cjpeg.1 13 KB

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  1. .TH CJPEG 1 "18 March 2017"
  2. .SH NAME
  3. cjpeg \- compress an image file to a JPEG file
  4. .SH SYNOPSIS
  5. .B cjpeg
  6. [
  7. .I options
  8. ]
  9. [
  10. .I filename
  11. ]
  12. .LP
  13. .SH DESCRIPTION
  14. .LP
  15. .B cjpeg
  16. compresses the named image file, or the standard input if no file is
  17. named, and produces a JPEG/JFIF file on the standard output.
  18. The currently supported input file formats are: PPM (PBMPLUS color
  19. format), PGM (PBMPLUS grayscale format), BMP, Targa, and RLE (Utah Raster
  20. Toolkit format). (RLE is supported only if the URT library is available.)
  21. .SH OPTIONS
  22. All switch names may be abbreviated; for example,
  23. .B \-grayscale
  24. may be written
  25. .B \-gray
  26. or
  27. .BR \-gr .
  28. Most of the "basic" switches can be abbreviated to as little as one letter.
  29. Upper and lower case are equivalent (thus
  30. .B \-BMP
  31. is the same as
  32. .BR \-bmp ).
  33. British spellings are also accepted (e.g.,
  34. .BR \-greyscale ),
  35. though for brevity these are not mentioned below.
  36. .PP
  37. The basic switches are:
  38. .TP
  39. .BI \-quality " N[,...]"
  40. Scale quantization tables to adjust image quality. Quality is 0 (worst) to
  41. 100 (best); default is 75. (See below for more info.)
  42. .TP
  43. .B \-grayscale
  44. Create monochrome JPEG file from color input. Be sure to use this switch when
  45. compressing a grayscale BMP file, because
  46. .B cjpeg
  47. isn't bright enough to notice whether a BMP file uses only shades of gray.
  48. By saying
  49. .BR \-grayscale,
  50. you'll get a smaller JPEG file that takes less time to process.
  51. .TP
  52. .B \-rgb
  53. Create RGB JPEG file.
  54. Using this switch suppresses the conversion from RGB
  55. colorspace input to the default YCbCr JPEG colorspace.
  56. .TP
  57. .B \-optimize
  58. Perform optimization of entropy encoding parameters. Without this, default
  59. encoding parameters are used.
  60. .B \-optimize
  61. usually makes the JPEG file a little smaller, but
  62. .B cjpeg
  63. runs somewhat slower and needs much more memory. Image quality and speed of
  64. decompression are unaffected by
  65. .BR \-optimize .
  66. .TP
  67. .B \-progressive
  68. Create progressive JPEG file (see below).
  69. .TP
  70. .B \-targa
  71. Input file is Targa format. Targa files that contain an "identification"
  72. field will not be automatically recognized by
  73. .BR cjpeg ;
  74. for such files you must specify
  75. .B \-targa
  76. to make
  77. .B cjpeg
  78. treat the input as Targa format.
  79. For most Targa files, you won't need this switch.
  80. .PP
  81. The
  82. .B \-quality
  83. switch lets you trade off compressed file size against quality of the
  84. reconstructed image: the higher the quality setting, the larger the JPEG file,
  85. and the closer the output image will be to the original input. Normally you
  86. want to use the lowest quality setting (smallest file) that decompresses into
  87. something visually indistinguishable from the original image. For this
  88. purpose the quality setting should generally be between 50 and 95 (the default
  89. is 75) for photographic images. If you see defects at
  90. .B \-quality
  91. 75, then go up 5 or 10 counts at a time until you are happy with the output
  92. image. (The optimal setting will vary from one image to another.)
  93. .PP
  94. .B \-quality
  95. 100 will generate a quantization table of all 1's, minimizing loss in the
  96. quantization step (but there is still information loss in subsampling, as well
  97. as roundoff error.) For most images, specifying a quality value above
  98. about 95 will increase the size of the compressed file dramatically, and while
  99. the quality gain from these higher quality values is measurable (using metrics
  100. such as PSNR or SSIM), it is rarely perceivable by human vision.
  101. .PP
  102. In the other direction, quality values below 50 will produce very small files
  103. of low image quality. Settings around 5 to 10 might be useful in preparing an
  104. index of a large image library, for example. Try
  105. .B \-quality
  106. 2 (or so) for some amusing Cubist effects. (Note: quality
  107. values below about 25 generate 2-byte quantization tables, which are
  108. considered optional in the JPEG standard.
  109. .B cjpeg
  110. emits a warning message when you give such a quality value, because some
  111. other JPEG programs may be unable to decode the resulting file. Use
  112. .B \-baseline
  113. if you need to ensure compatibility at low quality values.)
  114. .PP
  115. The \fB-quality\fR option has been extended in this version of \fBcjpeg\fR to
  116. support separate quality settings for luminance and chrominance (or, in
  117. general, separate settings for every quantization table slot.) The principle
  118. is the same as chrominance subsampling: since the human eye is more sensitive
  119. to spatial changes in brightness than spatial changes in color, the chrominance
  120. components can be quantized more than the luminance components without
  121. incurring any visible image quality loss. However, unlike subsampling, this
  122. feature reduces data in the frequency domain instead of the spatial domain,
  123. which allows for more fine-grained control. This option is useful in
  124. quality-sensitive applications, for which the artifacts generated by
  125. subsampling may be unacceptable.
  126. .PP
  127. The \fB-quality\fR option accepts a comma-separated list of parameters, which
  128. respectively refer to the quality levels that should be assigned to the
  129. quantization table slots. If there are more q-table slots than parameters,
  130. then the last parameter is replicated. Thus, if only one quality parameter is
  131. given, this is used for both luminance and chrominance (slots 0 and 1,
  132. respectively), preserving the legacy behavior of cjpeg v6b and prior.
  133. More (or customized) quantization tables can be set with the \fB-qtables\fR
  134. option and assigned to components with the \fB-qslots\fR option (see the
  135. "wizard" switches below.)
  136. .PP
  137. JPEG files generated with separate luminance and chrominance quality are fully
  138. compliant with standard JPEG decoders.
  139. .PP
  140. .BR CAUTION:
  141. For this setting to be useful, be sure to pass an argument of \fB-sample 1x1\fR
  142. to \fBcjpeg\fR to disable chrominance subsampling. Otherwise, the default
  143. subsampling level (2x2, AKA "4:2:0") will be used.
  144. .PP
  145. The
  146. .B \-progressive
  147. switch creates a "progressive JPEG" file. In this type of JPEG file, the data
  148. is stored in multiple scans of increasing quality. If the file is being
  149. transmitted over a slow communications link, the decoder can use the first
  150. scan to display a low-quality image very quickly, and can then improve the
  151. display with each subsequent scan. The final image is exactly equivalent to a
  152. standard JPEG file of the same quality setting, and the total file size is
  153. about the same --- often a little smaller.
  154. .PP
  155. Switches for advanced users:
  156. .TP
  157. .B \-arithmetic
  158. Use arithmetic coding.
  159. .B Caution:
  160. arithmetic coded JPEG is not yet widely implemented, so many decoders will be
  161. unable to view an arithmetic coded JPEG file at all.
  162. .TP
  163. .B \-dct int
  164. Use integer DCT method (default).
  165. .TP
  166. .B \-dct fast
  167. Use fast integer DCT (less accurate).
  168. In libjpeg-turbo, the fast method is generally about 5-15% faster than the int
  169. method when using the x86/x86-64 SIMD extensions (results may vary with other
  170. SIMD implementations, or when using libjpeg-turbo without SIMD extensions.)
  171. For quality levels of 90 and below, there should be little or no perceptible
  172. difference between the two algorithms. For quality levels above 90, however,
  173. the difference between the fast and the int methods becomes more pronounced.
  174. With quality=97, for instance, the fast method incurs generally about a 1-3 dB
  175. loss (in PSNR) relative to the int method, but this can be larger for some
  176. images. Do not use the fast method with quality levels above 97. The
  177. algorithm often degenerates at quality=98 and above and can actually produce a
  178. more lossy image than if lower quality levels had been used. Also, in
  179. libjpeg-turbo, the fast method is not fully accelerated for quality levels
  180. above 97, so it will be slower than the int method.
  181. .TP
  182. .B \-dct float
  183. Use floating-point DCT method.
  184. The float method is mainly a legacy feature. It does not produce significantly
  185. more accurate results than the int method, and it is much slower. The float
  186. method may also give different results on different machines due to varying
  187. roundoff behavior, whereas the integer methods should give the same results on
  188. all machines.
  189. .TP
  190. .BI \-icc " file"
  191. Embed ICC color management profile contained in the specified file.
  192. .TP
  193. .BI \-restart " N"
  194. Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
  195. attached to the number.
  196. .B \-restart 0
  197. (the default) means no restart markers.
  198. .TP
  199. .BI \-smooth " N"
  200. Smooth the input image to eliminate dithering noise. N, ranging from 1 to
  201. 100, indicates the strength of smoothing. 0 (the default) means no smoothing.
  202. .TP
  203. .BI \-maxmemory " N"
  204. Set limit for amount of memory to use in processing large images. Value is
  205. in thousands of bytes, or millions of bytes if "M" is attached to the
  206. number. For example,
  207. .B \-max 4m
  208. selects 4000000 bytes. If more space is needed, an error will occur.
  209. .TP
  210. .BI \-outfile " name"
  211. Send output image to the named file, not to standard output.
  212. .TP
  213. .BI \-memdst
  214. Compress to memory instead of a file. This feature was implemented mainly as a
  215. way of testing the in-memory destination manager (jpeg_mem_dest()), but it is
  216. also useful for benchmarking, since it reduces the I/O overhead.
  217. .TP
  218. .B \-verbose
  219. Enable debug printout. More
  220. .BR \-v 's
  221. give more output. Also, version information is printed at startup.
  222. .TP
  223. .B \-debug
  224. Same as
  225. .BR \-verbose .
  226. .TP
  227. .B \-version
  228. Print version information and exit.
  229. .PP
  230. The
  231. .B \-restart
  232. option inserts extra markers that allow a JPEG decoder to resynchronize after
  233. a transmission error. Without restart markers, any damage to a compressed
  234. file will usually ruin the image from the point of the error to the end of the
  235. image; with restart markers, the damage is usually confined to the portion of
  236. the image up to the next restart marker. Of course, the restart markers
  237. occupy extra space. We recommend
  238. .B \-restart 1
  239. for images that will be transmitted across unreliable networks such as Usenet.
  240. .PP
  241. The
  242. .B \-smooth
  243. option filters the input to eliminate fine-scale noise. This is often useful
  244. when converting dithered images to JPEG: a moderate smoothing factor of 10 to
  245. 50 gets rid of dithering patterns in the input file, resulting in a smaller
  246. JPEG file and a better-looking image. Too large a smoothing factor will
  247. visibly blur the image, however.
  248. .PP
  249. Switches for wizards:
  250. .TP
  251. .B \-baseline
  252. Force baseline-compatible quantization tables to be generated. This clamps
  253. quantization values to 8 bits even at low quality settings. (This switch is
  254. poorly named, since it does not ensure that the output is actually baseline
  255. JPEG. For example, you can use
  256. .B \-baseline
  257. and
  258. .B \-progressive
  259. together.)
  260. .TP
  261. .BI \-qtables " file"
  262. Use the quantization tables given in the specified text file.
  263. .TP
  264. .BI \-qslots " N[,...]"
  265. Select which quantization table to use for each color component.
  266. .TP
  267. .BI \-sample " HxV[,...]"
  268. Set JPEG sampling factors for each color component.
  269. .TP
  270. .BI \-scans " file"
  271. Use the scan script given in the specified text file.
  272. .PP
  273. The "wizard" switches are intended for experimentation with JPEG. If you
  274. don't know what you are doing, \fBdon't use them\fR. These switches are
  275. documented further in the file wizard.txt.
  276. .SH EXAMPLES
  277. .LP
  278. This example compresses the PPM file foo.ppm with a quality factor of
  279. 60 and saves the output as foo.jpg:
  280. .IP
  281. .B cjpeg \-quality
  282. .I 60 foo.ppm
  283. .B >
  284. .I foo.jpg
  285. .SH HINTS
  286. Color GIF files are not the ideal input for JPEG; JPEG is really intended for
  287. compressing full-color (24-bit) images. In particular, don't try to convert
  288. cartoons, line drawings, and other images that have only a few distinct
  289. colors. GIF works great on these, JPEG does not. If you want to convert a
  290. GIF to JPEG, you should experiment with
  291. .BR cjpeg 's
  292. .B \-quality
  293. and
  294. .B \-smooth
  295. options to get a satisfactory conversion.
  296. .B \-smooth 10
  297. or so is often helpful.
  298. .PP
  299. Avoid running an image through a series of JPEG compression/decompression
  300. cycles. Image quality loss will accumulate; after ten or so cycles the image
  301. may be noticeably worse than it was after one cycle. It's best to use a
  302. lossless format while manipulating an image, then convert to JPEG format when
  303. you are ready to file the image away.
  304. .PP
  305. The
  306. .B \-optimize
  307. option to
  308. .B cjpeg
  309. is worth using when you are making a "final" version for posting or archiving.
  310. It's also a win when you are using low quality settings to make very small
  311. JPEG files; the percentage improvement is often a lot more than it is on
  312. larger files. (At present,
  313. .B \-optimize
  314. mode is always selected when generating progressive JPEG files.)
  315. .SH ENVIRONMENT
  316. .TP
  317. .B JPEGMEM
  318. If this environment variable is set, its value is the default memory limit.
  319. The value is specified as described for the
  320. .B \-maxmemory
  321. switch.
  322. .B JPEGMEM
  323. overrides the default value specified when the program was compiled, and
  324. itself is overridden by an explicit
  325. .BR \-maxmemory .
  326. .SH SEE ALSO
  327. .BR djpeg (1),
  328. .BR jpegtran (1),
  329. .BR rdjpgcom (1),
  330. .BR wrjpgcom (1)
  331. .br
  332. .BR ppm (5),
  333. .BR pgm (5)
  334. .br
  335. Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
  336. Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
  337. .SH AUTHOR
  338. Independent JPEG Group
  339. .PP
  340. This file was modified by The libjpeg-turbo Project to include only information
  341. relevant to libjpeg-turbo, to wordsmith certain sections, and to describe
  342. features not present in libjpeg.
  343. .SH ISSUES
  344. Support for GIF input files was removed in cjpeg v6b due to concerns over
  345. the Unisys LZW patent. Although this patent expired in 2006, cjpeg still
  346. lacks GIF support, for these historical reasons. (Conversion of GIF files to
  347. JPEG is usually a bad idea anyway, since GIF is a 256-color format.)
  348. .PP
  349. Not all variants of BMP and Targa file formats are supported.
  350. .PP
  351. The
  352. .B \-targa
  353. switch is not a bug, it's a feature. (It would be a bug if the Targa format
  354. designers had not been clueless.)