| .TH DJPEG 1 "11 December 1991" |
| .SH NAME |
| djpeg \- decompress a JPEG file to an image file |
| .SH SYNOPSIS |
| .B djpeg |
| [ |
| .B \-GPRTbgD2d |
| ] |
| [ |
| .BI \-q " N" |
| ] |
| [ |
| .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.) |
| .LP |
| The color quantization algorithm is currently shoddy. Because of this, the |
| GIF output mode is not recommended in the current release, except for |
| gray-scale output (obtained with |
| .BR \-g ). |
| .SH OPTIONS |
| .TP |
| .B \-G |
| Select GIF output format (implies |
| .BR \-q , |
| with default of 256 colors). |
| Currently the color quantization uses a shoddy algorithm and external |
| quantization (e.g. |
| .IR ppmquant , |
| .IR rlequant ) |
| is recommended before conversion to GIF format. |
| .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 \-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 \-g |
| Force gray-scale output even if input is color. |
| .TP |
| .BI \-q " N" |
| Quantize to N 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 . |
| .TP |
| .B \-2 |
| Use two-pass color quantization (not yet supported). |
| .TP |
| .B \-d |
| Enable debug printout. More |
| .BR \-d 's |
| give more output. Also, version information is printed at startup. |
| .SH EXAMPLES |
| .LP |
| This example decompresses the JPEG file foo.jpg and saves the output |
| as a gray-scale image in foo.pgm: |
| .IP |
| .B djpeg \-g |
| .I foo.jpg |
| .B > |
| .I foo.pgm |
| .SH SEE ALSO |
| .BR cjpeg (1) |
| .br |
| .BR ppmquant (1) |
| [From the PBMplus distribution] |
| .br |
| .BR rlequant (1) |
| [From the Utah Raster Toolkit distribution] |
| .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 |
| .B djpeg |
| currently uses a shoddy color quantization algorithm. This leads to |
| poor GIF file output. Two-pass color quantization is not yet |
| supported. |
| .PP |
| Arithmetic coding is not supported for legal reasons. |
| .PP |
| Not as fast as we'd like. |
