All switch names may be abbreviated; for example, -grayscale may be written -gray or -gr. Most of the -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:
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.
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).
Input file is Targa format. Targa files that contain an 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 -progressive switch creates a Caution: progressive JPEG is not yet widely implemented, so many decoders will be unable to view a progressive JPEG file at all.
Switches for advanced users:
Use integer DCT method (default).
Use fast integer DCT (less accurate).
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.
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.
needed, temporary files will be used.
Send output image to the named file, not to standard output.
Enable debug printout. More -v's give more output. Also, version information is printed at startup.
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:
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.)
Use the quantization tables given in the specified text file.
Select which quantization table to use for each color component.
Set JPEG sampling factors for each color component.
Use the scan script given in the specified text file.
This example compresses the PPM file foo.ppm with a quality factor of 60 and saves the output as foo.jpg:
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.
Arithmetic coding is not supported for legal reasons.
GIF input files are no longer supported, to avoid the Unisys LZW patent. Use a Unisys-licensed program if you need to read a GIF file. (Conversion of GIF files to JPEG is usually a bad idea anyway.)
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.)