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 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 [legacy feature], and Targa
[legacy feature].
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. By specifying
-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). Implies -optimize
unless -arithmetic is also specified.
-targa Input file is Targa format [legacy feature]. 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:
-precision N
Create JPEG file with N-bit data precision. N is 2 to 16;
default is 8. If N is not 8 or 12, then -lossless must also be
specified. Note that only the PBMPLUS input file format
supports data precisions other than 8. Note also that PBMPLUS
input files are silently scaled to the target data precision,
even if it is lower than the precision of the input file.
Passing an argument of -verbose to cjpeg will cause it to print
information about the precision of the input file. Caution:
only 8-bit data precision is widely implemented, so many
decoders will be unable to handle JPEG files with other data
precisions.
-precision 12 implies -optimize unless -arithmetic is also
specified.
-lossless psv[,Pt]
Create a lossless JPEG file using the specified predictor
selection value (1 through 7) and optional point transform (0
through precision - 1, where precision is the JPEG data
precision in bits). A point transform value of 0 (the default)
is necessary in order to create a fully lossless JPEG file. (A
non-zero point transform value right-shifts the input samples by
the specified number of bits, which is effectively a form of
lossy color quantization.) Caution: lossless JPEG is not yet
widely implemented, so many decoders will be unable to handle a
lossless JPEG file at all. In most cases, compressing and
decompressing a lossless JPEG file is considerably slower than
compressing and decompressing a lossy JPEG file, and lossless
JPEG files are much larger than lossy JPEG files. Also note
that the following features will be unavailable when compressing
or decompressing a lossless JPEG file:
- Quality/quantization table selection
- Color space conversion (the JPEG image will use the same color
space as the input image)
- Color quantization
- DCT/IDCT algorithm selection
- Smoothing
- Downsampling/upsampling
- IDCT scaling
- Partial image decompression
- Transformations using jpegtran
Any switches used to enable or configure those features will be
ignored.
-arithmetic
Use arithmetic coding. Caution: arithmetic-coded JPEG is not
yet widely implemented, so many decoders will be unable to
handle 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 MCUs if
"B" is attached to the number.
In typical JPEG images, an MCU (Minimum Coded Unit) is the
minimum set of interleaved "data units" (8x8 DCT blocks if the
image is lossy or samples if the image is lossless) necessary to
represent at least one data unit per component. (For example,
an MCU in an interleaved lossy JPEG image that uses 4:2:2
subsampling consists of two luminance blocks followed by one
block for each chrominance component.) In single-component or
non-interleaved JPEG images, an MCU is the same as a data unit.
An MCU row is a row of MCUs spanning the entire width of the
image.
-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.
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 through 48-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.)
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.
30 August 2024 CJPEG(1)
cjpeg \- compress an image file to a JPEG file