Encyclopedia of Graphics File Formats Companion/GFF_CD.ISO/software/unix/jpeg/jpegv4a.tar/USAGE

Text File | 1993-02-17 | 15KB | 309 lines

USAGE instructions for the Independent JPEG Group's JPEG software
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INTRODUCTION

This distribution contains software to 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 this
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.

This file describes usage of the standard programs "cjpeg" and "djpeg" that
can be built directly from the distributed C code.  See the README file for
hints on incorporating the JPEG software into other programs.

If you are on a Unix machine you may prefer to read the Unix-style manual
pages in files cjpeg.1 and djpeg.1.  But also see the HINTS section below,
which is not present in either manual page.

NOTE: the switch syntax has been redesigned since the v3 release of
cjpeg/djpeg.  Switch names are now words instead of single letters.


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 SETUP.)

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), 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.

The only JPEG file format currently supported is the JFIF format.  Support for
the TIFF 6.0 JPEG format will probably be added at some future date.

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.

    -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.

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, eliminating 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 commercial JPEG programs may be unable to decode the resulting file.)

Switches for advanced users:

    -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.

    -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.

    -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:

    -arithmetic    Use arithmetic coding rather than Huffman coding.
            (Not currently supported for legal reasons.)

    -nointerleave    Generate noninterleaved JPEG file (not yet supported).

    -qtables file    Use the quantization tables given in the specified
            file.  The file should contain one to four tables
            (64 values each) as plain text.  Comments preceded by
            '#' may be included in the file.  The tables are
            implicitly numbered 0,1,etc.  If -quality N is also
            specified, the values in the file are scaled according
            to cjpeg's quality scaling curve.

    -sample HxV[,...]    Set JPEG sampling factors.  If you specify
            fewer H/V pairs than there are components, the
            remaining components are set to 1x1 sampling.  The
            default setting is equivalent to "-sample 2x2".

The "wizard" switches are intended for experimentation with JPEG.  If you
don't know what you are doing, DON'T USE THEM.  You can easily produce files
with worse image quality and/or poorer compression than you'll get from the
default settings.  Furthermore, these switches should not be used when making
files intended for general use, because not all JPEG implementations will
support unusual JPEG parameter settings.


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.)

    -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).

    -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:

    -blocksmooth    Perform cross-block smoothing.  This is slow, quite
            memory-intensive, and only seems to improve the image
            at very low quality settings (-quality 10 to 20 or so).
            At normal quality settings it may make things worse.

    -grayscale    Force gray-scale output even if JPEG file is color.
            Useful for viewing on monochrome displays.

    -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.

    -nodither    Do not use dithering in color quantization.
            By default, Floyd-Steinberg dithering is applied when
            quantizing colors, but on some images dithering may
            result in objectionable "graininess".  If that
            happens, you can turn off dithering with -nodither.
            -nodither is ignored unless you also say -colors N.

    -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).

    -verbose    Enable debug printout.  More -v's give more printout.
    or  -debug    Also, version information is printed at startup.


HINTS

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.

When making images to be posted on Usenet, we recommend using cjpeg's option
-restart 1.  This option limits the damage done to a compressed image by
netnews transmission errors.

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 -maxmemory.

djpeg with 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.

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 -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.