cjpeg - Online in the Cloud

PROGRAM:

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, Targa, and
RLE (Utah Raster Toolkit format). (RLE is supported only if the URT library is
available.)

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 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.

-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 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 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 integer DCT method (default).

-dct fast
Use fast integer DCT (less accurate). In libjpeg-turbo, the fast method is
generally about 5-15% faster than the int method when using the x86/x86-64 SIMD
extensions (results may vary with other SIMD implementations, or when using
libjpeg-turbo without SIMD extensions.) For quality levels of 90 and below, there
should be little or no perceptible difference between the two algorithms. For
quality levels above 90, however, the difference between the fast and the 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. The float method is mainly a legacy feature. It
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.

-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.

-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.

-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) 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.

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