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diff --git a/libjpegtwrp/djpeg.1 b/libjpegtwrp/djpeg.1 new file mode 100644 index 000000000..11beb6a51 --- /dev/null +++ b/libjpegtwrp/djpeg.1 @@ -0,0 +1,253 @@ +.TH DJPEG 1 "22 August 1997" +.SH NAME +djpeg \- decompress a JPEG file to an image file +.SH SYNOPSIS +.B djpeg +[ +.I options +] +[ +.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), BMP, +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 \-BMP +is the same as +.BR \-bmp ). +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 \-fast +Select recommended processing options for fast, low quality output. (The +default options are chosen for highest quality output.) Currently, this is +equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR. +.TP +.B \-grayscale +Force gray-scale output even if JPEG file is color. Useful for viewing on +monochrome displays; also, +.B djpeg +runs noticeably faster in this mode. +.TP +.BI \-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, +.B djpeg +runs much faster when scaling down the output. +.TP +.B \-bmp +Select BMP output format (Windows flavor). 8-bit colormapped format is +emitted if +.B \-colors +or +.B \-grayscale +is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color +format is emitted. +.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 \-os2 +Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is +emitted if +.B \-colors +or +.B \-grayscale +is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color +format is emitted. +.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 \-dct int +Use integer DCT method (default). +.TP +.B \-dct fast +Use fast integer DCT (less accurate). +.TP +.B \-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. +.TP +.B \-dither fs +Use Floyd-Steinberg dithering in color quantization. +.TP +.B \-dither ordered +Use ordered dithering in color quantization. +.TP +.B \-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 +.B \-onepass +mode. +.TP +.BI \-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 +.I file +must be a GIF or PPM file. This option overrides +.B \-colors +and +.BR \-onepass . +.TP +.B \-nosmooth +Use a faster, lower-quality upsampling routine. +.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 +.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 +.BI \-outfile " name" +Send output image to the named file, not to standard output. +.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, quantizes it to +256 colors, and saves the output in 8-bit BMP format in foo.bmp: +.IP +.B djpeg \-colors 256 \-bmp +.I foo.jpg +.B > +.I foo.bmp +.SH HINTS +To get a quick preview of an image, use the +.B \-grayscale +and/or +.B \-scale +switches. +.B \-grayscale \-scale 1/8 +is the fastest case. +.PP +Several options are available that trade off image quality to gain speed. +.B \-fast +turns on the recommended settings. +.PP +.B \-dct fast +and/or +.B \-nosmooth +gain speed at a small sacrifice in quality. +When producing a color-quantized image, +.B \-onepass \-dither ordered +is fast but much lower quality than the default behavior. +.B \-dither none +may give acceptable results in two-pass mode, but is seldom tolerable in +one-pass mode. +.PP +If you are fortunate enough to have very fast floating point hardware, +\fB\-dct float\fR may be even faster than \fB\-dct fast\fR. But on most +machines \fB\-dct float\fR is slower than \fB\-dct int\fR; in this case it is +not worth using, because its theoretical accuracy advantage is too small to be +significant in practice. +.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 jpegtran (1), +.BR rdjpgcom (1), +.BR wrjpgcom (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 +To avoid the Unisys LZW patent, +.B djpeg +produces uncompressed GIF files. These are larger than they should be, but +are readable by standard GIF decoders. +.PP +Still not as fast as we'd like. |