djpeg.1 7.1 KB

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  1. .TH DJPEG 1 "15 June 1995"
  2. .SH NAME
  3. djpeg \- decompress a JPEG file to an image file
  4. .SH SYNOPSIS
  5. .B djpeg
  6. [
  7. .I options
  8. ]
  9. [
  10. .I filename
  11. ]
  12. .LP
  13. .SH DESCRIPTION
  14. .LP
  15. .B djpeg
  16. decompresses the named JPEG file, or the standard input if no file is named,
  17. and produces an image file on the standard output. PBMPLUS (PPM/PGM), BMP,
  18. GIF, Targa, or RLE (Utah Raster Toolkit) output format can be selected.
  19. (RLE is supported only if the URT library is available.)
  20. .SH OPTIONS
  21. All switch names may be abbreviated; for example,
  22. .B \-grayscale
  23. may be written
  24. .B \-gray
  25. or
  26. .BR \-gr .
  27. Most of the "basic" switches can be abbreviated to as little as one letter.
  28. Upper and lower case are equivalent (thus
  29. .B \-GIF
  30. is the same as
  31. .BR \-gif ).
  32. British spellings are also accepted (e.g.,
  33. .BR \-greyscale ),
  34. though for brevity these are not mentioned below.
  35. .PP
  36. The basic switches are:
  37. .TP
  38. .BI \-colors " N"
  39. Reduce image to at most N colors. This reduces the number of colors used in
  40. the output image, so that it can be displayed on a colormapped display or
  41. stored in a colormapped file format. For example, if you have an 8-bit
  42. display, you'd need to reduce to 256 or fewer colors.
  43. .TP
  44. .BI \-quantize " N"
  45. Same as
  46. .BR \-colors .
  47. .B \-colors
  48. is the recommended name,
  49. .B \-quantize
  50. is provided only for backwards compatibility.
  51. .TP
  52. .B \-fast
  53. Select recommended processing options for fast, low quality output. (The
  54. default options are chosen for highest quality output.) Currently, this is
  55. equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.
  56. .TP
  57. .B \-grayscale
  58. Force gray-scale output even if JPEG file is color. Useful for viewing on
  59. monochrome displays; also,
  60. .B djpeg
  61. runs noticeably faster in this mode.
  62. .TP
  63. .BI \-scale " M/N"
  64. Scale the output image by a factor M/N. Currently the scale factor must be
  65. 1/1, 1/2, 1/4, or 1/8. Scaling is handy if the image is larger than your
  66. screen; also,
  67. .B djpeg
  68. runs much faster when scaling down the output.
  69. .TP
  70. .B \-bmp
  71. Select BMP output format (Windows flavor). 8-bit colormapped format is
  72. emitted if
  73. .B \-colors
  74. or
  75. .B \-grayscale
  76. is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color
  77. format is emitted.
  78. .TP
  79. .B \-gif
  80. Select GIF output format. Since GIF does not support more than 256 colors,
  81. .B \-colors 256
  82. is assumed (unless you specify a smaller number of colors).
  83. .TP
  84. .B \-os2
  85. Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is
  86. emitted if
  87. .B \-colors
  88. or
  89. .B \-grayscale
  90. is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color
  91. format is emitted.
  92. .TP
  93. .B \-pnm
  94. Select PBMPLUS (PPM/PGM) output format (this is the default format).
  95. PGM is emitted if the JPEG file is gray-scale or if
  96. .B \-grayscale
  97. is specified; otherwise PPM is emitted.
  98. .TP
  99. .B \-rle
  100. Select RLE output format. (Requires URT library.)
  101. .TP
  102. .B \-targa
  103. Select Targa output format. Gray-scale format is emitted if the JPEG file is
  104. gray-scale or if
  105. .B \-grayscale
  106. is specified; otherwise, colormapped format is emitted if
  107. .B \-colors
  108. is specified; otherwise, 24-bit full-color format is emitted.
  109. .PP
  110. Switches for advanced users:
  111. .TP
  112. .B \-dct int
  113. Use integer DCT method (default).
  114. .TP
  115. .B \-dct fast
  116. Use fast integer DCT (less accurate).
  117. .TP
  118. .B \-dct float
  119. Use floating-point DCT method.
  120. The float method is very slightly more accurate than the int method, but is
  121. much slower unless your machine has very fast floating-point hardware. Also
  122. note that results of the floating-point method may vary slightly across
  123. machines, while the integer methods should give the same results everywhere.
  124. The fast integer method is much less accurate than the other two.
  125. .TP
  126. .B \-dither fs
  127. Use Floyd-Steinberg dithering in color quantization.
  128. .TP
  129. .B \-dither ordered
  130. Use ordered dithering in color quantization.
  131. .TP
  132. .B \-dither none
  133. Do not use dithering in color quantization.
  134. By default, Floyd-Steinberg dithering is applied when quantizing colors; this
  135. is slow but usually produces the best results. Ordered dither is a compromise
  136. between speed and quality; no dithering is fast but usually looks awful. Note
  137. that these switches have no effect unless color quantization is being done.
  138. Ordered dither is only available in
  139. .B \-onepass
  140. mode.
  141. .TP
  142. .BI \-map " file"
  143. Quantize to the colors used in the specified image file. This is useful for
  144. producing multiple files with identical color maps, or for forcing a
  145. predefined set of colors to be used. The
  146. .I file
  147. must be a GIF or PPM file. This option overrides
  148. .B \-colors
  149. and
  150. .BR \-onepass .
  151. .TP
  152. .B \-nosmooth
  153. Use a faster, lower-quality upsampling routine.
  154. .TP
  155. .B \-onepass
  156. Use one-pass instead of two-pass color quantization. The one-pass method is
  157. faster and needs less memory, but it produces a lower-quality image.
  158. .B \-onepass
  159. is ignored unless you also say
  160. .B \-colors
  161. .IR N .
  162. Also, the one-pass method is always used for gray-scale output (the two-pass
  163. method is no improvement then).
  164. .TP
  165. .BI \-maxmemory " N"
  166. Set limit for amount of memory to use in processing large images. Value is
  167. in thousands of bytes, or millions of bytes if "M" is attached to the
  168. number. For example,
  169. .B \-max 4m
  170. selects 4000000 bytes. If more space is needed, temporary files will be used.
  171. .TP
  172. .BI \-outfile " name"
  173. Send output image to the named file, not to standard output.
  174. .TP
  175. .B \-verbose
  176. Enable debug printout. More
  177. .BR \-v 's
  178. give more output. Also, version information is printed at startup.
  179. .TP
  180. .B \-debug
  181. Same as
  182. .BR \-verbose .
  183. .SH EXAMPLES
  184. .LP
  185. This example decompresses the JPEG file foo.jpg, automatically quantizes to
  186. 256 colors, and saves the output in GIF format in foo.gif:
  187. .IP
  188. .B djpeg \-gif
  189. .I foo.jpg
  190. .B >
  191. .I foo.gif
  192. .SH HINTS
  193. To get a quick preview of an image, use the
  194. .B \-grayscale
  195. and/or
  196. .B \-scale
  197. switches.
  198. .B \-grayscale \-scale 1/8
  199. is the fastest case.
  200. .PP
  201. Several options are available that trade off image quality to gain speed.
  202. .B \-fast
  203. turns on the recommended settings.
  204. .PP
  205. .B \-dct fast
  206. and/or
  207. .B \-nosmooth
  208. gain speed at a small sacrifice in quality.
  209. When producing a color-quantized image,
  210. .B \-onepass \-dither ordered
  211. is fast but much lower quality than the default behavior.
  212. .B \-dither none
  213. may give acceptable results in two-pass mode, but is seldom tolerable in
  214. one-pass mode.
  215. .PP
  216. If you are fortunate enough to have very fast floating point hardware,
  217. \fB\-dct float\fR may be even faster than \fB\-dct fast\fR. But on most
  218. machines \fB\-dct float\fR is slower than \fB\-dct int\fR; in this case it is
  219. not worth using, because its theoretical accuracy advantage is too small to be
  220. significant in practice.
  221. .SH ENVIRONMENT
  222. .TP
  223. .B JPEGMEM
  224. If this environment variable is set, its value is the default memory limit.
  225. The value is specified as described for the
  226. .B \-maxmemory
  227. switch.
  228. .B JPEGMEM
  229. overrides the default value specified when the program was compiled, and
  230. itself is overridden by an explicit
  231. .BR \-maxmemory .
  232. .SH SEE ALSO
  233. .BR cjpeg (1),
  234. .BR jpegtran (1),
  235. .BR rdjpgcom (1),
  236. .BR wrjpgcom (1)
  237. .br
  238. .BR ppm (5),
  239. .BR pgm (5)
  240. .br
  241. Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
  242. Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
  243. .SH AUTHOR
  244. Independent JPEG Group
  245. .SH BUGS
  246. Arithmetic coding is not supported for legal reasons.
  247. .PP
  248. Still not as fast as we'd like.