cjpeg - compress an image file to a JPEG file
cjpeg [
options ] [
filename ]
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
grayscale format), BMP, GIF, and Targa.
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. 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 colorspace input to the default YCbCr JPEG
colorspace.
- -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 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 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
support 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 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-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 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 libjpeg-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
quality 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 varying
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 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
implemented mainly as a way of testing the in-memory destination manager
(jpeg_mem_dest()), but it is also useful 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 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 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 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.
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
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.)
- 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.
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.
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.
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.)