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.TH CJPEG 1 "28 August 2011"
.SH NAME
cjpeg \- compress an image file to a JPEG file
.SH SYNOPSIS
.B cjpeg
[
.I options
]
[
.I filename
]
.LP
.SH DESCRIPTION
.LP
.B cjpeg
compresses the named image file, or the standard input if no file is
named, and produces a JPEG/JFIF file on the standard output.
The currently supported input file formats are: PPM (PBMPLUS color
format), PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster
Toolkit format). (RLE is supported only if the URT library is available.)
.SH OPTIONS
All switch names may be abbreviated; for example,
.B \-grayscale
may be written
.B \-gray
or
.BR \-gr .
Most of the "basic" switches can be abbreviated to as little as one letter.
Upper and lower case are equivalent (thus
.B \-BMP
is the same as
.BR \-bmp ).
British spellings are also accepted (e.g.,
.BR \-greyscale ),
though for brevity these are not mentioned below.
.PP
The basic switches are:
.TP
.BI \-quality " N[,...]"
Scale quantization tables to adjust image quality. Quality is 0 (worst) to
100 (best); default is 75. (See below for more info.)
.TP
.B \-grayscale
Create monochrome JPEG file from color input. Be sure to use this switch when
compressing a grayscale BMP file, because
.B cjpeg
isn't bright enough to notice whether a BMP file uses only shades of gray.
By saying
.BR \-grayscale ,
you'll get a smaller JPEG file that takes less time to process.
.TP
.B \-rgb
Create RGB JPEG file.
Using this switch suppresses the conversion from RGB
colorspace input to the default YCbCr JPEG colorspace.
Use this switch in combination with the
.BI \-block " N"
switch (see below) for lossless JPEG coding.
.TP
.B \-optimize
Perform optimization of entropy encoding parameters. Without this, default
encoding parameters are used.
.B \-optimize
usually makes the JPEG file a little smaller, but
.B cjpeg
runs somewhat slower and needs much more memory. Image quality and speed of
decompression are unaffected by
.BR \-optimize .
.TP
.B \-progressive
Create progressive JPEG file (see below).
.TP
.BI \-scale " M/N"
Scale the output image by a factor M/N. Currently supported scale factors are
M/N with all N from 1 to 16, where M is the destination DCT size, which is 8
by default (see
.BI \-block " N"
switch below).
.TP
.B \-targa
Input file is Targa format. Targa files that contain an "identification"
field will not be automatically recognized by
.BR cjpeg ;
for such files you must specify
.B \-targa
to make
.B cjpeg
treat the input as Targa format.
For most Targa files, you won't need this switch.
.PP
The
.B \-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
.B \-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.)
.PP
.B \-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
.B not
recommended for normal use; the compressed file size goes up dramatically for
hardly any gain in output image quality.
.PP
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
.B \-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.
.B 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
.B \-baseline
if you need to ensure compatibility at low quality values.)
.PP
The
.B \-quality
option has been extended in IJG version 7 for support of separate quality
settings for luminance and chrominance (or in general, for every provided
quantization table slot). This feature is useful for high-quality
applications which cannot accept the damage of color data by coarse
subsampling settings. You can now easily reduce the color data amount more
smoothly with finer control without separate subsampling. The resulting file
is fully compliant with standard JPEG decoders.
Note that the
.B \-quality
ratings refer to the quantization table slots, and that the last value is
replicated if there are more q-table slots than parameters. The default
q-table slots are 0 for luminance and 1 for chrominance with default tables as
given in the JPEG standard. This is compatible with the old behaviour in case
that only one parameter is given, which is then used for both luminance and
chrominance (slots 0 and 1). More or custom quantization tables can be set
with
.B \-qtables
and assigned to components with
.B \-qslots
parameter (see the "wizard" switches below).
.B Caution:
You must explicitly add
.BI \-sample " 1x1"
for efficient separate color
quality selection, since the default value used by library is 2x2!
.PP
The
.B \-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.
.PP
Switches for advanced users:
.TP
.B \-arithmetic
Use arithmetic coding.
.B Caution:
arithmetic coded JPEG is not yet widely implemented, so many decoders will be
unable to view an arithmetic coded JPEG file at all.
.TP
.BI \-block " N"
Set DCT block size. All N from 1 to 16 are possible.
Default is 8 (baseline format).
Larger values produce higher compression,
smaller values produce higher quality
(exact DCT stage possible with 1 or 2; with the default quality of 75 and
default Luminance qtable the DCT+Quantization stage is lossless for N=1).
.B Caution:
An implementation of the JPEG SmartScale extension is required for this
feature. SmartScale enabled JPEG is not yet widely implemented, so many
decoders will be unable to view a SmartScale extended JPEG file at all.
.TP
.B \-dct int
Use integer DCT method (default).
.TP
.B \-dct fast
Use fast integer DCT (less accurate).
.TP
.B \-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.
.TP
.B \-nosmooth
Don't use high-quality downsampling.
.TP
.BI \-restart " N"
Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
attached to the number.
.B \-restart 0
(the default) means no restart markers.
.TP
.BI \-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.
.TP
.BI \-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,
.B \-max 4m
selects 4000000 bytes. If more space is needed, temporary files will be used.
.TP
.BI \-outfile " name"
Send output image to the named file, not to standard output.
.TP
.B \-verbose
Enable debug printout. More
.BR \-v 's
give more output. Also, version information is printed at startup.
.TP
.B \-debug
Same as
.BR \-verbose .
.PP
The
.B \-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
.B \-restart 1
for images that will be transmitted across unreliable networks such as Usenet.
.PP
The
.B \-smooth
option filters the input to eliminate fine-scale noise. This is often useful
when converting dithered images 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.
.PP
Switches for wizards:
.TP
.B \-baseline
Force baseline-compatible quantization tables to be generated. This clamps
quantization values to 8 bits even at low quality settings. (This switch is
poorly named, since it does not ensure that the output is actually baseline
JPEG. For example, you can use
.B \-baseline
and
.B \-progressive
together.)
.TP
.BI \-qtables " file"
Use the quantization tables given in the specified text file.
.TP
.BI \-qslots " N[,...]"
Select which quantization table to use for each color component.
.TP
.BI \-sample " HxV[,...]"
Set JPEG sampling factors for each color component.
.TP
.BI \-scans " file"
Use the scan script given in the specified text file.
.PP
The "wizard" switches are intended for experimentation with JPEG. If you
don't know what you are doing, \fBdon't use them\fR. These switches are
documented further in the file wizard.txt.
.SH EXAMPLES
.LP
This example compresses the PPM file foo.ppm with a quality factor of
60 and saves the output as foo.jpg:
.IP
.B cjpeg \-quality
.I 60 foo.ppm
.B >
.I foo.jpg
.SH 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
.BR cjpeg 's
.B \-quality
and
.B \-smooth
options to get a satisfactory conversion.
.B \-smooth 10
or so is often helpful.
.PP
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.
.PP
The
.B \-optimize
option to
.B 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,
.B \-optimize
mode is always selected when generating progressive JPEG files.)
.SH ENVIRONMENT
.TP
.B JPEGMEM
If this environment variable is set, its value is the default memory limit.
The value is specified as described for the
.B \-maxmemory
switch.
.B JPEGMEM
overrides the default value specified when the program was compiled, and
itself is overridden by an explicit
.BR \-maxmemory .
.SH SEE ALSO
.BR djpeg (1),
.BR jpegtran (1),
.BR rdjpgcom (1),
.BR wrjpgcom (1)
.br
.BR ppm (5),
.BR pgm (5)
.br
Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
.SH AUTHOR
Independent JPEG Group
.SH BUGS
GIF input files are no longer supported, to avoid the Unisys LZW patent.
(Conversion of GIF files to JPEG is usually a bad idea anyway.)
.PP
Not all variants of BMP and Targa file formats are supported.
.PP
The
.B \-targa
switch is not a bug, it's a feature. (It would be a bug if the Targa format
designers had not been clueless.)
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