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1USAGE instructions for the Independent JPEG Group's JPEG software 2================================================================= 3 4This file describes usage of the JPEG conversion programs cjpeg and djpeg, 5as well as the utility programs jpegtran, rdjpgcom and wrjpgcom. (See 6the other documentation files if you wish to use the JPEG library within 7your own programs.) 8 9If you are on a Unix machine you may prefer to read the Unix-style manual 10pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1. 11 12 13INTRODUCTION 14 15These programs implement JPEG image encoding, decoding, and transcoding. 16JPEG (pronounced "jay-peg") is a standardized compression method for 17full-color and gray-scale images. 18 19 20GENERAL USAGE 21 22We provide two programs, cjpeg to compress an image file into JPEG format, 23and djpeg to decompress a JPEG file back into a conventional image format. 24 25On Unix-like systems, you say: 26 cjpeg [switches] [imagefile] >jpegfile 27or 28 djpeg [switches] [jpegfile] >imagefile 29The programs read the specified input file, or standard input if none is 30named. They always write to standard output (with trace/error messages to 31standard error). These conventions are handy for piping images between 32programs. 33 34On most non-Unix systems, you say: 35 cjpeg [switches] imagefile jpegfile 36or 37 djpeg [switches] jpegfile imagefile 38i.e., both the input and output files are named on the command line. This 39style is a little more foolproof, and it loses no functionality if you don't 40have pipes. (You can get this style on Unix too, if you prefer, by defining 41TWO_FILE_COMMANDLINE when you compile the programs; see install.txt.) 42 43You can also say: 44 cjpeg [switches] -outfile jpegfile imagefile 45or 46 djpeg [switches] -outfile imagefile jpegfile 47This syntax works on all systems, so it is useful for scripts. 48 49The currently supported image file formats are: PPM (PBMPLUS color format), 50PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster Toolkit 51format). (RLE is supported only if the URT library is available.) 52cjpeg recognizes the input image format automatically, with the exception 53of some Targa-format files. You have to tell djpeg which format to generate. 54 55JPEG files are in the defacto standard JFIF file format. There are other, 56less widely used JPEG-based file formats, but we don't support them. 57 58All switch names may be abbreviated; for example, -grayscale may be written 59-gray or -gr. Most of the "basic" switches can be abbreviated to as little as 60one letter. Upper and lower case are equivalent (-BMP is the same as -bmp). 61British spellings are also accepted (e.g., -greyscale), though for brevity 62these are not mentioned below. 63 64 65CJPEG DETAILS 66 67The basic command line switches for cjpeg are: 68 69 -quality N[,...] Scale quantization tables to adjust image quality. 70 Quality is 0 (worst) to 100 (best); default is 75. 71 (See below for more info.) 72 73 -grayscale Create monochrome JPEG file from color input. 74 Be sure to use this switch when compressing a grayscale 75 BMP file, because cjpeg isn't bright enough to notice 76 whether a BMP file uses only shades of gray. By 77 saying -grayscale, you'll get a smaller JPEG file that 78 takes less time to process. 79 80 -rgb Create RGB JPEG file. 81 Using this switch suppresses the conversion from RGB 82 colorspace input to the default YCbCr JPEG colorspace. 83 Use this switch in combination with the -block N 84 switch (see below) for lossless JPEG coding. 85 86 -optimize Perform optimization of entropy encoding parameters. 87 Without this, default encoding parameters are used. 88 -optimize usually makes the JPEG file a little smaller, 89 but cjpeg runs somewhat slower and needs much more 90 memory. Image quality and speed of decompression are 91 unaffected by -optimize. 92 93 -progressive Create progressive JPEG file (see below). 94 95 -scale M/N Scale the output image by a factor M/N. Currently 96 supported scale factors are M/N with all N from 1 to 97 16, where M is the destination DCT size, which is 8 by 98 default (see -block N switch below). 99 100 -targa Input file is Targa format. Targa files that contain 101 an "identification" field will not be automatically 102 recognized by cjpeg; for such files you must specify 103 -targa to make cjpeg treat the input as Targa format. 104 For most Targa files, you won't need this switch. 105 106The -quality switch lets you trade off compressed file size against quality of 107the reconstructed image: the higher the quality setting, the larger the JPEG 108file, and the closer the output image will be to the original input. Normally 109you want to use the lowest quality setting (smallest file) that decompresses 110into something visually indistinguishable from the original image. For this 111purpose the quality setting should be between 50 and 95; the default of 75 is 112often about right. If you see defects at -quality 75, then go up 5 or 10 113counts at a time until you are happy with the output image. (The optimal 114setting will vary from one image to another.) 115 116-quality 100 will generate a quantization table of all 1's, minimizing loss 117in the quantization step (but there is still information loss in subsampling, 118as well as roundoff error). This setting is mainly of interest for 119experimental purposes. Quality values above about 95 are NOT recommended for 120normal use; the compressed file size goes up dramatically for hardly any gain 121in output image quality. 122 123In the other direction, quality values below 50 will produce very small files 124of low image quality. Settings around 5 to 10 might be useful in preparing an 125index of a large image library, for example. Try -quality 2 (or so) for some 126amusing Cubist effects. (Note: quality values below about 25 generate 2-byte 127quantization tables, which are considered optional in the JPEG standard. 128cjpeg emits a warning message when you give such a quality value, because some 129other JPEG programs may be unable to decode the resulting file. Use -baseline 130if you need to ensure compatibility at low quality values.) 131 132The -quality option has been extended in IJG version 7 for support of separate 133quality settings for luminance and chrominance (or in general, for every 134provided quantization table slot). This feature is useful for high-quality 135applications which cannot accept the damage of color data by coarse 136subsampling settings. You can now easily reduce the color data amount more 137smoothly with finer control without separate subsampling. The resulting file 138is fully compliant with standard JPEG decoders. 139Note that the -quality ratings refer to the quantization table slots, and that 140the last value is replicated if there are more q-table slots than parameters. 141The default q-table slots are 0 for luminance and 1 for chrominance with 142default tables as given in the JPEG standard. This is compatible with the old 143behaviour in case that only one parameter is given, which is then used for 144both luminance and chrominance (slots 0 and 1). More or custom quantization 145tables can be set with -qtables and assigned to components with -qslots 146parameter (see the "wizard" switches below). 147CAUTION: You must explicitly add -sample 1x1 for efficient separate color 148quality selection, since the default value used by library is 2x2! 149 150The -progressive switch creates a "progressive JPEG" file. In this type of 151JPEG file, the data is stored in multiple scans of increasing quality. If the 152file is being transmitted over a slow communications link, the decoder can use 153the first scan to display a low-quality image very quickly, and can then 154improve the display with each subsequent scan. The final image is exactly 155equivalent to a standard JPEG file of the same quality setting, and the total 156file size is about the same --- often a little smaller. 157 158Switches for advanced users: 159 160 -arithmetic Use arithmetic coding. CAUTION: arithmetic coded JPEG 161 is not yet widely implemented, so many decoders will 162 be unable to view an arithmetic coded JPEG file at 163 all. 164 165 -block N Set DCT block size. All N from 1 to 16 are possible. 166 Default is 8 (baseline format). 167 Larger values produce higher compression, 168 smaller values produce higher quality 169 (exact DCT stage possible with 1 or 2; with the 170 default quality of 75 and default Luminance qtable 171 the DCT+Quantization stage is lossless for N=1). 172 CAUTION: An implementation of the JPEG SmartScale 173 extension is required for this feature. SmartScale 174 enabled JPEG is not yet widely implemented, so many 175 decoders will be unable to view a SmartScale extended 176 JPEG file at all. 177 178 -dct int Use integer DCT method (default). 179 -dct fast Use fast integer DCT (less accurate). 180 -dct float Use floating-point DCT method. 181 The float method is very slightly more accurate than 182 the int method, but is much slower unless your machine 183 has very fast floating-point hardware. Also note that 184 results of the floating-point method may vary slightly 185 across machines, while the integer methods should give 186 the same results everywhere. The fast integer method 187 is much less accurate than the other two. 188 189 -nosmooth Don't use high-quality downsampling. 190 191 -restart N Emit a JPEG restart marker every N MCU rows, or every 192 N MCU blocks if "B" is attached to the number. 193 -restart 0 (the default) means no restart markers. 194 195 -smooth N Smooth the input image to eliminate dithering noise. 196 N, ranging from 1 to 100, indicates the strength of 197 smoothing. 0 (the default) means no smoothing. 198 199 -maxmemory N Set limit for amount of memory to use in processing 200 large images. Value is in thousands of bytes, or 201 millions of bytes if "M" is attached to the number. 202 For example, -max 4m selects 4000000 bytes. If more 203 space is needed, temporary files will be used. 204 205 -verbose Enable debug printout. More -v's give more printout. 206 or -debug Also, version information is printed at startup. 207 208The -restart option inserts extra markers that allow a JPEG decoder to 209resynchronize after a transmission error. Without restart markers, any damage 210to a compressed file will usually ruin the image from the point of the error 211to the end of the image; with restart markers, the damage is usually confined 212to the portion of the image up to the next restart marker. Of course, the 213restart markers occupy extra space. We recommend -restart 1 for images that 214will be transmitted across unreliable networks such as Usenet. 215 216The -smooth option filters the input to eliminate fine-scale noise. This is 217often useful when converting dithered images to JPEG: a moderate smoothing 218factor of 10 to 50 gets rid of dithering patterns in the input file, resulting 219in a smaller JPEG file and a better-looking image. Too large a smoothing 220factor will visibly blur the image, however. 221 222Switches for wizards: 223 224 -baseline Force baseline-compatible quantization tables to be 225 generated. This clamps quantization values to 8 bits 226 even at low quality settings. (This switch is poorly 227 named, since it does not ensure that the output is 228 actually baseline JPEG. For example, you can use 229 -baseline and -progressive together.) 230 231 -qtables file Use the quantization tables given in the specified 232 text file. 233 234 -qslots N[,...] Select which quantization table to use for each color 235 component. 236 237 -sample HxV[,...] Set JPEG sampling factors for each color component. 238 239 -scans file Use the scan script given in the specified text file. 240 241The "wizard" switches are intended for experimentation with JPEG. If you 242don't know what you are doing, DON'T USE THEM. These switches are documented 243further in the file wizard.txt. 244 245 246DJPEG DETAILS 247 248The basic command line switches for djpeg are: 249 250 -colors N Reduce image to at most N colors. This reduces the 251 or -quantize N number of colors used in the output image, so that it 252 can be displayed on a colormapped display or stored in 253 a colormapped file format. For example, if you have 254 an 8-bit display, you'd need to reduce to 256 or fewer 255 colors. (-colors is the recommended name, -quantize 256 is provided only for backwards compatibility.) 257 258 -fast Select recommended processing options for fast, low 259 quality output. (The default options are chosen for 260 highest quality output.) Currently, this is equivalent 261 to "-dct fast -nosmooth -onepass -dither ordered". 262 263 -grayscale Force gray-scale output even if JPEG file is color. 264 Useful for viewing on monochrome displays; also, 265 djpeg runs noticeably faster in this mode. 266 267 -scale M/N Scale the output image by a factor M/N. Currently 268 supported scale factors are M/N with all M from 1 to 269 16, where N is the source DCT size, which is 8 for 270 baseline JPEG. If the /N part is omitted, then M 271 specifies the DCT scaled size to be applied on the 272 given input. For baseline JPEG this is equivalent to 273 M/8 scaling, since the source DCT size for baseline 274 JPEG is 8. Scaling is handy if the image is larger 275 than your screen; also, djpeg runs much faster when 276 scaling down the output. 277 278 -bmp Select BMP output format (Windows flavor). 8-bit 279 colormapped format is emitted if -colors or -grayscale 280 is specified, or if the JPEG file is gray-scale; 281 otherwise, 24-bit full-color format is emitted. 282 283 -gif Select GIF output format. Since GIF does not support 284 more than 256 colors, -colors 256 is assumed (unless 285 you specify a smaller number of colors). If you 286 specify -fast, the default number of colors is 216. 287 288 -os2 Select BMP output format (OS/2 1.x flavor). 8-bit 289 colormapped format is emitted if -colors or -grayscale 290 is specified, or if the JPEG file is gray-scale; 291 otherwise, 24-bit full-color format is emitted. 292 293 -pnm Select PBMPLUS (PPM/PGM) output format (this is the 294 default format). PGM is emitted if the JPEG file is 295 gray-scale or if -grayscale is specified; otherwise 296 PPM is emitted. 297 298 -rle Select RLE output format. (Requires URT library.) 299 300 -targa Select Targa output format. Gray-scale format is 301 emitted if the JPEG file is gray-scale or if 302 -grayscale is specified; otherwise, colormapped format 303 is emitted if -colors is specified; otherwise, 24-bit 304 full-color format is emitted. 305 306Switches for advanced users: 307 308 -dct int Use integer DCT method (default). 309 -dct fast Use fast integer DCT (less accurate). 310 -dct float Use floating-point DCT method. 311 The float method is very slightly more accurate than 312 the int method, but is much slower unless your machine 313 has very fast floating-point hardware. Also note that 314 results of the floating-point method may vary slightly 315 across machines, while the integer methods should give 316 the same results everywhere. The fast integer method 317 is much less accurate than the other two. 318 319 -dither fs Use Floyd-Steinberg dithering in color quantization. 320 -dither ordered Use ordered dithering in color quantization. 321 -dither none Do not use dithering in color quantization. 322 By default, Floyd-Steinberg dithering is applied when 323 quantizing colors; this is slow but usually produces 324 the best results. Ordered dither is a compromise 325 between speed and quality; no dithering is fast but 326 usually looks awful. Note that these switches have 327 no effect unless color quantization is being done. 328 Ordered dither is only available in -onepass mode. 329 330 -map FILE Quantize to the colors used in the specified image 331 file. This is useful for producing multiple files 332 with identical color maps, or for forcing a predefined 333 set of colors to be used. The FILE must be a GIF 334 or PPM file. This option overrides -colors and 335 -onepass. 336 337 -nosmooth Don't use high-quality upsampling. 338 339 -onepass Use one-pass instead of two-pass color quantization. 340 The one-pass method is faster and needs less memory, 341 but it produces a lower-quality image. -onepass is 342 ignored unless you also say -colors N. Also, 343 the one-pass method is always used for gray-scale 344 output (the two-pass method is no improvement then). 345 346 -maxmemory N Set limit for amount of memory to use in processing 347 large images. Value is in thousands of bytes, or 348 millions of bytes if "M" is attached to the number. 349 For example, -max 4m selects 4000000 bytes. If more 350 space is needed, temporary files will be used. 351 352 -verbose Enable debug printout. More -v's give more printout. 353 or -debug Also, version information is printed at startup. 354 355 356HINTS FOR CJPEG 357 358Color GIF files are not the ideal input for JPEG; JPEG is really intended for 359compressing full-color (24-bit) images. In particular, don't try to convert 360cartoons, line drawings, and other images that have only a few distinct 361colors. GIF works great on these, JPEG does not. If you want to convert a 362GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options 363to get a satisfactory conversion. -smooth 10 or so is often helpful. 364 365Avoid running an image through a series of JPEG compression/decompression 366cycles. Image quality loss will accumulate; after ten or so cycles the image 367may be noticeably worse than it was after one cycle. It's best to use a 368lossless format while manipulating an image, then convert to JPEG format when 369you are ready to file the image away. 370 371The -optimize option to cjpeg is worth using when you are making a "final" 372version for posting or archiving. It's also a win when you are using low 373quality settings to make very small JPEG files; the percentage improvement 374is often a lot more than it is on larger files. (At present, -optimize 375mode is always selected when generating progressive JPEG files.) 376 377GIF input files are no longer supported, to avoid the Unisys LZW patent. 378(Conversion of GIF files to JPEG is usually a bad idea anyway.) 379 380 381HINTS FOR DJPEG 382 383To get a quick preview of an image, use the -grayscale and/or -scale switches. 384"-grayscale -scale 1/8" is the fastest case. 385 386Several options are available that trade off image quality to gain speed. 387"-fast" turns on the recommended settings. 388 389"-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality. 390When producing a color-quantized image, "-onepass -dither ordered" is fast but 391much lower quality than the default behavior. "-dither none" may give 392acceptable results in two-pass mode, but is seldom tolerable in one-pass mode. 393 394If you are fortunate enough to have very fast floating point hardware, 395"-dct float" may be even faster than "-dct fast". But on most machines 396"-dct float" is slower than "-dct int"; in this case it is not worth using, 397because its theoretical accuracy advantage is too small to be significant 398in practice. 399 400Two-pass color quantization requires a good deal of memory; on MS-DOS machines 401it may run out of memory even with -maxmemory 0. In that case you can still 402decompress, with some loss of image quality, by specifying -onepass for 403one-pass quantization. 404 405To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files. These 406are larger than they should be, but are readable by standard GIF decoders. 407 408 409HINTS FOR BOTH PROGRAMS 410 411If more space is needed than will fit in the available main memory (as 412determined by -maxmemory), temporary files will be used. (MS-DOS versions 413will try to get extended or expanded memory first.) The temporary files are 414often rather large: in typical cases they occupy three bytes per pixel, for 415example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough 416free disk space, leave out -progressive and -optimize (for cjpeg) or specify 417-onepass (for djpeg). 418 419On MS-DOS, the temporary files are created in the directory named by the TMP 420or TEMP environment variable, or in the current directory if neither of those 421exist. Amiga implementations put the temp files in the directory named by 422JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free 423space. 424 425The default memory usage limit (-maxmemory) is set when the software is 426compiled. If you get an "insufficient memory" error, try specifying a smaller 427-maxmemory value, even -maxmemory 0 to use the absolute minimum space. You 428may want to recompile with a smaller default value if this happens often. 429 430On machines that have "environment" variables, you can define the environment 431variable JPEGMEM to set the default memory limit. The value is specified as 432described for the -maxmemory switch. JPEGMEM overrides the default value 433specified when the program was compiled, and itself is overridden by an 434explicit -maxmemory switch. 435 436On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to 437use. (Extended or expanded memory is also used if available.) Most 438DOS-specific versions of this software do their own memory space estimation 439and do not need you to specify -maxmemory. 440 441 442JPEGTRAN 443 444jpegtran performs various useful transformations of JPEG files. 445It can translate the coded representation from one variant of JPEG to another, 446for example from baseline JPEG to progressive JPEG or vice versa. It can also 447perform some rearrangements of the image data, for example turning an image 448from landscape to portrait format by rotation. 449 450jpegtran works by rearranging the compressed data (DCT coefficients), without 451ever fully decoding the image. Therefore, its transformations are lossless: 452there is no image degradation at all, which would not be true if you used 453djpeg followed by cjpeg to accomplish the same conversion. But by the same 454token, jpegtran cannot perform lossy operations such as changing the image 455quality. 456 457jpegtran uses a command line syntax similar to cjpeg or djpeg. 458On Unix-like systems, you say: 459 jpegtran [switches] [inputfile] >outputfile 460On most non-Unix systems, you say: 461 jpegtran [switches] inputfile outputfile 462where both the input and output files are JPEG files. 463 464To specify the coded JPEG representation used in the output file, 465jpegtran accepts a subset of the switches recognized by cjpeg: 466 -optimize Perform optimization of entropy encoding parameters. 467 -progressive Create progressive JPEG file. 468 -arithmetic Use arithmetic coding. 469 -restart N Emit a JPEG restart marker every N MCU rows, or every 470 N MCU blocks if "B" is attached to the number. 471 -scans file Use the scan script given in the specified text file. 472See the previous discussion of cjpeg for more details about these switches. 473If you specify none of these switches, you get a plain baseline-JPEG output 474file. The quality setting and so forth are determined by the input file. 475 476The image can be losslessly transformed by giving one of these switches: 477 -flip horizontal Mirror image horizontally (left-right). 478 -flip vertical Mirror image vertically (top-bottom). 479 -rotate 90 Rotate image 90 degrees clockwise. 480 -rotate 180 Rotate image 180 degrees. 481 -rotate 270 Rotate image 270 degrees clockwise (or 90 ccw). 482 -transpose Transpose image (across UL-to-LR axis). 483 -transverse Transverse transpose (across UR-to-LL axis). 484 485The transpose transformation has no restrictions regarding image dimensions. 486The other transformations operate rather oddly if the image dimensions are not 487a multiple of the iMCU size (usually 8 or 16 pixels), because they can only 488transform complete blocks of DCT coefficient data in the desired way. 489 490jpegtran's default behavior when transforming an odd-size image is designed 491to preserve exact reversibility and mathematical consistency of the 492transformation set. As stated, transpose is able to flip the entire image 493area. Horizontal mirroring leaves any partial iMCU column at the right edge 494untouched, but is able to flip all rows of the image. Similarly, vertical 495mirroring leaves any partial iMCU row at the bottom edge untouched, but is 496able to flip all columns. The other transforms can be built up as sequences 497of transpose and flip operations; for consistency, their actions on edge 498pixels are defined to be the same as the end result of the corresponding 499transpose-and-flip sequence. 500 501For practical use, you may prefer to discard any untransformable edge pixels 502rather than having a strange-looking strip along the right and/or bottom edges 503of a transformed image. To do this, add the -trim switch: 504 -trim Drop non-transformable edge blocks. 505Obviously, a transformation with -trim is not reversible, so strictly speaking 506jpegtran with this switch is not lossless. Also, the expected mathematical 507equivalences between the transformations no longer hold. For example, 508"-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by 509"-rot 180 -trim" trims both edges. 510 511If you are only interested in perfect transformation, add the -perfect switch: 512 -perfect Fails with an error if the transformation is not 513 perfect. 514For example you may want to do 515 jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg 516to do a perfect rotation if available or an approximated one if not. 517 518We also offer a lossless-crop option, which discards data outside a given 519image region but losslessly preserves what is inside. Like the rotate and 520flip transforms, lossless crop is restricted by the current JPEG format: the 521upper left corner of the selected region must fall on an iMCU boundary. If 522this does not hold for the given crop parameters, we silently move the upper 523left corner up and/or left to make it so, simultaneously increasing the region 524dimensions to keep the lower right crop corner unchanged. (Thus, the output 525image covers at least the requested region, but may cover more.) 526 527The image can be losslessly cropped by giving the switch: 528 -crop WxH+X+Y Crop to a rectangular subarea of width W, height H 529 starting at point X,Y. 530 531Other not-strictly-lossless transformation switches are: 532 533 -grayscale Force grayscale output. 534This option discards the chrominance channels if the input image is YCbCr 535(ie, a standard color JPEG), resulting in a grayscale JPEG file. The 536luminance channel is preserved exactly, so this is a better method of reducing 537to grayscale than decompression, conversion, and recompression. This switch 538is particularly handy for fixing a monochrome picture that was mistakenly 539encoded as a color JPEG. (In such a case, the space savings from getting rid 540of the near-empty chroma channels won't be large; but the decoding time for 541a grayscale JPEG is substantially less than that for a color JPEG.) 542 543 -scale M/N Scale the output image by a factor M/N. 544Currently supported scale factors are M/N with all M from 1 to 16, where N is 545the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted, 546then M specifies the DCT scaled size to be applied on the given input. For 547baseline JPEG this is equivalent to M/8 scaling, since the source DCT size 548for baseline JPEG is 8. CAUTION: An implementation of the JPEG SmartScale 549extension is required for this feature. SmartScale enabled JPEG is not yet 550widely implemented, so many decoders will be unable to view a SmartScale 551extended JPEG file at all. 552 553jpegtran also recognizes these switches that control what to do with "extra" 554markers, such as comment blocks: 555 -copy none Copy no extra markers from source file. This setting 556 suppresses all comments and other excess baggage 557 present in the source file. 558 -copy comments Copy only comment markers. This setting copies 559 comments from the source file, but discards 560 any other inessential (for image display) data. 561 -copy all Copy all extra markers. This setting preserves 562 miscellaneous markers found in the source file, such 563 as JFIF thumbnails, Exif data, and Photoshop settings. 564 In some files these extra markers can be sizable. 565The default behavior is -copy comments. (Note: in IJG releases v6 and v6a, 566jpegtran always did the equivalent of -copy none.) 567 568Additional switches recognized by jpegtran are: 569 -outfile filename 570 -maxmemory N 571 -verbose 572 -debug 573These work the same as in cjpeg or djpeg. 574 575 576THE COMMENT UTILITIES 577 578The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file. 579Although the standard doesn't actually define what COM blocks are for, they 580are widely used to hold user-supplied text strings. This lets you add 581annotations, titles, index terms, etc to your JPEG files, and later retrieve 582them as text. COM blocks do not interfere with the image stored in the JPEG 583file. The maximum size of a COM block is 64K, but you can have as many of 584them as you like in one JPEG file. 585 586We provide two utility programs to display COM block contents and add COM 587blocks to a JPEG file. 588 589rdjpgcom searches a JPEG file and prints the contents of any COM blocks on 590standard output. The command line syntax is 591 rdjpgcom [-raw] [-verbose] [inputfilename] 592The switch "-raw" (or just "-r") causes rdjpgcom to also output non-printable 593characters in comments, which are normally escaped for security reasons. 594The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG 595image dimensions. If you omit the input file name from the command line, 596the JPEG file is read from standard input. (This may not work on some 597operating systems, if binary data can't be read from stdin.) 598 599wrjpgcom adds a COM block, containing text you provide, to a JPEG file. 600Ordinarily, the COM block is added after any existing COM blocks, but you 601can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG 602file; it does not modify the input file. DO NOT try to overwrite the input 603file by directing wrjpgcom's output back into it; on most systems this will 604just destroy your file. 605 606The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like 607systems, it is 608 wrjpgcom [switches] [inputfilename] 609The output file is written to standard output. The input file comes from 610the named file, or from standard input if no input file is named. 611 612On most non-Unix systems, the syntax is 613 wrjpgcom [switches] inputfilename outputfilename 614where both input and output file names must be given explicitly. 615 616wrjpgcom understands three switches: 617 -replace Delete any existing COM blocks from the file. 618 -comment "Comment text" Supply new COM text on command line. 619 -cfile name Read text for new COM block from named file. 620(Switch names can be abbreviated.) If you have only one line of comment text 621to add, you can provide it on the command line with -comment. The comment 622text must be surrounded with quotes so that it is treated as a single 623argument. Longer comments can be read from a text file. 624 625If you give neither -comment nor -cfile, then wrjpgcom will read the comment 626text from standard input. (In this case an input image file name MUST be 627supplied, so that the source JPEG file comes from somewhere else.) You can 628enter multiple lines, up to 64KB worth. Type an end-of-file indicator 629(usually control-D or control-Z) to terminate the comment text entry. 630 631wrjpgcom will not add a COM block if the provided comment string is empty. 632Therefore -replace -comment "" can be used to delete all COM blocks from a 633file. 634 635These utility programs do not depend on the IJG JPEG library. In 636particular, the source code for rdjpgcom is intended as an illustration of 637the minimum amount of code required to parse a JPEG file header correctly.