USAGE instructions for the Independent JPEG Group's JPEG software================================================================= This file describes usage of the JPEG conversion programs cjpeg and djpeg,as well as the utility programs jpegtran, rdjpgcom and wrjpgcom.  (Seethe other documentation files if you wish to use the JPEG library withinyour own programs.) If you are on a Unix machine you may prefer to read the Unix-style manualpages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1.  INTRODUCTION These programs implement JPEG image compression and decompression.  JPEG(pronounced "jay-peg") is a standardized compression method for full-colorand gray-scale images.  JPEG is designed to handle "real-world" scenes,for example scanned photographs.  Cartoons, line drawings, and othernon-realistic images are not JPEG's strong suit; on that sort of materialyou may get poor image quality and/or little compression. JPEG is lossy, meaning that the output image is not necessarily identical tothe input image.  Hence you should not use JPEG if you have to have identicaloutput bits.  However, on typical real-world images, very good compressionlevels can be obtained with no visible change, and amazingly high compressionis possible if you can tolerate a low-quality image.  You can trade off imagequality against file size by adjusting the compressor's "quality" setting.  GENERAL USAGE We provide two programs, cjpeg to compress an image file into JPEG format,and djpeg to decompress a JPEG file back into a conventional image format. On Unix-like systems, you say:	cjpeg [switches] [imagefile] >jpegfileor	djpeg [switches] [jpegfile]  >imagefileThe programs read the specified input file, or standard input if none isnamed.  They always write to standard output (with trace/error messages tostandard error).  These conventions are handy for piping images betweenprograms. On most non-Unix systems, you say:	cjpeg [switches] imagefile jpegfileor	djpeg [switches] jpegfile  imagefilei.e., both the input and output files are named on the command line.  Thisstyle is a little more foolproof, and it loses no functionality if you don'thave pipes.  (You can get this style on Unix too, if you prefer, by definingTWO_FILE_COMMANDLINE when you compile the programs; see install.doc.) You can also say:	cjpeg [switches] -outfile jpegfile  imagefileor	djpeg [switches] -outfile imagefile  jpegfileThis syntax works on all systems, so it is useful for scripts. The currently supported image file formats are: PPM (PBMPLUS color format),PGM (PBMPLUS gray-scale format), BMP, GIF, Targa, and RLE (Utah Raster Toolkitformat).  (RLE is supported only if the URT library is available.)cjpeg recognizes the input image format automatically, with the exceptionof some Targa-format files.  You have to tell djpeg which format to generate. JPEG files are in the defacto standard JFIF file format.  There are other,less widely used JPEG-based file formats, but we don't support them. All switch names may be abbreviated; for example, -grayscale may be written-gray or -gr.  Most of the "basic" switches can be abbreviated to as little asone letter.  Upper and lower case are equivalent (-GIF is the same as -gif).British spellings are also accepted (e.g., -greyscale), though for brevitythese are not mentioned below.  CJPEG DETAILS The basic command line switches for cjpeg are: 	-quality N	Scale quantization tables to adjust image quality.			Quality is 0 (worst) to 100 (best); default is 75.			(See below for more info.) 	-grayscale	Create monochrome JPEG file from color input.			Be sure to use this switch when compressing a grayscale			GIF file, because cjpeg isn't bright enough to notice			whether a GIF file uses only shades of gray.  By			saying -grayscale, you'll get a smaller JPEG file that			takes less time to process. 	-optimize	Perform optimization of entropy encoding parameters.			Without this, default encoding parameters are used.			-optimize usually makes the JPEG file a little smaller,			but cjpeg runs somewhat slower and needs much more			memory.  Image quality and speed of decompression are			unaffected by -optimize. 	-progressive	Create progressive JPEG file (see below). 	-targa		Input file is Targa format.  Targa files that contain			an "identification" field will not be automatically			recognized by cjpeg; for such files you must specify			-targa to make cjpeg treat the input as Targa format.			For most Targa files, you won't need this switch. The -quality switch lets you trade off compressed file size against quality ofthe reconstructed image: the higher the quality setting, the larger the JPEGfile, and the closer the output image will be to the original input.  Normallyyou want to use the lowest quality setting (smallest file) that decompressesinto something visually indistinguishable from the original image.  For thispurpose the quality setting should be between 50 and 95; the default of 75 isoften about right.  If you see defects at -quality 75, then go up 5 or 10counts at a time until you are happy with the output image.  (The optimalsetting will vary from one image to another.) -quality 100 will generate a quantization table of all 1's, minimizing lossin the quantization step (but there is still information loss in subsampling,as well as roundoff error).  This setting is mainly of interest forexperimental purposes.  Quality values above about 95 are NOT recommended fornormal use; the compressed file size goes up dramatically for hardly any gainin output image quality. In the other direction, quality values below 50 will produce very small filesof low image quality.  Settings around 5 to 10 might be useful in preparing anindex of a large image library, for example.  Try -quality 2 (or so) for someamusing Cubist effects.  (Note: quality values below about 25 generate 2-bytequantization tables, which are considered optional in the JPEG standard.cjpeg emits a warning message when you give such a quality value, because someother JPEG programs may be unable to decode the resulting file.  Use -baselineif you need to ensure compatibility at low quality values.) The -progressive switch creates a "progressive JPEG" file.  In this type ofJPEG file, the data is stored in multiple scans of increasing quality.  If thefile is being transmitted over a slow communications link, the decoder can usethe first scan to display a low-quality image very quickly, and can thenimprove the display with each subsequent scan.  The final image is exactlyequivalent to a standard JPEG file of the same quality setting, and the totalfile size is about the same --- often a little smaller.  CAUTION: progressiveJPEG is not yet widely implemented, so many decoders will be unable to view aprogressive JPEG file at all. Switches for advanced users: 	-dct int	Use integer DCT method (default).	-dct fast	Use fast integer DCT (less accurate).	-dct float	Use floating-point DCT method.			The float method is very slightly more accurate than			the int method, but is much slower unless your machine			has very fast floating-point hardware.  Also note that			results of the floating-point method may vary slightly			across machines, while the integer methods should give			the same results everywhere.  The fast integer method			is much less accurate than the other two. 	-restart N	Emit a JPEG restart marker every N MCU rows, or every			N MCU blocks if "B" is attached to the number.			-restart 0 (the default) means no restart markers. 	-smooth N	Smooth the input image to eliminate dithering noise.			N, ranging from 1 to 100, indicates the strength of			smoothing.  0 (the default) means no smoothing. 	-maxmemory N	Set limit for amount of memory to use in processing			large images.  Value is in thousands of bytes, or			millions of bytes if "M" is attached to the number.			For example, -max 4m selects 4000000 bytes.  If more			space is needed, temporary files will be used. 	-verbose	Enable debug printout.  More -v's give more printout.	or  -debug	Also, version information is printed at startup. The -restart option inserts extra markers that allow a JPEG decoder toresynchronize after a transmission error.  Without restart markers, any damageto a compressed file will usually ruin the image from the point of the errorto the end of the image; with restart markers, the damage is usually confinedto the portion of the image up to the next restart marker.  Of course, therestart markers occupy extra space.  We recommend -restart 1 for images thatwill be transmitted across unreliable networks such as Usenet. The -smooth option filters the input to eliminate fine-scale noise.  This isoften useful when converting GIF files to JPEG: a moderate smoothing factor of10 to 50 gets rid of dithering patterns in the input file, resulting in asmaller JPEG file and a better-looking image.  Too large a smoothing factorwill visibly blur the image, however. Switches for wizards: 	-baseline	Force a baseline JPEG file to be generated.  This			clamps quantization values to 8 bits even at low			quality settings. 	-qtables file	Use the quantization tables given in the specified			text file. 	-qslots N[,...] Select which quantization table to use for each color			component. 	-sample HxV[,...]  Set JPEG sampling factors for each color component. 	-scans file	Use the scan script given in the specified text file. The "wizard" switches are intended for experimentation with JPEG.  If youdon't know what you are doing, DON'T USE THEM.  These switches are documentedfurther in the file wizard.doc.  DJPEG DETAILS The basic command line switches for djpeg are: 	-colors N	Reduce image to at most N colors.  This reduces the	or -quantize N	number of colors used in the output image, so that it			can be displayed on a colormapped display or stored in			a colormapped file format.  For example, if you have			an 8-bit display, you'd need to reduce to 256 or fewer			colors.  (-colors is the recommended name, -quantize			is provided only for backwards compatibility.) 	-fast		Select recommended processing options for fast, low			quality output.  (The default options are chosen for			highest quality output.)  Currently, this is equivalent			to "-dct fast -nosmooth -onepass -dither ordered". 	-grayscale	Force gray-scale output even if JPEG file is color.			Useful for viewing on monochrome displays; also,			djpeg runs noticeably faster in this mode. 	-scale M/N	Scale the output image by a factor M/N.  Currently			the scale factor must be 1/1, 1/2, 1/4, or 1/8.			Scaling is handy if the image is larger than your			screen; also, djpeg runs much faster when scaling			down the output. 	-bmp		Select BMP output format (Windows flavor).  8-bit			colormapped format is emitted if -colors or -grayscale			is specified, or if the JPEG file is gray-scale;			otherwise, 24-bit full-color format is emitted. 	-gif		Select GIF output format.  Since GIF does not support			more than 256 colors, -colors 256 is assumed (unless			you specify a smaller number of colors).  If you			specify -fast, the default number of colors is 216. 	-os2		Select BMP output format (OS/2 1.x flavor).  8-bit			colormapped format is emitted if -colors or -grayscale			is specified, or if the JPEG file is gray-scale;			otherwise, 24-bit full-color format is emitted. 	-pnm		Select PBMPLUS (PPM/PGM) output format (this is the			default format).  PGM is emitted if the JPEG file is			gray-scale or if -grayscale is specified; otherwise			PPM is emitted. 	-rle		Select RLE output format.  (Requires URT library.) 	-targa		Select Targa output format.  Gray-scale format is			emitted if the JPEG file is gray-scale or if			-grayscale is specified; otherwise, colormapped format			is emitted if -colors is specified; otherwise, 24-bit			full-color format is emitted. Switches for advanced users: 	-dct int	Use integer DCT method (default).	-dct fast	Use fast integer DCT (less accurate).	-dct float	Use floating-point DCT method.			The float method is very slightly more accurate than			the int method, but is much slower unless your machine			has very fast floating-point hardware.  Also note that			results of the floating-point method may vary slightly			across machines, while the integer methods should give			the same results everywhere.  The fast integer method			is much less accurate than the other two. 	-dither fs	Use Floyd-Steinberg dithering in color quantization.	-dither ordered	Use ordered dithering in color quantization.	-dither none	Do not use dithering in color quantization.			By default, Floyd-Steinberg dithering is applied when			quantizing colors; this is slow but usually produces			the best results.  Ordered dither is a compromise			between speed and quality; no dithering is fast but			usually looks awful.  Note that these switches have			no effect unless color quantization is being done.			Ordered dither is only available in -onepass mode. 	-map FILE	Quantize to the colors used in the specified image			file.  This is useful for producing multiple files			with identical color maps, or for forcing a predefined			set of colors to be used.  The FILE must be a GIF			or PPM file.  This option overrides -colors and			-onepass. 	-nosmooth	Use a faster, lower-quality upsampling routine. 	-onepass	Use one-pass instead of two-pass color quantization.			The one-pass method is faster and needs less memory,			but it produces a lower-quality image.  -onepass is			ignored unless you also say -colors N.  Also,			the one-pass method is always used for gray-scale			output (the two-pass method is no improvement then). 	-maxmemory N	Set limit for amount of memory to use in processing			large images.  Value is in thousands of bytes, or			millions of bytes if "M" is attached to the number.			For example, -max 4m selects 4000000 bytes.  If more			space is needed, temporary files will be used. 	-verbose	Enable debug printout.  More -v's give more printout.	or  -debug	Also, version information is printed at startup.  HINTS FOR CJPEG Color GIF files are not the ideal input for JPEG; JPEG is really intended forcompressing full-color (24-bit) images.  In particular, don't try to convertcartoons, line drawings, and other images that have only a few distinctcolors.  GIF works great on these, JPEG does not.  If you want to convert aGIF to JPEG, you should experiment with cjpeg's -quality and -smooth optionsto get a satisfactory conversion.  -smooth 10 or so is often helpful. Avoid running an image through a series of JPEG compression/decompressioncycles.  Image quality loss will accumulate; after ten or so cycles the imagemay be noticeably worse than it was after one cycle.  It's best to use alossless format while manipulating an image, then convert to JPEG format whenyou are ready to file the image away. The -optimize option to cjpeg is worth using when you are making a "final"version for posting or archiving.  It's also a win when you are using lowquality settings to make very small JPEG files; the percentage improvementis often a lot more than it is on larger files.  (At present, -optimizemode is always selected when generating progressive JPEG files.)  HINTS FOR DJPEG To get a quick preview of an image, use the -grayscale and/or -scale switches."-grayscale -scale 1/8" is the fastest case. Several options are available that trade off image quality to gain speed."-fast" turns on the recommended settings. "-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.When producing a color-quantized image, "-onepass -dither ordered" is fast butmuch lower quality than the default behavior.  "-dither none" may giveacceptable results in two-pass mode, but is seldom tolerable in one-pass mode. If you are fortunate enough to have very fast floating point hardware,"-dct float" may be even faster than "-dct fast".  But on most machines"-dct float" is slower than "-dct int"; in this case it is not worth using,because its theoretical accuracy advantage is too small to be significantin practice. Two-pass color quantization requires a good deal of memory; on MS-DOS machinesit may run out of memory even with -maxmemory 0.  In that case you can stilldecompress, with some loss of image quality, by specifying -onepass forone-pass quantization.  HINTS FOR BOTH PROGRAMS If more space is needed than will fit in the available main memory (asdetermined by -maxmemory), temporary files will be used.  (MS-DOS versionswill try to get extended or expanded memory first.)  The temporary files areoften rather large: in typical cases they occupy three bytes per pixel, forexample 3*800*600 = 1.44Mb for an 800x600 image.  If you don't have enoughfree disk space, leave out -progressive and -optimize (for cjpeg) or specify-onepass (for djpeg). On MS-DOS, the temporary files are created in the directory named by the TMPor TEMP environment variable, or in the current directory if neither of thoseexist.  Amiga implementations put the temp files in the directory named byJPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate freespace. The default memory usage limit (-maxmemory) is set when the software iscompiled.  If you get an "insufficient memory" error, try specifying a smaller-maxmemory value, even -maxmemory 0 to use the absolute minimum space.  Youmay want to recompile with a smaller default value if this happens often. On machines that have "environment" variables, you can define the environmentvariable JPEGMEM to set the default memory limit.  The value is specified asdescribed for the -maxmemory switch.  JPEGMEM overrides the default valuespecified when the program was compiled, and itself is overridden by anexplicit -maxmemory switch. On MS-DOS machines, -maxmemory is the amount of main (conventional) memory touse.  (Extended or expanded memory is also used if available.)  MostDOS-specific versions of this software do their own memory space estimationand do not need you to specify -maxmemory.  JPEGTRAN jpegtran translates JPEG files from one variant of JPEG to another, forexample from baseline JPEG to progressive JPEG or vice versa.  Thetransformation is lossless: no image degradation occurs, which would notbe true if you used djpeg followed by cjpeg.  However, you cannot alterthe image quality, because that would not be a lossless operation. jpegtran operates similarly to cjpeg, except that it reads a JPEG fileand writes another JPEG file. jpegtran accepts a subset of the switches recognized by cjpeg:	-outfile filename	-optimize	-progressive	-restart N	-scans file	-maxmemory N	-verbose	-debugSee the previous discussion of cjpeg for details about these switches. If you specify no switches, you get a plain baseline-JPEG output file.  THE COMMENT UTILITIES The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.Although the standard doesn't actually define what COM blocks are for, theyare widely used to hold user-supplied text strings.  This lets you addannotations, titles, index terms, etc to your JPEG files, and later retrievethem as text.  COM blocks do not interfere with the image stored in the JPEGfile.  The maximum size of a COM block is 64K, but you can have as many ofthem as you like in one JPEG file. We provide two utility programs to display COM block contents and add COMblocks to a JPEG file. rdjpgcom searches a JPEG file and prints the contents of any COM blocks onstandard output.  The command line syntax is	rdjpgcom [-verbose] [inputfilename]The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEGimage dimensions.  If you omit the input file name from the command line,the JPEG file is read from standard input.  (This may not work on someoperating systems, if binary data can't be read from stdin.) wrjpgcom adds a COM block, containing text you provide, to a JPEG file.Ordinarily, the COM block is added after any existing COM blocks, but youcan delete the old COM blocks if you wish.  wrjpgcom produces a new JPEGfile; it does not modify the input file.  DO NOT try to overwrite the inputfile by directing wrjpgcom's output back into it; on most systems this willjust destroy your file. The command line syntax for wrjpgcom is similar to cjpeg's.  On Unix-likesystems, it is	wrjpgcom [switches] [inputfilename]The output file is written to standard output.  The input file comes fromthe named file, or from standard input if no input file is named. On most non-Unix systems, the syntax is	wrjpgcom [switches] inputfilename outputfilenamewhere both input and output file names must be given explicitly. wrjpgcom understands three switches:	-replace		 Delete any existing COM blocks from the file.	-comment "Comment text"	 Supply new COM text on command line.        -cfile name		 Read text for new COM block from named file.(Switch names can be abbreviated.)  If you have only one line of comment textto add, you can provide it on the command line with -comment.  The commenttext must be surrounded with quotes so that it is treated as a singleargument.  Longer comments can be read from a text file. If you give neither -comment nor -cfile, then wrjpgcom will read the commenttext from standard input.  (In this case an input image file name MUST besupplied, so that the source JPEG file comes from somewhere else.)  You canenter multiple lines, up to 64KB worth.  Type an end-of-file indicator(usually control-D or control-Z) to terminate the comment text entry. wrjpgcom will not add a COM block if the provided comment string is empty.Therefore -replace -comment "" can be used to delete all COM blocks from afile. These utility programs do not depend on the IJG JPEG library.  Inparticular, the source code for rdjpgcom is intended as an illustration ofthe minimum amount of code required to parse a JPEG file header correctly.