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), BMP, Targa, and RLE (Utah Raster Toolkit format). (RLE is supported only if the URT library is available.)
The basic switches are:
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:
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:
This example compresses the PPM file foo.ppm with a quality factor of 60 and saves the output as foo.jpg:
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.)
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.)