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MakeJPEG/!MakeJPEG/Resources/JPEG/doc/usage
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USAGE instructions for the Independent JPEG Group's JPEG software
=================================================================
This file describes usage of the JPEG conversion programs cjpeg and djpeg,
as well as the utility programs jpegtran, rdjpgcom and wrjpgcom. (See
the other documentation files if you wish to use the JPEG library within
your own programs.)
If you are on a Unix machine you may prefer to read the Unix-style manual
pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1.
INTRODUCTION
These programs implement JPEG image compression and decompression. JPEG
(pronounced "jay-peg") is a standardized compression method for full-color
and gray-scale images. JPEG is designed to handle "real-world" scenes,
for example scanned photographs. Cartoons, line drawings, and other
non-realistic images are not JPEG's strong suit; on that sort of material
you may get poor image quality and/or little compression.
JPEG is lossy, meaning that the output image is not necessarily identical to
the input image. Hence you should not use JPEG if you have to have identical
output bits. However, on typical real-world images, very good compression
levels can be obtained with no visible change, and amazingly high compression
is possible if you can tolerate a low-quality image. You can trade off image
quality against file size by adjusting the compressor's "quality" setting.
GENERAL USAGE
We provide two programs, cjpeg to compress an image file into JPEG format,
and djpeg to decompress a JPEG file back into a conventional image format.
On Unix-like systems, you say:
cjpeg [switches] [imagefile] >jpegfile
or
djpeg [switches] [jpegfile] >imagefile
The programs read the specified input file, or standard input if none is
named. They always write to standard output (with trace/error messages to
standard error). These conventions are handy for piping images between
programs.
On most non-Unix systems, you say:
cjpeg [switches] imagefile jpegfile
or
djpeg [switches] jpegfile imagefile
i.e., both the input and output files are named on the command line. This
style is a little more foolproof, and it loses no functionality if you don't
have pipes. (You can get this style on Unix too, if you prefer, by defining
TWO_FILE_COMMANDLINE when you compile the programs; see install.doc.)
You can also say:
cjpeg [switches] -outfile jpegfile imagefile
or
djpeg [switches] -outfile imagefile jpegfile
This syntax works on all systems, so it is useful for scripts.
The currently supported image file formats are: PPM (PBMPLUS color format),
PGM (PBMPLUS gray-scale format), BMP, GIF, Targa, and RLE (Utah Raster Toolkit
format). (RLE is supported only if the URT library is available.)
cjpeg recognizes the input image format automatically, with the exception
of some Targa-format files. You have to tell djpeg which format to generate.
JPEG files are in the defacto standard JFIF file format. There are other,
less widely used JPEG-based file formats, but we don't support them.
All switch names may be abbreviated; for example, -grayscale may be written
-gray or -gr. Most of the "basic" switches can be abbreviated to as little as
one letter. Upper and lower case are equivalent (-GIF is the same as -gif).
British spellings are also accepted (e.g., -greyscale), though for brevity
these are not mentioned below.
CJPEG DETAILS
The basic command line switches for cjpeg are:
-quality N Scale quantization tables to adjust image quality.
Quality is 0 (worst) to 100 (best); default is 75.
(See below for more info.)
-grayscale Create monochrome JPEG file from color input.
Be sure to use this switch when compressing a grayscale
GIF file, because cjpeg isn't bright enough to notice
whether a GIF file uses only shades of gray. By
saying -grayscale, you'll get a smaller JPEG file that
takes less time to process.
-optimize Perform optimization of entropy encoding parameters.
Without this, default encoding parameters are used.
-optimize usually makes the JPEG file a little smaller,
but cjpeg runs somewhat slower and needs much more
memory. Image quality and speed of decompression are
unaffected by -optimize.
-progressive Create progressive JPEG file (see below).
-targa Input file is Targa format. Targa files that contain
an "identification" field will not be automatically
recognized by cjpeg; for such files you must specify
-targa to make cjpeg treat the input as Targa format.
For most Targa files, you won't need this switch.
The -quality switch lets you trade off compressed file size against quality of
the reconstructed image: the higher the quality setting, the larger the JPEG
file, and the closer the output image will be to the original input. Normally
you want to use the lowest quality setting (smallest file) that decompresses
into something visually indistinguishable from the original image. For this
purpose the quality setting should be between 50 and 95; the default of 75 is
often about right. If you see defects at -quality 75, then go up 5 or 10
counts at a time until you are happy with the output image. (The optimal
setting will vary from one image to another.)
-quality 100 will generate a quantization table of all 1's, minimizing loss
in the quantization step (but there is still information loss in subsampling,
as well as roundoff error). This setting is mainly of interest for
experimental purposes. Quality values above about 95 are NOT recommended for
normal use; the compressed file size goes up dramatically for hardly any gain
in output image quality.
In the other direction, quality values below 50 will produce very small files
of low image quality. Settings around 5 to 10 might be useful in preparing an
index of a large image library, for example. Try -quality 2 (or so) for some
amusing Cubist effects. (Note: quality values below about 25 generate 2-byte
quantization tables, which are considered optional in the JPEG standard.
cjpeg emits a warning message when you give such a quality value, because some
other JPEG programs may be unable to decode the resulting file. Use -baseline
if you need to ensure compatibility at low quality values.)
The -progressive switch creates a "progressive JPEG" file. In this type of
JPEG file, the data is stored in multiple scans of increasing quality. If the
file is being transmitted over a slow communications link, the decoder can use
the first scan to display a low-quality image very quickly, and can then
improve the display with each subsequent scan. The final image is exactly
equivalent to a standard JPEG file of the same quality setting, and the total
file size is about the same --- often a little smaller. CAUTION: progressive
JPEG is not yet widely implemented, so many decoders will be unable to view a
progressive JPEG file at all.
Switches for advanced users:
-dct int Use integer DCT method (default).
-dct fast Use fast integer DCT (less accurate).
-dct float Use floating-point DCT method.
The float method is very slightly more accurate than
the int method, but is much slower unless your machine
has very fast floating-point hardware. Also note that
results of the floating-point method may vary slightly
across machines, while the integer methods should give
the same results everywhere. The fast integer method
is much less accurate than the other two.
-restart N Emit a JPEG restart marker every N MCU rows, or every
N MCU blocks if "B" is attached to the number.
-restart 0 (the default) means no restart markers.
-smooth N Smooth the input image to eliminate dithering noise.
N, ranging from 1 to 100, indicates the strength of
smoothing. 0 (the default) means no smoothing.
-maxmemory N Set limit for amount of memory to use in processing
large images. Value is in thousands of bytes, or
millions of bytes if "M" is attached to the number.
For example, -max 4m selects 4000000 bytes. If more
space is needed, temporary files will be used.
-verbose Enable debug printout. More -v's give more printout.
or -debug Also, version information is printed at startup.
The -restart option inserts extra markers that allow a JPEG decoder to
resynchronize after a transmission error. Without restart markers, any damage
to a compressed file will usually ruin the image from the point of the error
to the end of the image; with restart markers, the damage is usually confined
to the portion of the image up to the next restart marker. Of course, the
restart markers occupy extra space. We recommend -restart 1 for images that
will be transmitted across unreliable networks such as Usenet.
The -smooth option filters the input to eliminate fine-scale noise. This is
often useful when converting GIF files to JPEG: a moderate smoothing factor of
10 to 50 gets rid of dithering patterns in the input file, resulting in a
smaller JPEG file and a better-looking image. Too large a smoothing factor
will visibly blur the image, however.
Switches for wizards:
-baseline Force a baseline JPEG file to be generated. This
clamps quantization values to 8 bits even at low
quality settings.
-qtables file Use the quantization tables given in the specified
text file.
-qslots N[,...] Select which quantization table to use for each color
component.
-sample HxV[,...] Set JPEG sampling factors for each color component.
-scans file Use the scan script given in the specified text file.
The "wizard" switches are intended for experimentation with JPEG. If you
don't know what you are doing, DON'T USE THEM. These switches are documented
further in the file wizard.doc.
DJPEG DETAILS
The basic command line switches for djpeg are:
-colors N Reduce image to at most N colors. This reduces the
or -quantize N number of colors used in the output image, so that it
can be displayed on a colormapped display or stored in
a colormapped file format. For example, if you have
an 8-bit display, you'd need to reduce to 256 or fewer
colors. (-colors is the recommended name, -quantize
is provided only for backwards compatibility.)
-fast Select recommended processing options for fast, low
quality output. (The default options are chosen for
highest quality output.) Currently, this is equivalent
to "-dct fast -nosmooth -onepass -dither ordered".
-grayscale Force gray-scale output even if JPEG file is color.
Useful for viewing on monochrome displays; also,
djpeg runs noticeably faster in this mode.
-scale M/N Scale the output image by a factor M/N. Currently
the scale factor must be 1/1, 1/2, 1/4, or 1/8.
Scaling is handy if the image is larger than your
screen; also, djpeg runs much faster when scaling
down the output.
-bmp Select BMP output format (Windows flavor). 8-bit
colormapped format is emitted if -colors or -grayscale
is specified, or if the JPEG file is gray-scale;
otherwise, 24-bit full-color format is emitted.
-gif Select GIF output format. Since GIF does not support
more than 256 colors, -colors 256 is assumed (unless
you specify a smaller number of colors). If you
specify -fast, the default number of colors is 216.
-os2 Select BMP output format (OS/2 1.x flavor). 8-bit
colormapped format is emitted if -colors or -grayscale
is specified, or if the JPEG file is gray-scale;
otherwise, 24-bit full-color format is emitted.
-pnm Select PBMPLUS (PPM/PGM) output format (this is the
default format). PGM is emitted if the JPEG file is
gray-scale or if -grayscale is specified; otherwise
PPM is emitted.
-rle Select RLE output format. (Requires URT library.)
-targa Select Targa output format. Gray-scale format is
emitted if the JPEG file is gray-scale or if
-grayscale is specified; otherwise, colormapped format
is emitted if -colors is specified; otherwise, 24-bit
full-color format is emitted.
Switches for advanced users:
-dct int Use integer DCT method (default).
-dct fast Use fast integer DCT (less accurate).
-dct float Use floating-point DCT method.
The float method is very slightly more accurate than
the int method, but is much slower unless your machine
has very fast floating-point hardware. Also note that
results of the floating-point method may vary slightly
across machines, while the integer methods should give
the same results everywhere. The fast integer method
is much less accurate than the other two.
-dither fs Use Floyd-Steinberg dithering in color quantization.
-dither ordered Use ordered dithering in color quantization.
-dither none Do not use dithering in color quantization.
By default, Floyd-Steinberg dithering is applied when
quantizing colors; this is slow but usually produces
the best results. Ordered dither is a compromise
between speed and quality; no dithering is fast but
usually looks awful. Note that these switches have
no effect unless color quantization is being done.
Ordered dither is only available in -onepass mode.
-map FILE Quantize to the colors used in the specified image
file. This is useful for producing multiple files
with identical color maps, or for forcing a predefined
set of colors to be used. The FILE must be a GIF
or PPM file. This option overrides -colors and
-onepass.
-nosmooth Use a faster, lower-quality upsampling routine.
-onepass Use one-pass instead of two-pass color quantization.
The one-pass method is faster and needs less memory,
but it produces a lower-quality image. -onepass is
ignored unless you also say -colors N. Also,
the one-pass method is always used for gray-scale
output (the two-pass method is no improvement then).
-maxmemory N Set limit for amount of memory to use in processing
large images. Value is in thousands of bytes, or
millions of bytes if "M" is attached to the number.
For example, -max 4m selects 4000000 bytes. If more
space is needed, temporary files will be used.
-verbose Enable debug printout. More -v's give more printout.
or -debug Also, version information is printed at startup.
HINTS FOR CJPEG
Color GIF files are not the ideal input for JPEG; JPEG is really intended for
compressing full-color (24-bit) images. In particular, don't try to convert
cartoons, line drawings, and other images that have only a few distinct
colors. GIF works great on these, JPEG does not. If you want to convert a
GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
to get a satisfactory conversion. -smooth 10 or so is often helpful.
Avoid running an image through a series of JPEG compression/decompression
cycles. Image quality loss will accumulate; after ten or so cycles the image
may be noticeably worse than it was after one cycle. It's best to use a
lossless format while manipulating an image, then convert to JPEG format when
you are ready to file the image away.
The -optimize option to cjpeg is worth using when you are making a "final"
version for posting or archiving. It's also a win when you are using low
quality settings to make very small JPEG files; the percentage improvement
is often a lot more than it is on larger files. (At present, -optimize
mode is always selected when generating progressive JPEG files.)
HINTS FOR DJPEG
To get a quick preview of an image, use the -grayscale and/or -scale switches.
"-grayscale -scale 1/8" is the fastest case.
Several options are available that trade off image quality to gain speed.
"-fast" turns on the recommended settings.
"-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
When producing a color-quantized image, "-onepass -dither ordered" is fast but
much lower quality than the default behavior. "-dither none" may give
acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
If you are fortunate enough to have very fast floating point hardware,
"-dct float" may be even faster than "-dct fast". But on most machines
"-dct float" is slower than "-dct int"; in this case it is not worth using,
because its theoretical accuracy advantage is too small to be significant
in practice.
Two-pass color quantization requires a good deal of memory; on MS-DOS machines
it may run out of memory even with -maxmemory 0. In that case you can still
decompress, with some loss of image quality, by specifying -onepass for
one-pass quantization.
HINTS FOR BOTH PROGRAMS
If more space is needed than will fit in the available main memory (as
determined by -maxmemory), temporary files will be used. (MS-DOS versions
will try to get extended or expanded memory first.) The temporary files are
often rather large: in typical cases they occupy three bytes per pixel, for
example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough
free disk space, leave out -progressive and -optimize (for cjpeg) or specify
-onepass (for djpeg).
On MS-DOS, the temporary files are created in the directory named by the TMP
or TEMP environment variable, or in the current directory if neither of those
exist. Amiga implementations put the temp files in the directory named by
JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
space.
The default memory usage limit (-maxmemory) is set when the software is
compiled. If you get an "insufficient memory" error, try specifying a smaller
-maxmemory value, even -maxmemory 0 to use the absolute minimum space. You
may want to recompile with a smaller default value if this happens often.
On machines that have "environment" variables, you can define the environment
variable JPEGMEM to set the default memory limit. The value is specified as
described for the -maxmemory switch. JPEGMEM overrides the default value
specified when the program was compiled, and itself is overridden by an
explicit -maxmemory switch.
On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
use. (Extended or expanded memory is also used if available.) Most
DOS-specific versions of this software do their own memory space estimation
and do not need you to specify -maxmemory.
JPEGTRAN
jpegtran translates JPEG files from one variant of JPEG to another, for
example from baseline JPEG to progressive JPEG or vice versa. The
transformation is lossless: no image degradation occurs, which would not
be true if you used djpeg followed by cjpeg. However, you cannot alter
the image quality, because that would not be a lossless operation.
jpegtran operates similarly to cjpeg, except that it reads a JPEG file
and writes another JPEG file.
jpegtran accepts a subset of the switches recognized by cjpeg:
-outfile filename
-optimize
-progressive
-restart N
-scans file
-maxmemory N
-verbose
-debug
See the previous discussion of cjpeg for details about these switches.
If you specify no switches, you get a plain baseline-JPEG output file.
THE COMMENT UTILITIES
The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
Although the standard doesn't actually define what COM blocks are for, they
are widely used to hold user-supplied text strings. This lets you add
annotations, titles, index terms, etc to your JPEG files, and later retrieve
them as text. COM blocks do not interfere with the image stored in the JPEG
file. The maximum size of a COM block is 64K, but you can have as many of
them as you like in one JPEG file.
We provide two utility programs to display COM block contents and add COM
blocks to a JPEG file.
rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
standard output. The command line syntax is
rdjpgcom [-verbose] [inputfilename]
The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
image dimensions. If you omit the input file name from the command line,
the JPEG file is read from standard input. (This may not work on some
operating systems, if binary data can't be read from stdin.)
wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
Ordinarily, the COM block is added after any existing COM blocks, but you
can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG
file; it does not modify the input file. DO NOT try to overwrite the input
file by directing wrjpgcom's output back into it; on most systems this will
just destroy your file.
The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like
systems, it is
wrjpgcom [switches] [inputfilename]
The output file is written to standard output. The input file comes from
the named file, or from standard input if no input file is named.
On most non-Unix systems, the syntax is
wrjpgcom [switches] inputfilename outputfilename
where both input and output file names must be given explicitly.
wrjpgcom understands three switches:
-replace Delete any existing COM blocks from the file.
-comment "Comment text" Supply new COM text on command line.
-cfile name Read text for new COM block from named file.
(Switch names can be abbreviated.) If you have only one line of comment text
to add, you can provide it on the command line with -comment. The comment
text must be surrounded with quotes so that it is treated as a single
argument. Longer comments can be read from a text file.
If you give neither -comment nor -cfile, then wrjpgcom will read the comment
text from standard input. (In this case an input image file name MUST be
supplied, so that the source JPEG file comes from somewhere else.) You can
enter multiple lines, up to 64KB worth. Type an end-of-file indicator
(usually control-D or control-Z) to terminate the comment text entry.
wrjpgcom will not add a COM block if the provided comment string is empty.
Therefore -replace -comment "" can be used to delete all COM blocks from a
file.
These utility programs do not depend on the IJG JPEG library. In
particular, the source code for rdjpgcom is intended as an illustration of
the minimum amount of code required to parse a JPEG file header correctly.
00000000 55 53 41 47 45 20 69 6e 73 74 72 75 63 74 69 6f |USAGE instructio| 00000010 6e 73 20 66 6f 72 20 74 68 65 20 49 6e 64 65 70 |ns for the Indep| 00000020 65 6e 64 65 6e 74 20 4a 50 45 47 20 47 72 6f 75 |endent JPEG Grou| 00000030 70 27 73 20 4a 50 45 47 20 73 6f 66 74 77 61 72 |p's JPEG softwar| 00000040 65 0a 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d |e.==============| 00000050 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d |================| * 00000080 3d 3d 3d 0a 0a 54 68 69 73 20 66 69 6c 65 20 64 |===..This file d| 00000090 65 73 63 72 69 62 65 73 20 75 73 61 67 65 20 6f |escribes usage o| 000000a0 66 20 74 68 65 20 4a 50 45 47 20 63 6f 6e 76 65 |f the JPEG conve| 000000b0 72 73 69 6f 6e 20 70 72 6f 67 72 61 6d 73 20 63 |rsion programs c| 000000c0 6a 70 65 67 20 61 6e 64 20 64 6a 70 65 67 2c 0a |jpeg and djpeg,.| 000000d0 61 73 20 77 65 6c 6c 20 61 73 20 74 68 65 20 75 |as well as the u| 000000e0 74 69 6c 69 74 79 20 70 72 6f 67 72 61 6d 73 20 |tility programs | 000000f0 6a 70 65 67 74 72 61 6e 2c 20 72 64 6a 70 67 63 |jpegtran, rdjpgc| 00000100 6f 6d 20 61 6e 64 20 77 72 6a 70 67 63 6f 6d 2e |om and wrjpgcom.| 00000110 20 20 28 53 65 65 0a 74 68 65 20 6f 74 68 65 72 | (See.the other| 00000120 20 64 6f 63 75 6d 65 6e 74 61 74 69 6f 6e 20 66 | documentation f| 00000130 69 6c 65 73 20 69 66 20 79 6f 75 20 77 69 73 68 |iles if you wish| 00000140 20 74 6f 20 75 73 65 20 74 68 65 20 4a 50 45 47 | to use the JPEG| 00000150 20 6c 69 62 72 61 72 79 20 77 69 74 68 69 6e 0a | library within.| 00000160 79 6f 75 72 20 6f 77 6e 20 70 72 6f 67 72 61 6d |your own program| 00000170 73 2e 29 0a 0a 49 66 20 79 6f 75 20 61 72 65 20 |s.)..If you are | 00000180 6f 6e 20 61 20 55 6e 69 78 20 6d 61 63 68 69 6e |on a Unix machin| 00000190 65 20 79 6f 75 20 6d 61 79 20 70 72 65 66 65 72 |e you may prefer| 000001a0 20 74 6f 20 72 65 61 64 20 74 68 65 20 55 6e 69 | to read the Uni| 000001b0 78 2d 73 74 79 6c 65 20 6d 61 6e 75 61 6c 0a 70 |x-style manual.p| 000001c0 61 67 65 73 20 69 6e 20 66 69 6c 65 73 20 63 6a |ages in files cj| 000001d0 70 65 67 2e 31 2c 20 64 6a 70 65 67 2e 31 2c 20 |peg.1, djpeg.1, | 000001e0 6a 70 65 67 74 72 61 6e 2e 31 2c 20 72 64 6a 70 |jpegtran.1, rdjp| 000001f0 67 63 6f 6d 2e 31 2c 20 77 72 6a 70 67 63 6f 6d |gcom.1, wrjpgcom| 00000200 2e 31 2e 0a 0a 0a 49 4e 54 52 4f 44 55 43 54 49 |.1....INTRODUCTI| 00000210 4f 4e 0a 0a 54 68 65 73 65 20 70 72 6f 67 72 61 |ON..These progra| 00000220 6d 73 20 69 6d 70 6c 65 6d 65 6e 74 20 4a 50 45 |ms implement JPE| 00000230 47 20 69 6d 61 67 65 20 63 6f 6d 70 72 65 73 73 |G image compress| 00000240 69 6f 6e 20 61 6e 64 20 64 65 63 6f 6d 70 72 65 |ion and decompre| 00000250 73 73 69 6f 6e 2e 20 20 4a 50 45 47 0a 28 70 72 |ssion. JPEG.(pr| 00000260 6f 6e 6f 75 6e 63 65 64 20 22 6a 61 79 2d 70 65 |onounced "jay-pe| 00000270 67 22 29 20 69 73 20 61 20 73 74 61 6e 64 61 72 |g") is a standar| 00000280 64 69 7a 65 64 20 63 6f 6d 70 72 65 73 73 69 6f |dized compressio| 00000290 6e 20 6d 65 74 68 6f 64 20 66 6f 72 20 66 75 6c |n method for ful| 000002a0 6c 2d 63 6f 6c 6f 72 0a 61 6e 64 20 67 72 61 79 |l-color.and gray| 000002b0 2d 73 63 61 6c 65 20 69 6d 61 67 65 73 2e 20 20 |-scale images. | 000002c0 4a 50 45 47 20 69 73 20 64 65 73 69 67 6e 65 64 |JPEG is designed| 000002d0 20 74 6f 20 68 61 6e 64 6c 65 20 22 72 65 61 6c | to handle "real| 000002e0 2d 77 6f 72 6c 64 22 20 73 63 65 6e 65 73 2c 0a |-world" scenes,.| 000002f0 66 6f 72 20 65 78 61 6d 70 6c 65 20 73 63 61 6e |for example scan| 00000300 6e 65 64 20 70 68 6f 74 6f 67 72 61 70 68 73 2e |ned photographs.| 00000310 20 20 43 61 72 74 6f 6f 6e 73 2c 20 6c 69 6e 65 | Cartoons, line| 00000320 20 64 72 61 77 69 6e 67 73 2c 20 61 6e 64 20 6f | drawings, and o| 00000330 74 68 65 72 0a 6e 6f 6e 2d 72 65 61 6c 69 73 74 |ther.non-realist| 00000340 69 63 20 69 6d 61 67 65 73 20 61 72 65 20 6e 6f |ic images are no| 00000350 74 20 4a 50 45 47 27 73 20 73 74 72 6f 6e 67 20 |t JPEG's strong | 00000360 73 75 69 74 3b 20 6f 6e 20 74 68 61 74 20 73 6f |suit; on that so| 00000370 72 74 20 6f 66 20 6d 61 74 65 72 69 61 6c 0a 79 |rt of material.y| 00000380 6f 75 20 6d 61 79 20 67 65 74 20 70 6f 6f 72 20 |ou may get poor | 00000390 69 6d 61 67 65 20 71 75 61 6c 69 74 79 20 61 6e |image quality an| 000003a0 64 2f 6f 72 20 6c 69 74 74 6c 65 20 63 6f 6d 70 |d/or little comp| 000003b0 72 65 73 73 69 6f 6e 2e 0a 0a 4a 50 45 47 20 69 |ression...JPEG i| 000003c0 73 20 6c 6f 73 73 79 2c 20 6d 65 61 6e 69 6e 67 |s lossy, meaning| 000003d0 20 74 68 61 74 20 74 68 65 20 6f 75 74 70 75 74 | that the output| 000003e0 20 69 6d 61 67 65 20 69 73 20 6e 6f 74 20 6e 65 | image is not ne| 000003f0 63 65 73 73 61 72 69 6c 79 20 69 64 65 6e 74 69 |cessarily identi| 00000400 63 61 6c 20 74 6f 0a 74 68 65 20 69 6e 70 75 74 |cal to.the input| 00000410 20 69 6d 61 67 65 2e 20 20 48 65 6e 63 65 20 79 | image. Hence y| 00000420 6f 75 20 73 68 6f 75 6c 64 20 6e 6f 74 20 75 73 |ou should not us| 00000430 65 20 4a 50 45 47 20 69 66 20 79 6f 75 20 68 61 |e JPEG if you ha| 00000440 76 65 20 74 6f 20 68 61 76 65 20 69 64 65 6e 74 |ve to have ident| 00000450 69 63 61 6c 0a 6f 75 74 70 75 74 20 62 69 74 73 |ical.output bits| 00000460 2e 20 20 48 6f 77 65 76 65 72 2c 20 6f 6e 20 74 |. However, on t| 00000470 79 70 69 63 61 6c 20 72 65 61 6c 2d 77 6f 72 6c |ypical real-worl| 00000480 64 20 69 6d 61 67 65 73 2c 20 76 65 72 79 20 67 |d images, very g| 00000490 6f 6f 64 20 63 6f 6d 70 72 65 73 73 69 6f 6e 0a |ood compression.| 000004a0 6c 65 76 65 6c 73 20 63 61 6e 20 62 65 20 6f 62 |levels can be ob| 000004b0 74 61 69 6e 65 64 20 77 69 74 68 20 6e 6f 20 76 |tained with no v| 000004c0 69 73 69 62 6c 65 20 63 68 61 6e 67 65 2c 20 61 |isible change, a| 000004d0 6e 64 20 61 6d 61 7a 69 6e 67 6c 79 20 68 69 67 |nd amazingly hig| 000004e0 68 20 63 6f 6d 70 72 65 73 73 69 6f 6e 0a 69 73 |h compression.is| 000004f0 20 70 6f 73 73 69 62 6c 65 20 69 66 20 79 6f 75 | possible if you| 00000500 20 63 61 6e 20 74 6f 6c 65 72 61 74 65 20 61 20 | can tolerate a | 00000510 6c 6f 77 2d 71 75 61 6c 69 74 79 20 69 6d 61 67 |low-quality imag| 00000520 65 2e 20 20 59 6f 75 20 63 61 6e 20 74 72 61 64 |e. You can trad| 00000530 65 20 6f 66 66 20 69 6d 61 67 65 0a 71 75 61 6c |e off image.qual| 00000540 69 74 79 20 61 67 61 69 6e 73 74 20 66 69 6c 65 |ity against file| 00000550 20 73 69 7a 65 20 62 79 20 61 64 6a 75 73 74 69 | size by adjusti| 00000560 6e 67 20 74 68 65 20 63 6f 6d 70 72 65 73 73 6f |ng the compresso| 00000570 72 27 73 20 22 71 75 61 6c 69 74 79 22 20 73 65 |r's "quality" se| 00000580 74 74 69 6e 67 2e 0a 0a 0a 47 45 4e 45 52 41 4c |tting....GENERAL| 00000590 20 55 53 41 47 45 0a 0a 57 65 20 70 72 6f 76 69 | USAGE..We provi| 000005a0 64 65 20 74 77 6f 20 70 72 6f 67 72 61 6d 73 2c |de two programs,| 000005b0 20 63 6a 70 65 67 20 74 6f 20 63 6f 6d 70 72 65 | cjpeg to compre| 000005c0 73 73 20 61 6e 20 69 6d 61 67 65 20 66 69 6c 65 |ss an image file| 000005d0 20 69 6e 74 6f 20 4a 50 45 47 20 66 6f 72 6d 61 | into JPEG forma| 000005e0 74 2c 0a 61 6e 64 20 64 6a 70 65 67 20 74 6f 20 |t,.and djpeg to | 000005f0 64 65 63 6f 6d 70 72 65 73 73 20 61 20 4a 50 45 |decompress a JPE| 00000600 47 20 66 69 6c 65 20 62 61 63 6b 20 69 6e 74 6f |G file back into| 00000610 20 61 20 63 6f 6e 76 65 6e 74 69 6f 6e 61 6c 20 | a conventional | 00000620 69 6d 61 67 65 20 66 6f 72 6d 61 74 2e 0a 0a 4f |image format...O| 00000630 6e 20 55 6e 69 78 2d 6c 69 6b 65 20 73 79 73 74 |n Unix-like syst| 00000640 65 6d 73 2c 20 79 6f 75 20 73 61 79 3a 0a 09 63 |ems, you say:..c| 00000650 6a 70 65 67 20 5b 73 77 69 74 63 68 65 73 5d 20 |jpeg [switches] | 00000660 5b 69 6d 61 67 65 66 69 6c 65 5d 20 3e 6a 70 65 |[imagefile] >jpe| 00000670 67 66 69 6c 65 0a 6f 72 0a 09 64 6a 70 65 67 20 |gfile.or..djpeg | 00000680 5b 73 77 69 74 63 68 65 73 5d 20 5b 6a 70 65 67 |[switches] [jpeg| 00000690 66 69 6c 65 5d 20 20 3e 69 6d 61 67 65 66 69 6c |file] >imagefil| 000006a0 65 0a 54 68 65 20 70 72 6f 67 72 61 6d 73 20 72 |e.The programs r| 000006b0 65 61 64 20 74 68 65 20 73 70 65 63 69 66 69 65 |ead the specifie| 000006c0 64 20 69 6e 70 75 74 20 66 69 6c 65 2c 20 6f 72 |d input file, or| 000006d0 20 73 74 61 6e 64 61 72 64 20 69 6e 70 75 74 20 | standard input | 000006e0 69 66 20 6e 6f 6e 65 20 69 73 0a 6e 61 6d 65 64 |if none is.named| 000006f0 2e 20 20 54 68 65 79 20 61 6c 77 61 79 73 20 77 |. They always w| 00000700 72 69 74 65 20 74 6f 20 73 74 61 6e 64 61 72 64 |rite to standard| 00000710 20 6f 75 74 70 75 74 20 28 77 69 74 68 20 74 72 | output (with tr| 00000720 61 63 65 2f 65 72 72 6f 72 20 6d 65 73 73 61 67 |ace/error messag| 00000730 65 73 20 74 6f 0a 73 74 61 6e 64 61 72 64 20 65 |es to.standard e| 00000740 72 72 6f 72 29 2e 20 20 54 68 65 73 65 20 63 6f |rror). These co| 00000750 6e 76 65 6e 74 69 6f 6e 73 20 61 72 65 20 68 61 |nventions are ha| 00000760 6e 64 79 20 66 6f 72 20 70 69 70 69 6e 67 20 69 |ndy for piping i| 00000770 6d 61 67 65 73 20 62 65 74 77 65 65 6e 0a 70 72 |mages between.pr| 00000780 6f 67 72 61 6d 73 2e 0a 0a 4f 6e 20 6d 6f 73 74 |ograms...On most| 00000790 20 6e 6f 6e 2d 55 6e 69 78 20 73 79 73 74 65 6d | non-Unix system| 000007a0 73 2c 20 79 6f 75 20 73 61 79 3a 0a 09 63 6a 70 |s, you say:..cjp| 000007b0 65 67 20 5b 73 77 69 74 63 68 65 73 5d 20 69 6d |eg [switches] im| 000007c0 61 67 65 66 69 6c 65 20 6a 70 65 67 66 69 6c 65 |agefile jpegfile| 000007d0 0a 6f 72 0a 09 64 6a 70 65 67 20 5b 73 77 69 74 |.or..djpeg [swit| 000007e0 63 68 65 73 5d 20 6a 70 65 67 66 69 6c 65 20 20 |ches] jpegfile | 000007f0 69 6d 61 67 65 66 69 6c 65 0a 69 2e 65 2e 2c 20 |imagefile.i.e., | 00000800 62 6f 74 68 20 74 68 65 20 69 6e 70 75 74 20 61 |both the input a| 00000810 6e 64 20 6f 75 74 70 75 74 20 66 69 6c 65 73 20 |nd output files | 00000820 61 72 65 20 6e 61 6d 65 64 20 6f 6e 20 74 68 65 |are named on the| 00000830 20 63 6f 6d 6d 61 6e 64 20 6c 69 6e 65 2e 20 20 | command line. | 00000840 54 68 69 73 0a 73 74 79 6c 65 20 69 73 20 61 20 |This.style is a | 00000850 6c 69 74 74 6c 65 20 6d 6f 72 65 20 66 6f 6f 6c |little more fool| 00000860 70 72 6f 6f 66 2c 20 61 6e 64 20 69 74 20 6c 6f |proof, and it lo| 00000870 73 65 73 20 6e 6f 20 66 75 6e 63 74 69 6f 6e 61 |ses no functiona| 00000880 6c 69 74 79 20 69 66 20 79 6f 75 20 64 6f 6e 27 |lity if you don'| 00000890 74 0a 68 61 76 65 20 70 69 70 65 73 2e 20 20 28 |t.have pipes. (| 000008a0 59 6f 75 20 63 61 6e 20 67 65 74 20 74 68 69 73 |You can get this| 000008b0 20 73 74 79 6c 65 20 6f 6e 20 55 6e 69 78 20 74 | style on Unix t| 000008c0 6f 6f 2c 20 69 66 20 79 6f 75 20 70 72 65 66 65 |oo, if you prefe| 000008d0 72 2c 20 62 79 20 64 65 66 69 6e 69 6e 67 0a 54 |r, by defining.T| 000008e0 57 4f 5f 46 49 4c 45 5f 43 4f 4d 4d 41 4e 44 4c |WO_FILE_COMMANDL| 000008f0 49 4e 45 20 77 68 65 6e 20 79 6f 75 20 63 6f 6d |INE when you com| 00000900 70 69 6c 65 20 74 68 65 20 70 72 6f 67 72 61 6d |pile the program| 00000910 73 3b 20 73 65 65 20 69 6e 73 74 61 6c 6c 2e 64 |s; see install.d| 00000920 6f 63 2e 29 0a 0a 59 6f 75 20 63 61 6e 20 61 6c |oc.)..You can al| 00000930 73 6f 20 73 61 79 3a 0a 09 63 6a 70 65 67 20 5b |so say:..cjpeg [| 00000940 73 77 69 74 63 68 65 73 5d 20 2d 6f 75 74 66 69 |switches] -outfi| 00000950 6c 65 20 6a 70 65 67 66 69 6c 65 20 20 69 6d 61 |le jpegfile ima| 00000960 67 65 66 69 6c 65 0a 6f 72 0a 09 64 6a 70 65 67 |gefile.or..djpeg| 00000970 20 5b 73 77 69 74 63 68 65 73 5d 20 2d 6f 75 74 | [switches] -out| 00000980 66 69 6c 65 20 69 6d 61 67 65 66 69 6c 65 20 20 |file imagefile | 00000990 6a 70 65 67 66 69 6c 65 0a 54 68 69 73 20 73 79 |jpegfile.This sy| 000009a0 6e 74 61 78 20 77 6f 72 6b 73 20 6f 6e 20 61 6c |ntax works on al| 000009b0 6c 20 73 79 73 74 65 6d 73 2c 20 73 6f 20 69 74 |l systems, so it| 000009c0 20 69 73 20 75 73 65 66 75 6c 20 66 6f 72 20 73 | is useful for s| 000009d0 63 72 69 70 74 73 2e 0a 0a 54 68 65 20 63 75 72 |cripts...The cur| 000009e0 72 65 6e 74 6c 79 20 73 75 70 70 6f 72 74 65 64 |rently supported| 000009f0 20 69 6d 61 67 65 20 66 69 6c 65 20 66 6f 72 6d | image file form| 00000a00 61 74 73 20 61 72 65 3a 20 50 50 4d 20 28 50 42 |ats are: PPM (PB| 00000a10 4d 50 4c 55 53 20 63 6f 6c 6f 72 20 66 6f 72 6d |MPLUS color form| 00000a20 61 74 29 2c 0a 50 47 4d 20 28 50 42 4d 50 4c 55 |at),.PGM (PBMPLU| 00000a30 53 20 67 72 61 79 2d 73 63 61 6c 65 20 66 6f 72 |S gray-scale for| 00000a40 6d 61 74 29 2c 20 42 4d 50 2c 20 47 49 46 2c 20 |mat), BMP, GIF, | 00000a50 54 61 72 67 61 2c 20 61 6e 64 20 52 4c 45 20 28 |Targa, and RLE (| 00000a60 55 74 61 68 20 52 61 73 74 65 72 20 54 6f 6f 6c |Utah Raster Tool| 00000a70 6b 69 74 0a 66 6f 72 6d 61 74 29 2e 20 20 28 52 |kit.format). (R| 00000a80 4c 45 20 69 73 20 73 75 70 70 6f 72 74 65 64 20 |LE is supported | 00000a90 6f 6e 6c 79 20 69 66 20 74 68 65 20 55 52 54 20 |only if the URT | 00000aa0 6c 69 62 72 61 72 79 20 69 73 20 61 76 61 69 6c |library is avail| 00000ab0 61 62 6c 65 2e 29 0a 63 6a 70 65 67 20 72 65 63 |able.).cjpeg rec| 00000ac0 6f 67 6e 69 7a 65 73 20 74 68 65 20 69 6e 70 75 |ognizes the inpu| 00000ad0 74 20 69 6d 61 67 65 20 66 6f 72 6d 61 74 20 61 |t image format a| 00000ae0 75 74 6f 6d 61 74 69 63 61 6c 6c 79 2c 20 77 69 |utomatically, wi| 00000af0 74 68 20 74 68 65 20 65 78 63 65 70 74 69 6f 6e |th the exception| 00000b00 0a 6f 66 20 73 6f 6d 65 20 54 61 72 67 61 2d 66 |.of some Targa-f| 00000b10 6f 72 6d 61 74 20 66 69 6c 65 73 2e 20 20 59 6f |ormat files. Yo| 00000b20 75 20 68 61 76 65 20 74 6f 20 74 65 6c 6c 20 64 |u have to tell d| 00000b30 6a 70 65 67 20 77 68 69 63 68 20 66 6f 72 6d 61 |jpeg which forma| 00000b40 74 20 74 6f 20 67 65 6e 65 72 61 74 65 2e 0a 0a |t to generate...| 00000b50 4a 50 45 47 20 66 69 6c 65 73 20 61 72 65 20 69 |JPEG files are i| 00000b60 6e 20 74 68 65 20 64 65 66 61 63 74 6f 20 73 74 |n the defacto st| 00000b70 61 6e 64 61 72 64 20 4a 46 49 46 20 66 69 6c 65 |andard JFIF file| 00000b80 20 66 6f 72 6d 61 74 2e 20 20 54 68 65 72 65 20 | format. 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Most| 00000c40 20 6f 66 20 74 68 65 20 22 62 61 73 69 63 22 20 | of the "basic" | 00000c50 73 77 69 74 63 68 65 73 20 63 61 6e 20 62 65 20 |switches can be | 00000c60 61 62 62 72 65 76 69 61 74 65 64 20 74 6f 20 61 |abbreviated to a| 00000c70 73 20 6c 69 74 74 6c 65 20 61 73 0a 6f 6e 65 20 |s little as.one | 00000c80 6c 65 74 74 65 72 2e 20 20 55 70 70 65 72 20 61 |letter. Upper a| 00000c90 6e 64 20 6c 6f 77 65 72 20 63 61 73 65 20 61 72 |nd lower case ar| 00000ca0 65 20 65 71 75 69 76 61 6c 65 6e 74 20 28 2d 47 |e equivalent (-G| 00000cb0 49 46 20 69 73 20 74 68 65 20 73 61 6d 65 20 61 |IF is the same a| 00000cc0 73 20 2d 67 69 66 29 2e 0a 42 72 69 74 69 73 68 |s -gif)..British| 00000cd0 20 73 70 65 6c 6c 69 6e 67 73 20 61 72 65 20 61 | spellings are a| 00000ce0 6c 73 6f 20 61 63 63 65 70 74 65 64 20 28 65 2e |lso accepted (e.| 00000cf0 67 2e 2c 20 2d 67 72 65 79 73 63 61 6c 65 29 2c |g., -greyscale),| 00000d00 20 74 68 6f 75 67 68 20 66 6f 72 20 62 72 65 76 | though for brev| 00000d10 69 74 79 0a 74 68 65 73 65 20 61 72 65 20 6e 6f |ity.these are no| 00000d20 74 20 6d 65 6e 74 69 6f 6e 65 64 20 62 65 6c 6f |t mentioned belo| 00000d30 77 2e 0a 0a 0a 43 4a 50 45 47 20 44 45 54 41 49 |w....CJPEG DETAI| 00000d40 4c 53 0a 0a 54 68 65 20 62 61 73 69 63 20 63 6f |LS..The basic co| 00000d50 6d 6d 61 6e 64 20 6c 69 6e 65 20 73 77 69 74 63 |mmand line switc| 00000d60 68 65 73 20 66 6f 72 20 63 6a 70 65 67 20 61 72 |hes for cjpeg ar| 00000d70 65 3a 0a 0a 09 2d 71 75 61 6c 69 74 79 20 4e 09 |e:...-quality N.| 00000d80 53 63 61 6c 65 20 71 75 61 6e 74 69 7a 61 74 69 |Scale quantizati| 00000d90 6f 6e 20 74 61 62 6c 65 73 20 74 6f 20 61 64 6a |on tables to adj| 00000da0 75 73 74 20 69 6d 61 67 65 20 71 75 61 6c 69 74 |ust image qualit| 00000db0 79 2e 0a 09 09 09 51 75 61 6c 69 74 79 20 69 73 |y.....Quality is| 00000dc0 20 30 20 28 77 6f 72 73 74 29 20 74 6f 20 31 30 | 0 (worst) to 10| 00000dd0 30 20 28 62 65 73 74 29 3b 20 64 65 66 61 75 6c |0 (best); defaul| 00000de0 74 20 69 73 20 37 35 2e 0a 09 09 09 28 53 65 65 |t is 75.....(See| 00000df0 20 62 65 6c 6f 77 20 66 6f 72 20 6d 6f 72 65 20 | below for more | 00000e00 69 6e 66 6f 2e 29 0a 0a 09 2d 67 72 61 79 73 63 |info.)...-graysc| 00000e10 61 6c 65 09 43 72 65 61 74 65 20 6d 6f 6e 6f 63 |ale.Create monoc| 00000e20 68 72 6f 6d 65 20 4a 50 45 47 20 66 69 6c 65 20 |hrome JPEG file | 00000e30 66 72 6f 6d 20 63 6f 6c 6f 72 20 69 6e 70 75 74 |from color input| 00000e40 2e 0a 09 09 09 42 65 20 73 75 72 65 20 74 6f 20 |.....Be sure to | 00000e50 75 73 65 20 74 68 69 73 20 73 77 69 74 63 68 20 |use this switch | 00000e60 77 68 65 6e 20 63 6f 6d 70 72 65 73 73 69 6e 67 |when compressing| 00000e70 20 61 20 67 72 61 79 73 63 61 6c 65 0a 09 09 09 | a grayscale....| 00000e80 47 49 46 20 66 69 6c 65 2c 20 62 65 63 61 75 73 |GIF file, becaus| 00000e90 65 20 63 6a 70 65 67 20 69 73 6e 27 74 20 62 72 |e cjpeg isn't br| 00000ea0 69 67 68 74 20 65 6e 6f 75 67 68 20 74 6f 20 6e |ight enough to n| 00000eb0 6f 74 69 63 65 0a 09 09 09 77 68 65 74 68 65 72 |otice....whether| 00000ec0 20 61 20 47 49 46 20 66 69 6c 65 20 75 73 65 73 | a GIF file uses| 00000ed0 20 6f 6e 6c 79 20 73 68 61 64 65 73 20 6f 66 20 | only shades of | 00000ee0 67 72 61 79 2e 20 20 42 79 0a 09 09 09 73 61 79 |gray. 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