CJPEG(1) User Commands 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 8, 12, or
16; default is 8. If N is 16, then -lossless must also be
specified. Note that only the PBMPLUS input file format
supports data precisions other than 8. (For historical
reasons, cjpeg allows GIF input files to be converted into
12-bit-per-sample JPEG files, but this is not a useful
conversion.) 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: 12-bit and 16-bit data
precision is not yet widely implemented, so many decoders will
be unable to handle a 12-bit-per-sample or 16-bit-per-sample
JPEG file at all.

-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)