CJPEG(1)CJPEG(1)NAMEcjpeg - compress an image file to a JPEG file
SYNOPSIScjpeg [ options ] [ filename ]
DESCRIPTIONcjpeg 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 grayscale format), BMP, Targa, and RLE (Utah Raster Tool‐
kit format). (RLE is supported only if the URT library is available.)
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 accepted (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 compressing a grayscale BMP file, because cjpeg
isn't bright enough to notice whether a BMP 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 conver‐
sion from RGB colorspace input to the default YCbCr JPEG col‐
orspace.
-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 "iden‐
tification" 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 indistin‐
guishable from the original image. For this purpose the quality set‐
ting should generally be between 50 and 95 (the default is 75) for pho‐
tographic 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
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.) 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 human 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 sup‐
port separate quality settings for luminance and chrominance (or, in
general, separate settings for every quantization 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 feature 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 applica‐
tions, for which the artifacts generated by subsampling may be unac‐
ceptable.
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 parame‐
ters, 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-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.
Switches for advanced users:
-arithmetic
Use arithmetic coding. Caution: arithmetic coded JPEG is not
yet widely implemented, so many decoders will be unable to view
an arithmetic coded JPEG file at all.
-dct int
Use integer DCT method (default).
-dct fast
Use fast integer DCT (less accurate). In libjpeg-turbo, the
fast method is generally about 5-15% faster than the int method
when using the x86/x86-64 SIMD extensions (results may vary with
other SIMD implementations, or when using libjpeg-turbo without
SIMD extensions.) For quality levels of 90 and below, there
should be little or no perceptible difference between the two
algorithms. For quality levels above 90, however, the differ‐
ence between the fast and the int methods becomes more pro‐
nounced. 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. The float method is mainly a
legacy feature. It 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 varying roundoff behavior, whereas the integer methods should
give the same results on all machines.
-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, an error will occur.
-outfile name
Send output image to the named file, not to standard output.
-memdst
Compress to memory instead of a file. This feature was imple‐
mented mainly as a way of testing the in-memory destination man‐
ager (jpeg_mem_dest()), but it is also useful for benchmarking,
since it reduces the I/O overhead.
-verbose
Enable debug printout. More -v's give more output. Also, ver‐
sion 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 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 recom‐
mend -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 moder‐
ate 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, how‐
ever.
Switches for wizards:
-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 -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 wizard.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 -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/decompres‐
sion 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 percent‐
age 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 speci‐
fied when the program was compiled, and itself is overridden by
an explicit -maxmemory.
SEE ALSOdjpeg(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
Support for GIF input files was removed in cjpeg v6b due to concerns
over the Unisys LZW patent. Although this patent expired in 2006,
cjpeg still lacks GIF support, for these historical reasons. (Conver‐
sion of GIF files to JPEG is usually a bad idea anyway, since GIF is a
256-color format.)
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 March 2017 CJPEG(1)
