djpeg decompresses the named JPEG file, or the standard input if no file is named, and produces an image file on the standard output. PBMPLUS (PPM/PGM), BMP, GIF, Targa, or RLE (Utah Raster Toolkit) output format can be selected. (RLE is supported only if the URT library is available.)

The basic switches are:

-colors N

Reduce image to at most N colors. This reduces the number of colors used in the output image, so that it can be displayed on a colormapped display or stored in a colormapped file format. For example, if you have an 8-bit display, you'd need to reduce to 256 or fewer colors.

-quantize N

Same as -colors. -colors is the recommended name, -quantize is provided only for backwards compatibility.

-fast

Select recommended processing options for fast, low quality output. (The default options are chosen for highest quality output.) Currently, this is equivalent to -dct fast -nosmooth -onepass -dither ordered.

-grayscale

Force gray-scale output even if JPEG file is color. Useful for viewing on monochrome displays; also, djpeg runs noticeably faster in this mode.

-scale M/N

Scale the output image by a factor M/N. Currently the scale factor must be 1/1, 1/2, 1/4, or 1/8. Scaling is handy if the image is larger than your screen; also, djpeg runs much faster when scaling down the output.

-bmp

Select BMP output format (Windows flavor). 8-bit colormapped format is emitted if -colors or -grayscale is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color format is emitted.

-gif

Select GIF output format. Since GIF does not support more than 256 colors, -colors 256 is assumed (unless you specify a smaller number of colors).

-os2

Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is emitted if -colors or -grayscale is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color format is emitted.

-pnm

Select PBMPLUS (PPM/PGM) output format (this is the default format). PGM is emitted if the JPEG file is gray-scale or if -grayscale is specified; otherwise PPM is emitted.

-rle

Select RLE output format. (Requires URT library.)

-targa

Select Targa output format. Gray-scale format is emitted if the JPEG file is gray-scale or if -grayscale is specified; otherwise, colormapped format is emitted if -colors is specified; otherwise, 24-bit full-color format is emitted.

Switches for advanced users:

-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.

-dither fs

Use Floyd-Steinberg dithering in color quantization.

-dither ordered

Use ordered dithering in color quantization.

-dither none

Do not use dithering in color quantization. By default, Floyd-Steinberg dithering is applied when quantizing colors; this is slow but usually produces the best results. Ordered dither is a compromise between speed and quality; no dithering is fast but usually looks awful. Note that these switches have no effect unless color quantization is being done. Ordered dither is only available in -onepass mode.

-map file

Quantize to the colors used in the specified image file. This is useful for producing multiple files with identical color maps, or for forcing a predefined set of colors to be used. The file must be a GIF or PPM file. This option overrides -colors and -onepass.

-nosmooth

Use a faster, lower-quality upsampling routine.

-onepass

Use one-pass instead of two-pass color quantization. The one-pass method is faster and needs less memory, but it produces a lower-quality image. -onepass is ignored unless you also say -colors N. Also, the one-pass method is always used for gray-scale output (the two-pass method is no improvement then).

-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.

-verbose

Enable debug printout. More -v's give more output. Also, version information is printed at startup.

-debug

Same as -verbose.

This example decompresses the JPEG file foo.jpg, quantizes it to 256 colors, and saves the output in 8-bit BMP format in foo.bmp:

djpeg -colors 256 -bmp foo.jpg > foo.bmp

Several options are available that trade off image quality to gain speed. -fast turns on the recommended settings.

-dct fast and/or -nosmooth gain speed at a small sacrifice in quality. When producing a color-quantized image, -onepass -dither ordered is fast but much lower quality than the default behavior. -dither none may give acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.

If you are fortunate enough to have very fast floating point hardware, -dct float may be even faster than -dct fast. But on most machines -dct float is slower than -dct int; in this case it is not worth using, because its theoretical accuracy advantage is too small to be significant in practice.

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.

To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files. These are larger than they should be, but are readable by standard GIF decoders.

Still not as fast as we'd like.