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CJPEG(1) General Commands Manual CJPEG(1)
NAME
cjpeg - compress an image file to a JPEG file
SYNOPSIS
cjpeg [ options ] [ filename ]
DESCRIPTION
cjpeg compresses the named image file, or the standard input if no file is named, and pro-
duces a JPEG/JFIF file on the standard output. The currently supported input file formats
are: PPM (PBMPLUS color format), PGM (PBMPLUS grayscale format), BMP, GIF, and Targa.
OPTIONS
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 (thus -BMP is the same as -bmp). British spellings are also ac-
cepted (e.g., -greyscale), though for brevity these are not mentioned below.
The basic switches 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 com-
pressing a grayscale BMP or GIF file, because cjpeg isn't bright enough to notice
whether a BMP or GIF file uses only shades of gray. By saying -grayscale, you'll
get a smaller JPEG file that takes less time to process.
-rgb Create RGB JPEG file. Using this switch suppresses the conversion from RGB col-
orspace input to the default YCbCr JPEG colorspace.
-optimize
Perform optimization of entropy encoding parameters. Without this, default encod-
ing 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 recon-
structed 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 qual-
ity setting (smallest file) that decompresses into something visually indistinguishable
from the original image. For this purpose the quality setting should generally be between
50 and 95 (the default is 75) for photographic images. 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 opti-
mal setting will vary from one image to another.)
-quality 100 will generate a quantization table of all 1's, minimizing loss in the quanti-
zation step (but there is still information loss in subsampling, as well as roundoff er-
ror.) For most images, specifying a quality value above about 95 will increase the size
of the compressed file dramatically, and while the quality gain from these higher quality
values is measurable (using metrics such as PSNR or SSIM), it is rarely perceivable by hu-
man vision.
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 -quality option has been extended in this version of cjpeg to support separate quality
settings for luminance and chrominance (or, in general, separate settings for every quan-
tization table slot.) The principle is the same as chrominance subsampling: since the
human eye is more sensitive to spatial changes in brightness than spatial changes in
color, the chrominance components can be quantized more than the luminance components
without incurring any visible image quality loss. However, unlike subsampling, this fea-
ture reduces data in the frequency domain instead of the spatial domain, which allows for
more fine-grained control. This option is useful in quality-sensitive applications, for
which the artifacts generated by subsampling may be unacceptable.
The -quality option accepts a comma-separated list of parameters, which respectively refer
to the quality levels that should be assigned to the quantization table slots. If there
are more q-table slots than parameters, then the last parameter is replicated. Thus, if
only one quality parameter is given, this is used for both luminance and chrominance
(slots 0 and 1, respectively), preserving the legacy behavior of cjpeg v6b and prior.
More (or customized) quantization tables can be set with the -qtables option and assigned
to components with the -qslots option (see the "wizard" switches below.)
JPEG files generated with separate luminance and chrominance quality are fully compliant
with standard JPEG decoders.
CAUTION: For this setting to be useful, be sure to pass an argument of -sample 1x1 to
cjpeg to disable chrominance subsampling. Otherwise, the default subsampling level (2x2,
AKA "4:2:0") will be used.
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-qual-
ity 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.
Switches for advanced users:
-arithmetic
Use arithmetic coding. Caution: arithmetic coded JPEG is not yet widely imple-
mented, so many decoders will be unable to view an arithmetic coded JPEG file at
all.
-dct int
Use accurate integer DCT method (default).
-dct fast
Use less accurate integer DCT method [legacy feature]. When the Independent JPEG
Group's software was first released in 1991, the compression time for a 1-megapixel
JPEG image on a mainstream PC was measured in minutes. Thus, the fast integer DCT
algorithm provided noticeable performance benefits. On modern CPUs running lib-
jpeg-turbo, however, the compression time for a 1-megapixel JPEG image is measured
in milliseconds, and thus the performance benefits of the fast algorithm are much
less noticeable. On modern x86/x86-64 CPUs that support AVX2 instructions, the
fast and int methods have similar performance. On other types of CPUs, the fast
method is generally about 5-15% faster than the int method.
For quality levels of 90 and below, there should be little or no perceptible qual-
ity difference between the two algorithms. For quality levels above 90, however,
the difference between the fast and int methods becomes more pronounced. With
quality=97, for instance, the fast method incurs generally about a 1-3 dB loss in
PSNR relative to the int method, but this can be larger for some images. Do not
use the fast method with quality levels above 97. The algorithm often degenerates
at quality=98 and above and can actually produce a more lossy image than if lower
quality levels had been used. Also, in libjpeg-turbo, the fast method is not fully
accelerated for quality levels above 97, so it will be slower than the int method.
-dct float
Use floating-point DCT method [legacy feature]. The float method does not produce
significantly more accurate results than the int method, and it is much slower.
The float method may also give different results on different machines due to vary-
ing roundoff behavior, whereas the integer methods should give the same results on
all machines.
-icc file
Embed ICC color management profile contained in the specified file.
-restart N
Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is at-
tached 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, in-
dicates 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 ex-
ample, -max 4m selects 4000000 bytes. If more space is needed, an error will oc-
cur.
-outfile name
Send output image to the named file, not to standard output.
-memdst
Compress to memory instead of a file. This feature was implemented mainly as a way
of testing the in-memory destination manager (jpeg_mem_dest()), but it is also use-
ful for benchmarking, since it reduces the I/O overhead.
-report
Report compression progress.
-strict
Treat all warnings as fatal. Enabling this option will cause the compressor to
abort if an LZW-compressed GIF input image contains incomplete or corrupt image
data.
-verbose
Enable debug printout. More -v's give more output. Also, version information is
printed at startup.
-debug Same as -verbose.
-version
Print version information and exit.
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 usu-
ally 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 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.
Switches for wizards:
-baseline
Force baseline-compatible quantization tables to be generated. This clamps quanti-
zation 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 ex-
ample, you can use -baseline and -progressive together.)
-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 wiz-
ard.txt.
EXAMPLES
This example compresses the PPM file foo.ppm with a quality factor of 60 and saves the
output as foo.jpg:
cjpeg -quality 60 foo.ppm > foo.jpg
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 -qual-
ity and -smooth options to get a satisfactory conversion. -smooth 10 or so is often help-
ful.
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 im-
age, 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.)
ENVIRONMENT
JPEGMEM
If this environment variable is set, its value is 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.
SEE ALSO
djpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)
ppm(5), pgm(5)
Wallace, Gregory K. "The JPEG Still Picture Compression Standard", Communications of the
ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
AUTHOR
Independent JPEG Group
This file was modified by The libjpeg-turbo Project to include only information relevant
to libjpeg-turbo, to wordsmith certain sections, and to describe features not present in
libjpeg.
ISSUES
Not all variants of BMP and Targa file formats are supported.
The -targa switch is not a bug, it's a feature. (It would be a bug if the Targa format
designers had not been clueless.)
18 November 2021 CJPEG(1)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache
Under GNU General Public License
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