| .TH DJPEG 1 "28 February 1992" |
| .SH NAME |
| djpeg \- decompress a JPEG file to an image file |
| .SH SYNOPSIS |
| .B djpeg |
| [ |
| .B \-GPRTgD1bd |
| ] |
| [ |
| .BI \-q " colors" |
| ] |
| [ |
| .BI \-m " memory" |
| ] |
| [ |
| .I filename |
| ] |
| .LP |
| .SH DESCRIPTION |
| .LP |
| .B djpeg |
| decompresses the named JPEG file, or the standard input if no file is named, |
| and produces an image file on the standard output. PPM, GIF, Targa, or RLE |
| output format can be selected. (RLE is supported only if the URT library is |
| available.) |
| .SH OPTIONS |
| .TP |
| .B \-G |
| Select GIF output format (implies |
| .BR \-q , |
| with default of 256 colors). |
| .TP |
| .B \-P |
| Select PPM or PGM output format (this is the default). PGM is emitted if the |
| JPEG file is gray-scale or if |
| .B \-g |
| is specified. |
| .TP |
| .B \-R |
| Select RLE output format. Requires URT library. |
| .TP |
| .B \-T |
| Select Targa output format. Gray-scale format is emitted if the JPEG file is |
| gray-scale or if |
| .B \-g |
| is specified; otherwise, colormapped format is emitted if |
| .B \-q |
| is specified; otherwise, 24-bit full-color format is emitted. |
| .TP |
| .B \-g |
| Force gray-scale output even if input is color. |
| .TP |
| .BI \-q " N" |
| Quantize to N colors. This reduces the number of colors 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 quantize to 256 or fewer colors. |
| .TP |
| .B \-D |
| Do not use dithering in color quantization. By default, Floyd-Steinberg |
| dithering is applied when quantizing colors, but on some images dithering may |
| result in objectionable "graininess". If that happens, you can turn off |
| dithering with |
| .BR \-D . |
| .B \-D |
| is ignored unless you also say |
| .B \-q |
| or |
| .BR \-G . |
| .TP |
| .B \-1 |
| 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. |
| .B \-1 |
| is ignored unless you also say |
| .B \-q |
| or |
| .BR \-G . |
| Also, the one-pass method is always used for gray-scale output (the two-pass |
| method is no improvement then). |
| .TP |
| .B \-b |
| Perform cross-block smoothing. This is quite memory-intensive and only seems |
| to improve the image at low quality settings (\fB\-Q\fR 10 to 20 or so). |
| At normal |
| .B \-Q |
| settings it may make the image worse. |
| .TP |
| .B \-d |
| Enable debug printout. More |
| .BR \-d 's |
| give more output. Also, version information is printed at startup. |
| .TP |
| .BI \-m " memory" |
| 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, |
| .B \-m 4m |
| selects 4000000 bytes. If more space is needed, temporary files will be used. |
| .SH EXAMPLES |
| .LP |
| This example decompresses the JPEG file foo.jpg, quantizes to 256 colors, |
| and saves the output in GIF format in foo.gif: |
| .IP |
| .B djpeg \-G |
| .I foo.jpg |
| .B > |
| .I foo.gif |
| .SH SEE ALSO |
| .BR cjpeg (1) |
| .br |
| .BR ppm (5), |
| .BR pgm (5) |
| .br |
| Wallace, Gregory K. "The JPEG Still Picture Compression Standard", |
| Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44. |
| .SH AUTHOR |
| Independent JPEG Group |
| .SH BUGS |
| Arithmetic coding is not supported for legal reasons. |
| .PP |
| Not as fast as we'd like. |
