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      1 USAGE instructions for the Independent JPEG Group's JPEG software
      2 =================================================================
      3 
      4 This file describes usage of the JPEG conversion programs cjpeg and djpeg,
      5 as well as the utility programs jpegtran, rdjpgcom and wrjpgcom.  (See
      6 the other documentation files if you wish to use the JPEG library within
      7 your own programs.)
      8 
      9 If you are on a Unix machine you may prefer to read the Unix-style manual
     10 pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1.
     11 
     12 
     13 INTRODUCTION
     14 
     15 These programs implement JPEG image compression and decompression.  JPEG
     16 (pronounced "jay-peg") is a standardized compression method for full-color
     17 and gray-scale images.  JPEG is designed to handle "real-world" scenes,
     18 for example scanned photographs.  Cartoons, line drawings, and other
     19 non-realistic images are not JPEG's strong suit; on that sort of material
     20 you may get poor image quality and/or little compression.
     21 
     22 JPEG is lossy, meaning that the output image is not necessarily identical to
     23 the input image.  Hence you should not use JPEG if you have to have identical
     24 output bits.  However, on typical real-world images, very good compression
     25 levels can be obtained with no visible change, and amazingly high compression
     26 is possible if you can tolerate a low-quality image.  You can trade off image
     27 quality against file size by adjusting the compressor's "quality" setting.
     28 
     29 
     30 GENERAL USAGE
     31 
     32 We provide two programs, cjpeg to compress an image file into JPEG format,
     33 and djpeg to decompress a JPEG file back into a conventional image format.
     34 
     35 On Unix-like systems, you say:
     36 	cjpeg [switches] [imagefile] >jpegfile
     37 or
     38 	djpeg [switches] [jpegfile]  >imagefile
     39 The programs read the specified input file, or standard input if none is
     40 named.  They always write to standard output (with trace/error messages to
     41 standard error).  These conventions are handy for piping images between
     42 programs.
     43 
     44 On most non-Unix systems, you say:
     45 	cjpeg [switches] imagefile jpegfile
     46 or
     47 	djpeg [switches] jpegfile  imagefile
     48 i.e., both the input and output files are named on the command line.  This
     49 style is a little more foolproof, and it loses no functionality if you don't
     50 have pipes.  (You can get this style on Unix too, if you prefer, by defining
     51 TWO_FILE_COMMANDLINE when you compile the programs; see install.doc.)
     52 
     53 You can also say:
     54 	cjpeg [switches] -outfile jpegfile  imagefile
     55 or
     56 	djpeg [switches] -outfile imagefile  jpegfile
     57 This syntax works on all systems, so it is useful for scripts.
     58 
     59 The currently supported image file formats are: PPM (PBMPLUS color format),
     60 PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster Toolkit
     61 format).  (RLE is supported only if the URT library is available.)
     62 cjpeg recognizes the input image format automatically, with the exception
     63 of some Targa-format files.  You have to tell djpeg which format to generate.
     64 
     65 JPEG files are in the defacto standard JFIF file format.  There are other,
     66 less widely used JPEG-based file formats, but we don't support them.
     67 
     68 All 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
     70 one letter.  Upper and lower case are equivalent (-BMP is the same as -bmp).
     71 British spellings are also accepted (e.g., -greyscale), though for brevity
     72 these are not mentioned below.
     73 
     74 
     75 CJPEG DETAILS
     76 
     77 The 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 
    105 The -quality switch lets you trade off compressed file size against quality of
    106 the reconstructed image: the higher the quality setting, the larger the JPEG
    107 file, and the closer the output image will be to the original input.  Normally
    108 you want to use the lowest quality setting (smallest file) that decompresses
    109 into something visually indistinguishable from the original image.  For this
    110 purpose the quality setting should be between 50 and 95; the default of 75 is
    111 often about right.  If you see defects at -quality 75, then go up 5 or 10
    112 counts at a time until you are happy with the output image.  (The optimal
    113 setting will vary from one image to another.)
    114 
    115 -quality 100 will generate a quantization table of all 1's, minimizing loss
    116 in the quantization step (but there is still information loss in subsampling,
    117 as well as roundoff error).  This setting is mainly of interest for
    118 experimental purposes.  Quality values above about 95 are NOT recommended for
    119 normal use; the compressed file size goes up dramatically for hardly any gain
    120 in output image quality.
    121 
    122 In the other direction, quality values below 50 will produce very small files
    123 of low image quality.  Settings around 5 to 10 might be useful in preparing an
    124 index of a large image library, for example.  Try -quality 2 (or so) for some
    125 amusing Cubist effects.  (Note: quality values below about 25 generate 2-byte
    126 quantization tables, which are considered optional in the JPEG standard.
    127 cjpeg emits a warning message when you give such a quality value, because some
    128 other JPEG programs may be unable to decode the resulting file.  Use -baseline
    129 if you need to ensure compatibility at low quality values.)
    130 
    131 The -progressive switch creates a "progressive JPEG" file.  In this type of
    132 JPEG file, the data is stored in multiple scans of increasing quality.  If the
    133 file is being transmitted over a slow communications link, the decoder can use
    134 the first scan to display a low-quality image very quickly, and can then
    135 improve the display with each subsequent scan.  The final image is exactly
    136 equivalent to a standard JPEG file of the same quality setting, and the total
    137 file size is about the same --- often a little smaller.  CAUTION: progressive
    138 JPEG is not yet widely implemented, so many decoders will be unable to view a
    139 progressive JPEG file at all.
    140 
    141 Switches 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 
    171 The -restart option inserts extra markers that allow a JPEG decoder to
    172 resynchronize after a transmission error.  Without restart markers, any damage
    173 to a compressed file will usually ruin the image from the point of the error
    174 to the end of the image; with restart markers, the damage is usually confined
    175 to the portion of the image up to the next restart marker.  Of course, the
    176 restart markers occupy extra space.  We recommend -restart 1 for images that
    177 will be transmitted across unreliable networks such as Usenet.
    178 
    179 The -smooth option filters the input to eliminate fine-scale noise.  This is
    180 often useful when converting dithered images to JPEG: a moderate smoothing
    181 factor of 10 to 50 gets rid of dithering patterns in the input file, resulting
    182 in a smaller JPEG file and a better-looking image.  Too large a smoothing
    183 factor will visibly blur the image, however.
    184 
    185 Switches 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 
    204 The "wizard" switches are intended for experimentation with JPEG.  If you
    205 don't know what you are doing, DON'T USE THEM.  These switches are documented
    206 further in the file wizard.doc.
    207 
    208 
    209 DJPEG DETAILS
    210 
    211 The 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 
    264 Switches 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 
    314 HINTS FOR CJPEG
    315 
    316 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
    317 compressing full-color (24-bit) images.  In particular, don't try to convert
    318 cartoons, line drawings, and other images that have only a few distinct
    319 colors.  GIF works great on these, JPEG does not.  If you want to convert a
    320 GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
    321 to get a satisfactory conversion.  -smooth 10 or so is often helpful.
    322 
    323 Avoid running an image through a series of JPEG compression/decompression
    324 cycles.  Image quality loss will accumulate; after ten or so cycles the image
    325 may be noticeably worse than it was after one cycle.  It's best to use a
    326 lossless format while manipulating an image, then convert to JPEG format when
    327 you are ready to file the image away.
    328 
    329 The -optimize option to cjpeg is worth using when you are making a "final"
    330 version for posting or archiving.  It's also a win when you are using low
    331 quality settings to make very small JPEG files; the percentage improvement
    332 is often a lot more than it is on larger files.  (At present, -optimize
    333 mode is always selected when generating progressive JPEG files.)
    334 
    335 GIF input files are no longer supported, to avoid the Unisys LZW patent.
    336 Use a Unisys-licensed program if you need to read a GIF file.  (Conversion
    337 of GIF files to JPEG is usually a bad idea anyway.)
    338 
    339 
    340 HINTS FOR DJPEG
    341 
    342 To get a quick preview of an image, use the -grayscale and/or -scale switches.
    343 "-grayscale -scale 1/8" is the fastest case.
    344 
    345 Several 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.
    349 When producing a color-quantized image, "-onepass -dither ordered" is fast but
    350 much lower quality than the default behavior.  "-dither none" may give
    351 acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
    352 
    353 If 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,
    356 because its theoretical accuracy advantage is too small to be significant
    357 in practice.
    358 
    359 Two-pass color quantization requires a good deal of memory; on MS-DOS machines
    360 it may run out of memory even with -maxmemory 0.  In that case you can still
    361 decompress, with some loss of image quality, by specifying -onepass for
    362 one-pass quantization.
    363 
    364 To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files.  These
    365 are larger than they should be, but are readable by standard GIF decoders.
    366 
    367 
    368 HINTS FOR BOTH PROGRAMS
    369 
    370 If more space is needed than will fit in the available main memory (as
    371 determined by -maxmemory), temporary files will be used.  (MS-DOS versions
    372 will try to get extended or expanded memory first.)  The temporary files are
    373 often rather large: in typical cases they occupy three bytes per pixel, for
    374 example 3*800*600 = 1.44Mb for an 800x600 image.  If you don't have enough
    375 free disk space, leave out -progressive and -optimize (for cjpeg) or specify
    376 -onepass (for djpeg).
    377 
    378 On MS-DOS, the temporary files are created in the directory named by the TMP
    379 or TEMP environment variable, or in the current directory if neither of those
    380 exist.  Amiga implementations put the temp files in the directory named by
    381 JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
    382 space.
    383 
    384 The default memory usage limit (-maxmemory) is set when the software is
    385 compiled.  If you get an "insufficient memory" error, try specifying a smaller
    386 -maxmemory value, even -maxmemory 0 to use the absolute minimum space.  You
    387 may want to recompile with a smaller default value if this happens often.
    388 
    389 On machines that have "environment" variables, you can define the environment
    390 variable JPEGMEM to set the default memory limit.  The value is specified as
    391 described for the -maxmemory switch.  JPEGMEM overrides the default value
    392 specified when the program was compiled, and itself is overridden by an
    393 explicit -maxmemory switch.
    394 
    395 On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
    396 use.  (Extended or expanded memory is also used if available.)  Most
    397 DOS-specific versions of this software do their own memory space estimation
    398 and do not need you to specify -maxmemory.
    399 
    400 
    401 JPEGTRAN
    402 
    403 jpegtran performs various useful transformations of JPEG files.
    404 It can translate the coded representation from one variant of JPEG to another,
    405 for example from baseline JPEG to progressive JPEG or vice versa.  It can also
    406 perform some rearrangements of the image data, for example turning an image
    407 from landscape to portrait format by rotation.
    408 
    409 jpegtran works by rearranging the compressed data (DCT coefficients), without
    410 ever fully decoding the image.  Therefore, its transformations are lossless:
    411 there is no image degradation at all, which would not be true if you used
    412 djpeg followed by cjpeg to accomplish the same conversion.  But by the same
    413 token, jpegtran cannot perform lossy operations such as changing the image
    414 quality.
    415 
    416 jpegtran uses a command line syntax similar to cjpeg or djpeg.
    417 On Unix-like systems, you say:
    418 	jpegtran [switches] [inputfile] >outputfile
    419 On most non-Unix systems, you say:
    420 	jpegtran [switches] inputfile outputfile
    421 where both the input and output files are JPEG files.
    422 
    423 To specify the coded JPEG representation used in the output file,
    424 jpegtran 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.
    430 See the previous discussion of cjpeg for more details about these switches.
    431 If you specify none of these switches, you get a plain baseline-JPEG output
    432 file.  The quality setting and so forth are determined by the input file.
    433 
    434 The 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 
    443 The transpose transformation has no restrictions regarding image dimensions.
    444 The other transformations operate rather oddly if the image dimensions are not
    445 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
    446 transform complete blocks of DCT coefficient data in the desired way.
    447 
    448 jpegtran's default behavior when transforming an odd-size image is designed
    449 to preserve exact reversibility and mathematical consistency of the
    450 transformation set.  As stated, transpose is able to flip the entire image
    451 area.  Horizontal mirroring leaves any partial iMCU column at the right edge
    452 untouched, but is able to flip all rows of the image.  Similarly, vertical
    453 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
    454 able to flip all columns.  The other transforms can be built up as sequences
    455 of transpose and flip operations; for consistency, their actions on edge
    456 pixels are defined to be the same as the end result of the corresponding
    457 transpose-and-flip sequence.
    458 
    459 For practical use, you may prefer to discard any untransformable edge pixels
    460 rather than having a strange-looking strip along the right and/or bottom edges
    461 of a transformed image.  To do this, add the -trim switch:
    462 	-trim		Drop non-transformable edge blocks.
    463 Obviously, a transformation with -trim is not reversible, so strictly speaking
    464 jpegtran with this switch is not lossless.  Also, the expected mathematical
    465 equivalences 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 
    469 Another not-strictly-lossless transformation switch is:
    470 	-grayscale	Force grayscale output.
    471 This option discards the chrominance channels if the input image is YCbCr
    472 (ie, a standard color JPEG), resulting in a grayscale JPEG file.  The
    473 luminance channel is preserved exactly, so this is a better method of reducing
    474 to grayscale than decompression, conversion, and recompression.  This switch
    475 is particularly handy for fixing a monochrome picture that was mistakenly
    476 encoded as a color JPEG.  (In such a case, the space savings from getting rid
    477 of the near-empty chroma channels won't be large; but the decoding time for
    478 a grayscale JPEG is substantially less than that for a color JPEG.)
    479 
    480 jpegtran also recognizes these switches that control what to do with "extra"
    481 markers, 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.
    492 The default behavior is -copy comments.  (Note: in IJG releases v6 and v6a,
    493 jpegtran always did the equivalent of -copy none.)
    494 
    495 Additional switches recognized by jpegtran are:
    496 	-outfile filename
    497 	-maxmemory N
    498 	-verbose
    499 	-debug
    500 These work the same as in cjpeg or djpeg.
    501 
    502 
    503 THE COMMENT UTILITIES
    504 
    505 The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
    506 Although the standard doesn't actually define what COM blocks are for, they
    507 are widely used to hold user-supplied text strings.  This lets you add
    508 annotations, titles, index terms, etc to your JPEG files, and later retrieve
    509 them as text.  COM blocks do not interfere with the image stored in the JPEG
    510 file.  The maximum size of a COM block is 64K, but you can have as many of
    511 them as you like in one JPEG file.
    512 
    513 We provide two utility programs to display COM block contents and add COM
    514 blocks to a JPEG file.
    515 
    516 rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
    517 standard output.  The command line syntax is
    518 	rdjpgcom [-verbose] [inputfilename]
    519 The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
    520 image dimensions.  If you omit the input file name from the command line,
    521 the JPEG file is read from standard input.  (This may not work on some
    522 operating systems, if binary data can't be read from stdin.)
    523 
    524 wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
    525 Ordinarily, the COM block is added after any existing COM blocks, but you
    526 can delete the old COM blocks if you wish.  wrjpgcom produces a new JPEG
    527 file; it does not modify the input file.  DO NOT try to overwrite the input
    528 file by directing wrjpgcom's output back into it; on most systems this will
    529 just destroy your file.
    530 
    531 The command line syntax for wrjpgcom is similar to cjpeg's.  On Unix-like
    532 systems, it is
    533 	wrjpgcom [switches] [inputfilename]
    534 The output file is written to standard output.  The input file comes from
    535 the named file, or from standard input if no input file is named.
    536 
    537 On most non-Unix systems, the syntax is
    538 	wrjpgcom [switches] inputfilename outputfilename
    539 where both input and output file names must be given explicitly.
    540 
    541 wrjpgcom 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
    546 to add, you can provide it on the command line with -comment.  The comment
    547 text must be surrounded with quotes so that it is treated as a single
    548 argument.  Longer comments can be read from a text file.
    549 
    550 If you give neither -comment nor -cfile, then wrjpgcom will read the comment
    551 text from standard input.  (In this case an input image file name MUST be
    552 supplied, so that the source JPEG file comes from somewhere else.)  You can
    553 enter 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 
    556 wrjpgcom will not add a COM block if the provided comment string is empty.
    557 Therefore -replace -comment "" can be used to delete all COM blocks from a
    558 file.
    559 
    560 These utility programs do not depend on the IJG JPEG library.  In
    561 particular, the source code for rdjpgcom is intended as an illustration of
    562 the minimum amount of code required to parse a JPEG file header correctly.
    563