| 1 | .TH JPEGTRAN 1 "3 August 1997" |
| 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 | .B jpegtran |
| 23 | works by rearranging the compressed data (DCT coefficients), without |
| 24 | ever fully decoding the image. Therefore, its transformations are lossless: |
| 25 | there is no image degradation at all, which would not be true if you used |
| 26 | .B djpeg |
| 27 | followed by |
| 28 | .B cjpeg |
| 29 | to accomplish the same conversion. But by the same token, |
| 30 | .B jpegtran |
| 31 | cannot perform lossy operations such as changing the image quality. |
| 32 | .PP |
| 33 | .B jpegtran |
| 34 | reads the named JPEG/JFIF file, or the standard input if no file is |
| 35 | named, and produces a JPEG/JFIF file on the standard output. |
| 36 | .SH OPTIONS |
| 37 | All switch names may be abbreviated; for example, |
| 38 | .B \-optimize |
| 39 | may be written |
| 40 | .B \-opt |
| 41 | or |
| 42 | .BR \-o . |
| 43 | Upper and lower case are equivalent. |
| 44 | British spellings are also accepted (e.g., |
| 45 | .BR \-optimise ), |
| 46 | though for brevity these are not mentioned below. |
| 47 | .PP |
| 48 | To specify the coded JPEG representation used in the output file, |
| 49 | .B jpegtran |
| 50 | accepts a subset of the switches recognized by |
| 51 | .BR cjpeg : |
| 52 | .TP |
| 53 | .B \-optimize |
| 54 | Perform optimization of entropy encoding parameters. |
| 55 | .TP |
| 56 | .B \-progressive |
| 57 | Create progressive JPEG file. |
| 58 | .TP |
| 59 | .BI \-restart " N" |
| 60 | Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is |
| 61 | attached to the number. |
| 62 | .TP |
| 63 | .BI \-scans " file" |
| 64 | Use the scan script given in the specified text file. |
| 65 | .PP |
| 66 | See |
| 67 | .BR cjpeg (1) |
| 68 | for more details about these switches. |
| 69 | If you specify none of these switches, you get a plain baseline-JPEG output |
| 70 | file. The quality setting and so forth are determined by the input file. |
| 71 | .PP |
| 72 | The image can be losslessly transformed by giving one of these switches: |
| 73 | .TP |
| 74 | .B \-flip horizontal |
| 75 | Mirror image horizontally (left-right). |
| 76 | .TP |
| 77 | .B \-flip vertical |
| 78 | Mirror image vertically (top-bottom). |
| 79 | .TP |
| 80 | .B \-rotate 90 |
| 81 | Rotate image 90 degrees clockwise. |
| 82 | .TP |
| 83 | .B \-rotate 180 |
| 84 | Rotate image 180 degrees. |
| 85 | .TP |
| 86 | .B \-rotate 270 |
| 87 | Rotate image 270 degrees clockwise (or 90 ccw). |
| 88 | .TP |
| 89 | .B \-transpose |
| 90 | Transpose image (across UL-to-LR axis). |
| 91 | .TP |
| 92 | .B \-transverse |
| 93 | Transverse transpose (across UR-to-LL axis). |
| 94 | .PP |
| 95 | The transpose transformation has no restrictions regarding image dimensions. |
| 96 | The other transformations operate rather oddly if the image dimensions are not |
| 97 | a multiple of the iMCU size (usually 8 or 16 pixels), because they can only |
| 98 | transform complete blocks of DCT coefficient data in the desired way. |
| 99 | .PP |
| 100 | .BR jpegtran 's |
| 101 | default behavior when transforming an odd-size image is designed |
| 102 | to preserve exact reversibility and mathematical consistency of the |
| 103 | transformation set. As stated, transpose is able to flip the entire image |
| 104 | area. Horizontal mirroring leaves any partial iMCU column at the right edge |
| 105 | untouched, but is able to flip all rows of the image. Similarly, vertical |
| 106 | mirroring leaves any partial iMCU row at the bottom edge untouched, but is |
| 107 | able to flip all columns. The other transforms can be built up as sequences |
| 108 | of transpose and flip operations; for consistency, their actions on edge |
| 109 | pixels are defined to be the same as the end result of the corresponding |
| 110 | transpose-and-flip sequence. |
| 111 | .PP |
| 112 | For practical use, you may prefer to discard any untransformable edge pixels |
| 113 | rather than having a strange-looking strip along the right and/or bottom edges |
| 114 | of a transformed image. To do this, add the |
| 115 | .B \-trim |
| 116 | switch: |
| 117 | .TP |
| 118 | .B \-trim |
| 119 | Drop non-transformable edge blocks. |
| 120 | .PP |
| 121 | Obviously, a transformation with |
| 122 | .B \-trim |
| 123 | is not reversible, so strictly speaking |
| 124 | .B jpegtran |
| 125 | with this switch is not lossless. Also, the expected mathematical |
| 126 | equivalences between the transformations no longer hold. For example, |
| 127 | .B \-rot 270 -trim |
| 128 | trims only the bottom edge, but |
| 129 | .B \-rot 90 -trim |
| 130 | followed by |
| 131 | .B \-rot 180 -trim |
| 132 | trims both edges. |
| 133 | .PP |
| 134 | Another not-strictly-lossless transformation switch is: |
| 135 | .TP |
| 136 | .B \-grayscale |
| 137 | Force grayscale output. |
| 138 | .PP |
| 139 | This option discards the chrominance channels if the input image is YCbCr |
| 140 | (ie, a standard color JPEG), resulting in a grayscale JPEG file. The |
| 141 | luminance channel is preserved exactly, so this is a better method of reducing |
| 142 | to grayscale than decompression, conversion, and recompression. This switch |
| 143 | is particularly handy for fixing a monochrome picture that was mistakenly |
| 144 | encoded as a color JPEG. (In such a case, the space savings from getting rid |
| 145 | of the near-empty chroma channels won't be large; but the decoding time for |
| 146 | a grayscale JPEG is substantially less than that for a color JPEG.) |
| 147 | .PP |
| 148 | .B jpegtran |
| 149 | also recognizes these switches that control what to do with "extra" markers, |
| 150 | such as comment blocks: |
| 151 | .TP |
| 152 | .B \-copy none |
| 153 | Copy no extra markers from source file. This setting suppresses all |
| 154 | comments and other excess baggage present in the source file. |
| 155 | .TP |
| 156 | .B \-copy comments |
| 157 | Copy only comment markers. This setting copies comments from the source file, |
| 158 | but discards any other inessential data. |
| 159 | .TP |
| 160 | .B \-copy all |
| 161 | Copy all extra markers. This setting preserves miscellaneous markers |
| 162 | found in the source file, such as JFIF thumbnails and Photoshop settings. |
| 163 | In some files these extra markers can be sizable. |
| 164 | .PP |
| 165 | The default behavior is |
| 166 | .BR "\-copy comments" . |
| 167 | (Note: in IJG releases v6 and v6a, |
| 168 | .B jpegtran |
| 169 | always did the equivalent of |
| 170 | .BR "\-copy none" .) |
| 171 | .PP |
| 172 | Additional switches recognized by jpegtran are: |
| 173 | .TP |
| 174 | .BI \-maxmemory " N" |
| 175 | Set limit for amount of memory to use in processing large images. Value is |
| 176 | in thousands of bytes, or millions of bytes if "M" is attached to the |
| 177 | number. For example, |
| 178 | .B \-max 4m |
| 179 | selects 4000000 bytes. If more space is needed, temporary files will be used. |
| 180 | .TP |
| 181 | .BI \-outfile " name" |
| 182 | Send output image to the named file, not to standard output. |
| 183 | .TP |
| 184 | .B \-verbose |
| 185 | Enable debug printout. More |
| 186 | .BR \-v 's |
| 187 | give more output. Also, version information is printed at startup. |
| 188 | .TP |
| 189 | .B \-debug |
| 190 | Same as |
| 191 | .BR \-verbose . |
| 192 | .SH EXAMPLES |
| 193 | .LP |
| 194 | This example converts a baseline JPEG file to progressive form: |
| 195 | .IP |
| 196 | .B jpegtran \-progressive |
| 197 | .I foo.jpg |
| 198 | .B > |
| 199 | .I fooprog.jpg |
| 200 | .PP |
| 201 | This example rotates an image 90 degrees clockwise, discarding any |
| 202 | unrotatable edge pixels: |
| 203 | .IP |
| 204 | .B jpegtran \-rot 90 -trim |
| 205 | .I foo.jpg |
| 206 | .B > |
| 207 | .I foo90.jpg |
| 208 | .SH ENVIRONMENT |
| 209 | .TP |
| 210 | .B JPEGMEM |
| 211 | If this environment variable is set, its value is the default memory limit. |
| 212 | The value is specified as described for the |
| 213 | .B \-maxmemory |
| 214 | switch. |
| 215 | .B JPEGMEM |
| 216 | overrides the default value specified when the program was compiled, and |
| 217 | itself is overridden by an explicit |
| 218 | .BR \-maxmemory . |
| 219 | .SH SEE ALSO |
| 220 | .BR cjpeg (1), |
| 221 | .BR djpeg (1), |
| 222 | .BR rdjpgcom (1), |
| 223 | .BR wrjpgcom (1) |
| 224 | .br |
| 225 | Wallace, Gregory K. "The JPEG Still Picture Compression Standard", |
| 226 | Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44. |
| 227 | .SH AUTHOR |
| 228 | Independent JPEG Group |
| 229 | .SH BUGS |
| 230 | Arithmetic coding is not supported for legal reasons. |
| 231 | .PP |
| 232 | The transform options can't transform odd-size images perfectly. Use |
| 233 | .B \-trim |
| 234 | if you don't like the results without it. |
| 235 | .PP |
| 236 | The entire image is read into memory and then written out again, even in |
| 237 | cases where this isn't really necessary. Expect swapping on large images, |
| 238 | especially when using the more complex transform options. |