| 1 | .TH DJPEG 1 "3 October 2009"
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| 2 | .SH NAME
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| 3 | djpeg \- decompress a JPEG file to an image file
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| 4 | .SH SYNOPSIS
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| 5 | .B djpeg
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| 6 | [
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| 7 | .I options
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| 8 | ]
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| 9 | [
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| 10 | .I filename
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| 11 | ]
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| 12 | .LP
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| 13 | .SH DESCRIPTION
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| 14 | .LP
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| 15 | .B djpeg
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| 16 | decompresses the named JPEG file, or the standard input if no file is named,
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| 17 | and produces an image file on the standard output. PBMPLUS (PPM/PGM), BMP,
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| 18 | GIF, Targa, or RLE (Utah Raster Toolkit) output format can be selected.
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| 19 | (RLE is supported only if the URT library is available.)
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| 20 | .SH OPTIONS
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| 21 | All switch names may be abbreviated; for example,
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| 22 | .B \-grayscale
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| 23 | may be written
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| 24 | .B \-gray
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| 25 | or
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| 26 | .BR \-gr .
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| 27 | Most of the "basic" switches can be abbreviated to as little as one letter.
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| 28 | Upper and lower case are equivalent (thus
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| 29 | .B \-BMP
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| 30 | is the same as
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| 31 | .BR \-bmp ).
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| 32 | British spellings are also accepted (e.g.,
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| 33 | .BR \-greyscale ),
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| 34 | though for brevity these are not mentioned below.
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| 35 | .PP
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| 36 | The basic switches are:
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| 37 | .TP
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| 38 | .BI \-colors " N"
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| 39 | Reduce image to at most N colors. This reduces the number of colors used in
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| 40 | the output image, so that it can be displayed on a colormapped display or
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| 41 | stored in a colormapped file format. For example, if you have an 8-bit
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| 42 | display, you'd need to reduce to 256 or fewer colors.
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| 43 | .TP
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| 44 | .BI \-quantize " N"
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| 45 | Same as
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| 46 | .BR \-colors .
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| 47 | .B \-colors
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| 48 | is the recommended name,
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| 49 | .B \-quantize
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| 50 | is provided only for backwards compatibility.
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| 51 | .TP
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| 52 | .B \-fast
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| 53 | Select recommended processing options for fast, low quality output. (The
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| 54 | default options are chosen for highest quality output.) Currently, this is
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| 55 | equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.
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| 56 | .TP
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| 57 | .B \-grayscale
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| 58 | Force gray-scale output even if JPEG file is color. Useful for viewing on
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| 59 | monochrome displays; also,
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| 60 | .B djpeg
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| 61 | runs noticeably faster in this mode.
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| 62 | .TP
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| 63 | .BI \-scale " M/N"
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| 64 | Scale the output image by a factor M/N. Currently supported scale factors are
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| 65 | M/N with all M from 1 to 16, where N is the source DCT size, which is 8 for
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| 66 | baseline JPEG. If the /N part is omitted, then M specifies the DCT scaled
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| 67 | size to be applied on the given input. For baseline JPEG this is equivalent
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| 68 | to M/8 scaling, since the source DCT size for baseline JPEG is 8.
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| 69 | Scaling is handy if the image is larger than your screen; also,
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| 70 | .B djpeg
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| 71 | runs much faster when scaling down the output.
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| 72 | .TP
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| 73 | .B \-bmp
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| 74 | Select BMP output format (Windows flavor). 8-bit colormapped format is
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| 75 | emitted if
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| 76 | .B \-colors
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| 77 | or
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| 78 | .B \-grayscale
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| 79 | is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color
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| 80 | format is emitted.
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| 81 | .TP
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| 82 | .B \-gif
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| 83 | Select GIF output format. Since GIF does not support more than 256 colors,
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| 84 | .B \-colors 256
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| 85 | is assumed (unless you specify a smaller number of colors).
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| 86 | .TP
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| 87 | .B \-os2
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| 88 | Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is
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| 89 | emitted if
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| 90 | .B \-colors
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| 91 | or
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| 92 | .B \-grayscale
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| 93 | is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color
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| 94 | format is emitted.
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| 95 | .TP
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| 96 | .B \-pnm
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| 97 | Select PBMPLUS (PPM/PGM) output format (this is the default format).
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| 98 | PGM is emitted if the JPEG file is gray-scale or if
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| 99 | .B \-grayscale
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| 100 | is specified; otherwise PPM is emitted.
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| 101 | .TP
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| 102 | .B \-rle
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| 103 | Select RLE output format. (Requires URT library.)
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| 104 | .TP
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| 105 | .B \-targa
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| 106 | Select Targa output format. Gray-scale format is emitted if the JPEG file is
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| 107 | gray-scale or if
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| 108 | .B \-grayscale
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| 109 | is specified; otherwise, colormapped format is emitted if
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| 110 | .B \-colors
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| 111 | is specified; otherwise, 24-bit full-color format is emitted.
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| 112 | .PP
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| 113 | Switches for advanced users:
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| 114 | .TP
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| 115 | .B \-dct int
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| 116 | Use integer DCT method (default).
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| 117 | .TP
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| 118 | .B \-dct fast
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| 119 | Use fast integer DCT (less accurate).
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| 120 | .TP
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| 121 | .B \-dct float
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| 122 | Use floating-point DCT method.
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| 123 | The float method is very slightly more accurate than the int method, but is
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| 124 | much slower unless your machine has very fast floating-point hardware. Also
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| 125 | note that results of the floating-point method may vary slightly across
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| 126 | machines, while the integer methods should give the same results everywhere.
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| 127 | The fast integer method is much less accurate than the other two.
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| 128 | .TP
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| 129 | .B \-dither fs
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| 130 | Use Floyd-Steinberg dithering in color quantization.
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| 131 | .TP
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| 132 | .B \-dither ordered
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| 133 | Use ordered dithering in color quantization.
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| 134 | .TP
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| 135 | .B \-dither none
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| 136 | Do not use dithering in color quantization.
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| 137 | By default, Floyd-Steinberg dithering is applied when quantizing colors; this
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| 138 | is slow but usually produces the best results. Ordered dither is a compromise
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| 139 | between speed and quality; no dithering is fast but usually looks awful. Note
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| 140 | that these switches have no effect unless color quantization is being done.
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| 141 | Ordered dither is only available in
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| 142 | .B \-onepass
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| 143 | mode.
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| 144 | .TP
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| 145 | .BI \-map " file"
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| 146 | Quantize to the colors used in the specified image file. This is useful for
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| 147 | producing multiple files with identical color maps, or for forcing a
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| 148 | predefined set of colors to be used. The
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| 149 | .I file
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| 150 | must be a GIF or PPM file. This option overrides
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| 151 | .B \-colors
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| 152 | and
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| 153 | .BR \-onepass .
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| 154 | .TP
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| 155 | .B \-nosmooth
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| 156 | Don't use high-quality upsampling.
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| 157 | .TP
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| 158 | .B \-onepass
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| 159 | Use one-pass instead of two-pass color quantization. The one-pass method is
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| 160 | faster and needs less memory, but it produces a lower-quality image.
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| 161 | .B \-onepass
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| 162 | is ignored unless you also say
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| 163 | .B \-colors
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| 164 | .IR N .
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| 165 | Also, the one-pass method is always used for gray-scale output (the two-pass
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| 166 | method is no improvement then).
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| 167 | .TP
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| 168 | .BI \-maxmemory " N"
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| 169 | Set limit for amount of memory to use in processing large images. Value is
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| 170 | in thousands of bytes, or millions of bytes if "M" is attached to the
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| 171 | number. For example,
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| 172 | .B \-max 4m
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| 173 | selects 4000000 bytes. If more space is needed, temporary files will be used.
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| 174 | .TP
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| 175 | .BI \-outfile " name"
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| 176 | Send output image to the named file, not to standard output.
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| 177 | .TP
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| 178 | .B \-verbose
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| 179 | Enable debug printout. More
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| 180 | .BR \-v 's
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| 181 | give more output. Also, version information is printed at startup.
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| 182 | .TP
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| 183 | .B \-debug
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| 184 | Same as
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| 185 | .BR \-verbose .
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| 186 | .SH EXAMPLES
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| 187 | .LP
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| 188 | This example decompresses the JPEG file foo.jpg, quantizes it to
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| 189 | 256 colors, and saves the output in 8-bit BMP format in foo.bmp:
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| 190 | .IP
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| 191 | .B djpeg \-colors 256 \-bmp
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| 192 | .I foo.jpg
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| 193 | .B >
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| 194 | .I foo.bmp
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| 195 | .SH HINTS
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| 196 | To get a quick preview of an image, use the
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| 197 | .B \-grayscale
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| 198 | and/or
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| 199 | .B \-scale
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| 200 | switches.
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| 201 | .B \-grayscale \-scale 1/8
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| 202 | is the fastest case.
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| 203 | .PP
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| 204 | Several options are available that trade off image quality to gain speed.
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| 205 | .B \-fast
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| 206 | turns on the recommended settings.
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| 207 | .PP
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| 208 | .B \-dct fast
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| 209 | and/or
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| 210 | .B \-nosmooth
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| 211 | gain speed at a small sacrifice in quality.
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| 212 | When producing a color-quantized image,
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| 213 | .B \-onepass \-dither ordered
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| 214 | is fast but much lower quality than the default behavior.
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| 215 | .B \-dither none
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| 216 | may give acceptable results in two-pass mode, but is seldom tolerable in
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| 217 | one-pass mode.
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| 218 | .PP
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| 219 | If you are fortunate enough to have very fast floating point hardware,
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| 220 | \fB\-dct float\fR may be even faster than \fB\-dct fast\fR. But on most
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| 221 | machines \fB\-dct float\fR is slower than \fB\-dct int\fR; in this case it is
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| 222 | not worth using, because its theoretical accuracy advantage is too small to be
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| 223 | significant in practice.
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| 224 | .SH ENVIRONMENT
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| 225 | .TP
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| 226 | .B JPEGMEM
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| 227 | If this environment variable is set, its value is the default memory limit.
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| 228 | The value is specified as described for the
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| 229 | .B \-maxmemory
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| 230 | switch.
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| 231 | .B JPEGMEM
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| 232 | overrides the default value specified when the program was compiled, and
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| 233 | itself is overridden by an explicit
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| 234 | .BR \-maxmemory .
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| 235 | .SH SEE ALSO
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| 236 | .BR cjpeg (1),
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| 237 | .BR jpegtran (1),
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| 238 | .BR rdjpgcom (1),
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| 239 | .BR wrjpgcom (1)
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| 240 | .br
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| 241 | .BR ppm (5),
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| 242 | .BR pgm (5)
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| 243 | .br
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| 244 | Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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| 245 | Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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| 246 | .SH AUTHOR
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| 247 | Independent JPEG Group
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| 248 | .SH BUGS
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| 249 | To avoid the Unisys LZW patent,
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| 250 | .B djpeg
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| 251 | produces uncompressed GIF files. These are larger than they should be, but
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| 252 | are readable by standard GIF decoders.
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