CJPEG

Section: User Commands (1)
Updated: 4 November 1992
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NAME

cjpeg - compress an image file to a JPEG file

SYNOPSIS

cjpeg [ -quality N ] [ -grayscale ] [ -optimize ] [ -targa ] [ -maxmemory N ] [ -restart N ] [ -smooth N ] [ -verbose ] [ -debug ] [ -arithmetic ] [ -nointerleave ] [ -qtables file ] [ -sample HxV[,...] ] [ 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 gray-scale format), GIF, 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 -GIF is the same as -gif). 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 GIF file, because cjpeg isn't bright enough to notice whether a GIF file uses only shades of gray. By saying -grayscale, you'll get a smaller JPEG file that takes less time to process.
-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.
-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.

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, eliminating 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 commercial JPEG programs may be unable to decode the resulting file.)

Switches for advanced users:

-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.
-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.
-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 GIF files 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:

-arithmetic: Use arithmetic coding rather than Huffman coding. (Not currently supported for legal reasons.)
-nointerleave: Generate noninterleaved JPEG file (not yet supported).
-qtables file: Use the quantization tables given in the specified file. The file should contain one to four tables (64 values each) as plain text. Comments preceded by '#' may be included in the file. The tables are implicitly numbered 0,1,etc. If -quality N is also specified, the values in the file are scaled according to cjpeg's quality scaling curve.
-sample HxV[,...]: Set JPEG sampling factors. If you specify fewer H/V pairs than there are components, the remaining components are set to 1x1 sampling. The default setting is equivalent to -sample 2x2.

The "wizard" switches are intended for experimentation with JPEG. If you don't know what you are doing, don't use them. You can easily produce files with worse image quality and/or poorer compression than you'll get from the default settings. Furthermore, these switches should not be used when making files intended for general use, because not all JPEG implementations will support unusual JPEG parameter settings.

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

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.

AUTHOR

Independent JPEG Group

BUGS

Arithmetic coding and interleaved output not yet supported.

Not all variants of Targa file format 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.)

Still not as fast as we'd like.

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Time: 23:15:24 GMT, September 29, 2022