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Text File | 1996-08-15 | 23KB | 476 lines

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.