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 rdjpgcom and wrjpgcom. (See the other 6documentation files if you wish to use the JPEG library within your own 7programs.) 8 9If you are on a Unix machine you may prefer to read the Unix-style manual 10pages in files cjpeg.1, djpeg.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, GIF, 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 (-GIF is the same as -gif). 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 GIF file, because cjpeg isn't bright enough to notice 86 whether a GIF 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 -targa Input file is Targa format. Targa files that contain 98 an "identification" field will not be automatically 99 recognized by cjpeg; for such files you must specify 100 -targa to make cjpeg treat the input as Targa format. 101 For most Targa files, you won't need this switch. 102 103The -quality switch lets you trade off compressed file size against quality of 104the reconstructed image: the higher the quality setting, the larger the JPEG 105file, and the closer the output image will be to the original input. Normally 106you want to use the lowest quality setting (smallest file) that decompresses 107into something visually indistinguishable from the original image. For this 108purpose the quality setting should be between 50 and 95; the default of 75 is 109often about right. If you see defects at -quality 75, then go up 5 or 10 110counts at a time until you are happy with the output image. (The optimal 111setting will vary from one image to another.) 112 113-quality 100 will generate a quantization table of all 1's, eliminating loss 114in the quantization step (but there is still information loss in subsampling, 115as well as roundoff error). This setting is mainly of interest for 116experimental purposes. Quality values above about 95 are NOT recommended for 117normal use; the compressed file size goes up dramatically for hardly any gain 118in output image quality. 119 120In the other direction, quality values below 50 will produce very small files 121of low image quality. Settings around 5 to 10 might be useful in preparing an 122index of a large image library, for example. Try -quality 2 (or so) for some 123amusing Cubist effects. (Note: quality values below about 25 generate 2-byte 124quantization tables, which are considered optional in the JPEG standard. 125cjpeg emits a warning message when you give such a quality value, because 126some commercial JPEG programs may be unable to decode the resulting file. 127Use -baseline if you need to ensure compatibility at low quality values.) 128 129Switches for advanced users: 130 131 -dct int Use integer DCT method (default). 132 -dct fast Use fast integer DCT (less accurate). 133 -dct float Use floating-point DCT method. 134 The floating-point method is the most accurate, but 135 will be the slowest unless your machine has very fast 136 floating-point hardware. Also note that results of 137 the floating-point method may vary slightly across 138 machines, while the integer methods should give the 139 same results everywhere. The fast integer method is 140 much less accurate than the other two. 141 142 -restart N Emit a JPEG restart marker every N MCU rows, or every 143 N MCU blocks if "B" is attached to the number. 144 -restart 0 (the default) means no restart markers. 145 146 -smooth N Smooth the input image to eliminate dithering noise. 147 N, ranging from 1 to 100, indicates the strength of 148 smoothing. 0 (the default) means no smoothing. 149 150 -maxmemory N Set limit for amount of memory to use in processing 151 large images. Value is in thousands of bytes, or 152 millions of bytes if "M" is attached to the number. 153 For example, -max 4m selects 4000000 bytes. If more 154 space is needed, temporary files will be used. 155 156 -verbose Enable debug printout. More -v's give more printout. 157 or -debug Also, version information is printed at startup. 158 159The -restart option inserts extra markers that allow a JPEG decoder to 160resynchronize after a transmission error. Without restart markers, any damage 161to a compressed file will usually ruin the image from the point of the error 162to the end of the image; with restart markers, the damage is usually confined 163to the portion of the image up to the next restart marker. Of course, the 164restart markers occupy extra space. We recommend -restart 1 for images that 165will be transmitted across unreliable networks such as Usenet. 166 167The -smooth option filters the input to eliminate fine-scale noise. This is 168often useful when converting GIF files to JPEG: a moderate smoothing factor of 16910 to 50 gets rid of dithering patterns in the input file, resulting in a 170smaller JPEG file and a better-looking image. Too large a smoothing factor 171will visibly blur the image, however. 172 173Switches for wizards: 174 175 -arithmetic Use arithmetic coding rather than Huffman coding. 176 (Not currently supported for legal reasons.) 177 178 -baseline Force a baseline JPEG file to be generated. This 179 clamps quantization values to 8 bits even at low 180 quality settings. 181 182 -nointerleave Generate noninterleaved JPEG file (not yet supported). 183 184 -qtables file Use the quantization tables given in the specified 185 file. The file should contain one to four tables 186 (64 values each) as plain text. Comments preceded by 187 '#' may be included in the file. The tables are 188 implicitly numbered 0,1,etc. If -quality N is also 189 specified, the values in the file are scaled according 190 to cjpeg's quality scaling curve. 191 192 -qslots N[,...] Select which quantization table to use for each color 193 component. By default, table 0 is used for luminance 194 and table 1 for chrominance components. 195 196 -sample HxV[,...] Set JPEG sampling factors. If you specify 197 fewer H/V pairs than there are components, the 198 remaining components are set to 1x1 sampling. The 199 default setting is equivalent to "-sample 2x2". 200 201The "wizard" switches are intended for experimentation with JPEG. If you 202don't know what you are doing, DON'T USE THEM. You can easily produce files 203with worse image quality and/or poorer compression than you'll get from the 204default settings. Furthermore, these switches should not be used when making 205files intended for general use, because not all JPEG implementations will 206support unusual JPEG parameter settings. 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 floating-point method is the most accurate, but 270 will be the slowest unless your machine has very fast 271 floating-point hardware. Also note that results of 272 the floating-point method may vary slightly across 273 machines, while the integer methods should give the 274 same results everywhere. The fast integer method is 275 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. 333 334 335HINTS FOR DJPEG 336 337To get a quick preview of an image, use the -grayscale and/or -scale switches. 338"-grayscale -scale 1/8" is the fastest case. 339 340Several options are available that trade off image quality to gain speed. 341"-fast" turns on the recommended settings. 342 343"-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality. 344When producing a color-quantized image, "-onepass -dither ordered" is fast but 345much lower quality than the default behavior. "-dither none" may give 346acceptable results in two-pass mode, but is seldom tolerable in one-pass mode. 347 348If you are fortunate enough to have very fast floating point hardware, 349"-dct float" may be even faster than "-dct fast". 350 351Two-pass color quantization requires a good deal of memory; on MS-DOS machines 352it may run out of memory even with -maxmemory 0. In that case you can still 353decompress, with some loss of image quality, by specifying -onepass for 354one-pass quantization. 355 356 357HINTS FOR BOTH PROGRAMS 358 359If more space is needed than will fit in the available main memory (as 360determined by -maxmemory), temporary files will be used. (MS-DOS versions 361will try to get extended or expanded memory first.) The temporary files are 362often rather large: in typical cases they occupy three bytes per pixel, for 363example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough 364free disk space, leave out -optimize (for cjpeg) or specify -onepass (for 365djpeg). 366 367On MS-DOS, the temporary files are created in the directory named by the TMP 368or TEMP environment variable, or in the current directory if neither of those 369exist. Amiga implementations put the temp files in the directory named by 370JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free 371space. 372 373The default memory usage limit (-maxmemory) is set when the software is 374compiled. If you get an "insufficient memory" error, try specifying a smaller 375-maxmemory value, even -maxmemory 0 to use the absolute minimum space. You 376may want to recompile with a smaller default value if this happens often. 377 378On machines that have "environment" variables, you can define the environment 379variable JPEGMEM to set the default memory limit. The value is specified as 380described for the -maxmemory switch. JPEGMEM overrides the default value 381specified when the program was compiled, and itself is overridden by an 382explicit -maxmemory switch. 383 384On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to 385use. (Extended or expanded memory is also used if available.) Most 386DOS-specific versions of this software do their own memory space estimation 387and do not need you to specify -maxmemory. 388 389 390THE COMMENT UTILITIES 391 392The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file. 393Although the standard doesn't actually define what COM blocks are for, they 394are widely used to hold user-supplied text strings. This lets you add 395annotations, titles, index terms, etc to your JPEG files, and later retrieve 396them as text. COM blocks do not interfere with the image stored in the JPEG 397file. The maximum size of a COM block is 64K, but you can have as many of 398them as you like in one JPEG file. 399 400We provide two utility programs to display COM block contents and add COM 401blocks to a JPEG file. 402 403rdjpgcom searches a JPEG file and prints the contents of any COM blocks on 404standard output. The command line syntax is 405 rdjpgcom [-verbose] [inputfilename] 406The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG 407image dimensions. If you omit the input file name from the command line, 408the JPEG file is read from standard input. (This may not work on some 409operating systems, if binary data can't be read from stdin.) 410 411wrjpgcom adds a COM block, containing text you provide, to a JPEG file. 412Ordinarily, the COM block is added after any existing COM blocks, but you 413can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG 414file; it does not modify the input file. DO NOT try to overwrite the input 415file by directing wrjpgcom's output back into it; on most systems this will 416just destroy your file. 417 418The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like 419systems, it is 420 wrjpgcom [switches] [inputfilename] 421The output file is written to standard output. The input file comes from 422the named file, or from standard input if no input file is named. 423 424On most non-Unix systems, the syntax is 425 wrjpgcom [switches] inputfilename outputfilename 426where both input and output file names must be given explicitly. 427 428wrjpgcom understands three switches: 429 -replace Delete any existing COM blocks from the file. 430 -comment "Comment text" Supply new COM text on command line. 431 -cfile name Read text for new COM block from named file. 432(Switch names can be abbreviated.) If you have only one line of comment text 433to add, you can provide it on the command line with -comment. The comment 434text must be surrounded with quotes so that it is treated as a single 435argument. Longer comments can be read from a text file. 436 437If you give neither -comment nor -cfile, then wrjpgcom will read the comment 438text from standard input. (In this case an input image file name MUST be 439supplied, so that the source JPEG file comes from somewhere else.) You can 440enter multiple lines, up to 64KB worth. Type an end-of-file indicator 441(usually control-D or control-Z) to terminate the comment text entry. 442 443wrjpgcom will not add a COM block if the provided comment string is empty. 444Therefore -replace -comment "" can be used to delete all COM blocks from a 445file. 446 447These utility programs do not depend on the IJG JPEG library. In 448particular, the source code for rdjpgcom is intended as an illustration of 449the minimum amount of code required to parse a JPEG file header correctly. 450