| .TH DJPEG 1 "2 August 1992" |
| .SH NAME |
| djpeg \- decompress a JPEG file to an image file |
| .SH SYNOPSIS |
| .B djpeg |
| [ |
| .BI \-colors " N" |
| ] |
| [ |
| .B \-gif |
| ] |
| [ |
| .B \-pnm |
| ] |
| [ |
| .B \-rle |
| ] |
| [ |
| .B \-targa |
| ] |
| [ |
| .B \-blocksmooth |
| ] |
| [ |
| .B \-grayscale |
| ] |
| [ |
| .BI \-maxmemory " N" |
| ] |
| [ |
| .B \-nodither |
| ] |
| [ |
| .B \-onepass |
| ] |
| [ |
| .B \-verbose |
| ] |
| [ |
| .B \-debug |
| ] |
| [ |
| .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. PBMPLUS (PPM/PGM), GIF, |
| Targa, or RLE (Utah Raster Toolkit) output format can be selected. (RLE is |
| supported only if the URT library is available.) |
| .SH OPTIONS |
| All switch names may be abbreviated; for example, |
| .B \-grayscale |
| may be written |
| .B \-gray |
| or |
| .BR \-gr . |
| Most of the "basic" switches can be abbreviated to as little as one letter. |
| Upper and lower case are equivalent (thus |
| .B \-GIF |
| is the same as |
| .BR \-gif ). |
| British spellings are also accepted (e.g., |
| .BR \-greyscale ), |
| though for brevity these are not mentioned below. |
| .PP |
| The basic switches are: |
| .TP |
| .BI \-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. |
| .TP |
| .BI \-quantize " N" |
| Same as |
| .BR \-colors . |
| .B \-colors |
| is the recommended name, |
| .B \-quantize |
| is provided only for backwards compatibility. |
| .TP |
| .B \-gif |
| Select GIF output format. Since GIF does not support more than 256 colors, |
| .B \-colors 256 |
| is assumed (unless you specify a smaller number of colors). |
| .TP |
| .B \-pnm |
| Select PBMPLUS (PPM/PGM) output format (this is the default format). |
| PGM is emitted if the JPEG file is gray-scale or if |
| .B \-grayscale |
| is specified; otherwise PPM is emitted. |
| .TP |
| .B \-rle |
| Select RLE output format. (Requires URT library.) |
| .TP |
| .B \-targa |
| Select Targa output format. Gray-scale format is emitted if the JPEG file is |
| gray-scale or if |
| .B \-grayscale |
| is specified; otherwise, colormapped format is emitted if |
| .B \-colors |
| is specified; otherwise, 24-bit full-color format is emitted. |
| .PP |
| Switches for advanced users: |
| .TP |
| .B \-blocksmooth |
| Perform cross-block smoothing. This is quite memory-intensive and only seems |
| to improve the image at very low quality settings (\fB\-quality\fR 10 to 20 or |
| so). At normal quality settings it may make the image worse. |
| .TP |
| .B \-grayscale |
| Force gray-scale output even if JPEG file is color. |
| Useful for viewing on monochrome displays. |
| .TP |
| .BI \-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, |
| .B \-max 4m |
| selects 4000000 bytes. If more space is needed, temporary files will be used. |
| .TP |
| .B \-nodither |
| 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 \-nodither . |
| .B \-nodither |
| is ignored unless you also say |
| .B \-colors |
| .IR N . |
| .TP |
| .B \-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. |
| .B \-onepass |
| is ignored unless you also say |
| .B \-colors |
| .IR N . |
| Also, the one-pass method is always used for gray-scale output (the two-pass |
| method is no improvement then). |
| .TP |
| .B \-verbose |
| Enable debug printout. More |
| .BR \-v 's |
| give more output. Also, version information is printed at startup. |
| .TP |
| .B \-debug |
| Same as |
| .BR \-verbose . |
| .SH EXAMPLES |
| .LP |
| This example decompresses the JPEG file foo.jpg, automatically quantizes to |
| 256 colors, and saves the output in GIF format in foo.gif: |
| .IP |
| .B djpeg \-gif |
| .I foo.jpg |
| .B > |
| .I foo.gif |
| .SH ENVIRONMENT |
| .TP |
| .B JPEGMEM |
| If this environment variable is set, its value is the default memory limit. |
| The value is specified as described for the |
| .B \-maxmemory |
| switch. |
| .B JPEGMEM |
| overrides the default value specified when the program was compiled, and |
| itself is overridden by an explicit |
| .BR \-maxmemory . |
| .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 |
| Still not as fast as we'd like. |