting; incoming
data is automatically rescaled to either maxval=255 or maxval=4095 as
appropriate for the cjpeg bit depth.

The PPM writer (wrppm.c) will normally write 2-byte binary PPM or PGM
format, maxval 4095, when compiled with BITS_IN_JSAMPLE=12.  Since this
format is not yet widely supported, you can disable it by compiling wrppm.c
with PPM_NORAWWORD defined; then the data is scaled down to 8 bits to make a
standard 1-byte/sample PPM or PGM file.  (Yes, this means still another copy
of djpeg to keep around.  But hopefully you won't need it for very long.
Poskanzer's supposed to get that new PBMPLUS release out Real Soon Now.)

Of course, if you are working with 12-bit data, you probably have it stored
in some other, nonstandard format.  In that case you'll probably want to
write your own I/O modules to read and write your format.

Note that a 12-bit version of cjpeg always runs in "-optimize" mode, in
order to generate valid Huffman tables.  This is necessary because our
default Huffman tables only cover 8-bit data.

Removing code:

If you need to make a smaller version of the JPEG software, some optional
functions can be removed at compile time.  See the xxx_SUPPORTED #defines in
jconfig.h and jmorecfg.h.  If at all possible, we recommend that you leave in
decoder support for all valid JPEG files, to ensure that you can read anyone's
output.  Taking out support for image file formats that you don't use is the
most painless way to make the programs smaller.  Another possibility is to
remove some of the DCT methods: in particular, the "IFAST" method may not be
enough faster than the others to be worth keeping on your machine.  (If you
do remove ISLOW or IFAST, be sure to redefine JDCT_DEFAULT or JDCT_FASTEST
to a supported method, by adding a #define in jconfig.h.)


OPTIMIZATION
============

Unless you own a Cray, you'll probably be interested in making the JPEG
software go as fast as possible.  This section covers some machine-dependent
optimizations you may want to try.  We suggest that before trying any of
this, you first get the basic installation to pass the self-test step.
Repeat the self-test after any optimization to make sure that you haven't
broken anything.

The integer DCT routines perform a lot of multiplications.  These
multiplications must yield 32-bit results, but none of their input values
are more than 16 bits wide.  On many machines, notably the 680x0 and 80x86
CPUs, a 16x16=>32 bit multiply instruction is faster than a full 32x32=>32
bit multiply.  Unfortunately there is no portable way to specify such a
multiplication in C, but some compilers can generate one when you use the
right combination of casts.  See the MULTIPLYxxx macro definitions in
jdct.h.  If your compiler makes "int" be 32 bits and "short" be 16 bits,
defining SHORTxSHORT_32 is fairly likely to work.  When experimenting with
alternate definitions, be sure to test not only whether the code still works
(use the self-test), but also whether it is actually faster --- on some
compilers, alternate definitions may compute the right answer, yet be slower
than the default.  Timing cjpeg on a large PGM (grayscale) input file is the
best way to check this, as the DCT will be the largest fraction of the runtime
in that mode.  (Note: some of the distributed compiler-specific jconfig files
already contain #define switches to select appropriate MULTIPLYxxx
definitions.)

If your machine has sufficiently fast floating point hardware, you may find
that the float DCT method is faster than the integer DCT methods, even
after tweaking the integer multiply macros.  In that case you may want to
make the float DCT be the default method.  (The only objection to this is
that float DCT results may vary slightly across machines.)  To do that, add
"#define JDCT_DEFAULT JDCT_FLOAT" to jconfig.h.  Even if you don't change
the default, you should redefine JDCT_FASTEST, which is the method selected
by djpeg's -fast switch.  Don't forget to update the documentation files
(usage.doc and/or cjpeg.1, djpeg.1) to agree with what you've done.

If access to "short" arrays is slow on your machine, it may be a win to
define type JCOEF as int rather than short.  This will cost a good deal of
memory though, particularly in some multi-pass modes, so don't do it unless
you have memory to burn and short is REALLY slow.

If your compiler can compile function calls in-line, make sure the INLINE
macro in jmorecfg.h is defined as the keyword that marks a function