cjpeg - compress an image file to a JPEG file
] [ filename
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 Toolkit format). (RLE is
supported only if the URT library is available.)
All switch names may be abbreviated; for example, -grayscale
. Most of the "basic" switches can
be abbreviated to as little as one letter. Upper and lower case are equivalent
is the same as -bmp
). British spellings are also
accepted (e.g., -greyscale
), though for brevity these are not mentioned
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.)
- 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.
- 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
- 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.
- 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
- 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.
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
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.)
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
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
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
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
parameter (see the "wizard" switches below).
You must explicitly add -sample 1x1
efficient separate color quality selection, since the default value used by
library is 2x2!
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:
- 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
- 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
- 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.
- 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
- -outfile name
- Send output image to the named file, not to standard
- Enable debug printout. More -v's give more output.
Also, version information is printed at startup.
- Same as -verbose.
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
that will be transmitted across unreliable networks such as Usenet.
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:
- 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
- -qslots N[,...]
- Select which quantization table to use for each color
- -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 >
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
options to get a satisfactory conversion. -smooth 10
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.
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,
mode is always selected when generating progressive JPEG
- 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
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
GIF input files are no longer supported, to avoid the Unisys LZW patent (now
expired). (Conversion of GIF files to JPEG is usually a bad idea anyway.)
Not all variants of BMP and Targa file formats are supported.
switch is not a bug, it's a feature. (It would be a bug if the
Targa format designers had not been clueless.)