Provided by: libjpeg-progs_9e-1_amd64 
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, Targa,
and RLE (Utah Raster Toolkit format). (RLE is supported only if the URT library is
available, which it isn't on most non-Unix systems.)
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 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
colorspace input to the default YCbCr JPEG colorspace. You can use this switch in
combination with the -block N switch (see below) for lossless JPEG coding. See
also the -rgb1 switch below.
-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).
-scale M/N
Scale the output image by a factor M/N. Currently supported scale factors are M/N
with all N from 1 to 16, where M is the destination DCT size, which is 8 by default
(see -block N switch 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 be
between 50 and 95; the default of 75 is often about right. 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). This setting is mainly of interest for experimental purposes. Quality values
above about 95 are not recommended for normal use; the compressed file size goes up
dramatically for hardly any gain in output image quality.
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 IJG version 7 for support of separate quality
settings for luminance and chrominance (or in general, for every provided quantization
table slot). This feature is useful for high-quality applications which cannot accept the
damage of color data by coarse subsampling settings. You can now easily reduce the color
data amount more smoothly with finer control without separate subsampling. The resulting
file is fully compliant with standard JPEG decoders. Note that the -quality ratings refer
to the quantization table slots, and that the last value is replicated if there are more
q-table slots than parameters. The default q-table slots are 0 for luminance and 1 for
chrominance with default tables as given in the JPEG standard. This is compatible with
the old behaviour in case that only one parameter is given, which is then used for both
luminance and chrominance (slots 0 and 1). More or custom quantization tables can be set
with -qtables and assigned to components with -qslots parameter (see the "wizard" switches
below). Caution: You must explicitly add -sample 1x1 for efficient separate color quality
selection, since the default value used by library is 2x2!
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.
-block N
Set DCT block size. All N from 1 to 16 are possible. Default is 8 (baseline
format). Larger values produce higher compression, smaller values produce higher
quality (exact DCT stage possible with 1 or 2; with the default quality of 75 and
default Luminance qtable the DCT+Quantization stage is lossless for N=1). Caution:
An implementation of the JPEG SmartScale extension is required for this feature.
SmartScale enabled JPEG is not yet widely implemented, so many decoders will be
unable to view a SmartScale extended JPEG file at all.
-rgb1 Create RGB JPEG file with reversible color transform. Works like the -rgb switch
(see above) and inserts a simple reversible color transform into the processing
which significantly improves the compression. Use this switch in combination with
the -block N switch (see above) for lossless JPEG coding. Caution: A decoder with
inverse color transform support is required for this feature. Reversible color
transform support is not yet widely implemented, so many decoders will be unable to
view a reversible color transformed JPEG file at all.
-bgycc Create big gamut YCC JPEG file. In this type of encoding the color difference
components are quantized further by a factor of 2 compared to the normal Cb/Cr
values, thus creating space to allow larger color values with higher saturation
than the normal gamut limits to be encoded. In order to compensate for the loss of
color fidelity compared to a normal YCC encoded file, the color quantization tables
can be adjusted accordingly. For example, cjpeg -bgycc -quality 80,90 will give
similar results as cjpeg -quality 80. Caution: For correct decompression a decoder
with big gamut YCC support (JFIF version 2) is required. An old decoder may or may
not display a big gamut YCC encoded JPEG file, depending on JFIF version check and
corresponding warning/error configuration. In case of a granted decompression the
old decoder will display the image with half saturated colors.
-dct int
Use integer DCT method (default).
-dct fast
Use fast integer DCT (less accurate).
-dct float
Use floating-point DCT method. The float method is very slightly more accurate
than the int method, but is much slower unless your machine has very fast floating-
point hardware. Also note that results of the floating-point method may vary
slightly across machines, while the integer methods should give the same results
everywhere. The fast integer method is much less accurate than the other two.
-nosmooth
Don't use high-quality downsampling.
-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, temporary files
will be used.
-outfile name
Send output image to the named file, not to standard output.
-verbose
Enable debug printout. More -v's give more output. Also, version information is
printed at startup.
-debug Same as -verbose.
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) 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
BUGS
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.)
28 April 2019 CJPEG(1)