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