jpegtran.1 9.6 KB

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  1. .TH JPEGTRAN 1 "18 March 2017"
  2. .SH NAME
  3. jpegtran \- lossless transformation of JPEG files
  4. .SH SYNOPSIS
  5. .B jpegtran
  6. [
  7. .I options
  8. ]
  9. [
  10. .I filename
  11. ]
  12. .LP
  13. .SH DESCRIPTION
  14. .LP
  15. .B jpegtran
  16. performs various useful transformations of JPEG files.
  17. It can translate the coded representation from one variant of JPEG to another,
  18. for example from baseline JPEG to progressive JPEG or vice versa. It can also
  19. perform some rearrangements of the image data, for example turning an image
  20. from landscape to portrait format by rotation.
  21. .PP
  22. For EXIF files and JPEG files containing Exif data, you may prefer to use
  23. .B exiftran
  24. instead.
  25. .PP
  26. .B jpegtran
  27. works by rearranging the compressed data (DCT coefficients), without
  28. ever fully decoding the image. Therefore, its transformations are lossless:
  29. there is no image degradation at all, which would not be true if you used
  30. .B djpeg
  31. followed by
  32. .B cjpeg
  33. to accomplish the same conversion. But by the same token,
  34. .B jpegtran
  35. cannot perform lossy operations such as changing the image quality. However,
  36. while the image data is losslessly transformed, metadata can be removed. See
  37. the
  38. .B \-copy
  39. option for specifics.
  40. .PP
  41. .B jpegtran
  42. reads the named JPEG/JFIF file, or the standard input if no file is
  43. named, and produces a JPEG/JFIF file on the standard output.
  44. .SH OPTIONS
  45. All switch names may be abbreviated; for example,
  46. .B \-optimize
  47. may be written
  48. .B \-opt
  49. or
  50. .BR \-o .
  51. Upper and lower case are equivalent.
  52. British spellings are also accepted (e.g.,
  53. .BR \-optimise ),
  54. though for brevity these are not mentioned below.
  55. .PP
  56. To specify the coded JPEG representation used in the output file,
  57. .B jpegtran
  58. accepts a subset of the switches recognized by
  59. .BR cjpeg :
  60. .TP
  61. .B \-optimize
  62. Perform optimization of entropy encoding parameters.
  63. .TP
  64. .B \-progressive
  65. Create progressive JPEG file.
  66. .TP
  67. .BI \-restart " N"
  68. Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
  69. attached to the number.
  70. .TP
  71. .B \-arithmetic
  72. Use arithmetic coding.
  73. .TP
  74. .BI \-scans " file"
  75. Use the scan script given in the specified text file.
  76. .PP
  77. See
  78. .BR cjpeg (1)
  79. for more details about these switches.
  80. If you specify none of these switches, you get a plain baseline-JPEG output
  81. file. The quality setting and so forth are determined by the input file.
  82. .PP
  83. The image can be losslessly transformed by giving one of these switches:
  84. .TP
  85. .B \-flip horizontal
  86. Mirror image horizontally (left-right).
  87. .TP
  88. .B \-flip vertical
  89. Mirror image vertically (top-bottom).
  90. .TP
  91. .B \-rotate 90
  92. Rotate image 90 degrees clockwise.
  93. .TP
  94. .B \-rotate 180
  95. Rotate image 180 degrees.
  96. .TP
  97. .B \-rotate 270
  98. Rotate image 270 degrees clockwise (or 90 ccw).
  99. .TP
  100. .B \-transpose
  101. Transpose image (across UL-to-LR axis).
  102. .TP
  103. .B \-transverse
  104. Transverse transpose (across UR-to-LL axis).
  105. .PP
  106. The transpose transformation has no restrictions regarding image dimensions.
  107. The other transformations operate rather oddly if the image dimensions are not
  108. a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
  109. transform complete blocks of DCT coefficient data in the desired way.
  110. .PP
  111. .BR jpegtran 's
  112. default behavior when transforming an odd-size image is designed
  113. to preserve exact reversibility and mathematical consistency of the
  114. transformation set. As stated, transpose is able to flip the entire image
  115. area. Horizontal mirroring leaves any partial iMCU column at the right edge
  116. untouched, but is able to flip all rows of the image. Similarly, vertical
  117. mirroring leaves any partial iMCU row at the bottom edge untouched, but is
  118. able to flip all columns. The other transforms can be built up as sequences
  119. of transpose and flip operations; for consistency, their actions on edge
  120. pixels are defined to be the same as the end result of the corresponding
  121. transpose-and-flip sequence.
  122. .PP
  123. For practical use, you may prefer to discard any untransformable edge pixels
  124. rather than having a strange-looking strip along the right and/or bottom edges
  125. of a transformed image. To do this, add the
  126. .B \-trim
  127. switch:
  128. .TP
  129. .B \-trim
  130. Drop non-transformable edge blocks.
  131. .IP
  132. Obviously, a transformation with
  133. .B \-trim
  134. is not reversible, so strictly speaking
  135. .B jpegtran
  136. with this switch is not lossless. Also, the expected mathematical
  137. equivalences between the transformations no longer hold. For example,
  138. .B \-rot 270 -trim
  139. trims only the bottom edge, but
  140. .B \-rot 90 -trim
  141. followed by
  142. .B \-rot 180 -trim
  143. trims both edges.
  144. .TP
  145. .B \-perfect
  146. If you are only interested in perfect transformations, add the
  147. .B \-perfect
  148. switch. This causes
  149. .B jpegtran
  150. to fail with an error if the transformation is not perfect.
  151. .IP
  152. For example, you may want to do
  153. .IP
  154. .B (jpegtran \-rot 90 -perfect
  155. .I foo.jpg
  156. .B || djpeg
  157. .I foo.jpg
  158. .B | pnmflip \-r90 | cjpeg)
  159. .IP
  160. to do a perfect rotation, if available, or an approximated one if not.
  161. .PP
  162. This version of \fBjpegtran\fR also offers a lossless crop option, which
  163. discards data outside of a given image region but losslessly preserves what is
  164. inside. Like the rotate and flip transforms, lossless crop is restricted by the
  165. current JPEG format; the upper left corner of the selected region must fall on
  166. an iMCU boundary. If it doesn't, then it is silently moved up and/or left to
  167. the nearest iMCU boundary (the lower right corner is unchanged.) Thus, the
  168. output image covers at least the requested region, but it may cover more. The
  169. adjustment of the region dimensions may be optionally disabled by attaching
  170. an 'f' character ("force") to the width or height number.
  171. The image can be losslessly cropped by giving the switch:
  172. .TP
  173. .B \-crop WxH+X+Y
  174. Crop the image to a rectangular region of width W and height H, starting at
  175. point X,Y. The lossless crop feature discards data outside of a given image
  176. region but losslessly preserves what is inside. Like the rotate and flip
  177. transforms, lossless crop is restricted by the current JPEG format; the upper
  178. left corner of the selected region must fall on an iMCU boundary. If it
  179. doesn't, then it is silently moved up and/or left to the nearest iMCU boundary
  180. (the lower right corner is unchanged.)
  181. .PP
  182. Other not-strictly-lossless transformation switches are:
  183. .TP
  184. .B \-grayscale
  185. Force grayscale output.
  186. .IP
  187. This option discards the chrominance channels if the input image is YCbCr
  188. (ie, a standard color JPEG), resulting in a grayscale JPEG file. The
  189. luminance channel is preserved exactly, so this is a better method of reducing
  190. to grayscale than decompression, conversion, and recompression. This switch
  191. is particularly handy for fixing a monochrome picture that was mistakenly
  192. encoded as a color JPEG. (In such a case, the space savings from getting rid
  193. of the near-empty chroma channels won't be large; but the decoding time for
  194. a grayscale JPEG is substantially less than that for a color JPEG.)
  195. .PP
  196. .B jpegtran
  197. also recognizes these switches that control what to do with "extra" markers,
  198. such as comment blocks:
  199. .TP
  200. .B \-copy none
  201. Copy no extra markers from source file. This setting suppresses all
  202. comments and other metadata in the source file.
  203. .TP
  204. .B \-copy comments
  205. Copy only comment markers. This setting copies comments from the source file
  206. but discards any other metadata.
  207. .TP
  208. .B \-copy all
  209. Copy all extra markers. This setting preserves miscellaneous markers
  210. found in the source file, such as JFIF thumbnails, Exif data, and Photoshop
  211. settings. In some files, these extra markers can be sizable. Note that this
  212. option will copy thumbnails as-is; they will not be transformed.
  213. .PP
  214. The default behavior is \fB-copy comments\fR. (Note: in IJG releases v6 and
  215. v6a, \fBjpegtran\fR always did the equivalent of \fB-copy none\fR.)
  216. .PP
  217. Additional switches recognized by jpegtran are:
  218. .TP
  219. .BI \-icc " file"
  220. Embed ICC color management profile contained in the specified file. Note that
  221. this will cause \fBjpegtran\fR to ignore any APP2 markers in the input file,
  222. even if \fB-copy all\fR is specified.
  223. .TP
  224. .BI \-maxmemory " N"
  225. Set limit for amount of memory to use in processing large images. Value is
  226. in thousands of bytes, or millions of bytes if "M" is attached to the
  227. number. For example,
  228. .B \-max 4m
  229. selects 4000000 bytes. If more space is needed, an error will occur.
  230. .TP
  231. .BI \-outfile " name"
  232. Send output image to the named file, not to standard output.
  233. .TP
  234. .B \-verbose
  235. Enable debug printout. More
  236. .BR \-v 's
  237. give more output. Also, version information is printed at startup.
  238. .TP
  239. .B \-debug
  240. Same as
  241. .BR \-verbose .
  242. .TP
  243. .B \-version
  244. Print version information and exit.
  245. .SH EXAMPLES
  246. .LP
  247. This example converts a baseline JPEG file to progressive form:
  248. .IP
  249. .B jpegtran \-progressive
  250. .I foo.jpg
  251. .B >
  252. .I fooprog.jpg
  253. .PP
  254. This example rotates an image 90 degrees clockwise, discarding any
  255. unrotatable edge pixels:
  256. .IP
  257. .B jpegtran \-rot 90 -trim
  258. .I foo.jpg
  259. .B >
  260. .I foo90.jpg
  261. .SH ENVIRONMENT
  262. .TP
  263. .B JPEGMEM
  264. If this environment variable is set, its value is the default memory limit.
  265. The value is specified as described for the
  266. .B \-maxmemory
  267. switch.
  268. .B JPEGMEM
  269. overrides the default value specified when the program was compiled, and
  270. itself is overridden by an explicit
  271. .BR \-maxmemory .
  272. .SH SEE ALSO
  273. .BR cjpeg (1),
  274. .BR djpeg (1),
  275. .BR rdjpgcom (1),
  276. .BR wrjpgcom (1)
  277. .br
  278. Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
  279. Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
  280. .SH AUTHOR
  281. Independent JPEG Group
  282. .PP
  283. This file was modified by The libjpeg-turbo Project to include only information
  284. relevant to libjpeg-turbo and to wordsmith certain sections.
  285. .SH BUGS
  286. The transform options can't transform odd-size images perfectly. Use
  287. .B \-trim
  288. or
  289. .B \-perfect
  290. if you don't like the results.
  291. .PP
  292. The entire image is read into memory and then written out again, even in
  293. cases where this isn't really necessary. Expect swapping on large images,
  294. especially when using the more complex transform options.