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diff --git a/jpeg/usage.txt b/jpeg/usage.txt new file mode 100644 index 0000000..de9def7 --- /dev/null +++ b/jpeg/usage.txt @@ -0,0 +1,637 @@ +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. (See +the other documentation files if you wish to use the JPEG library within +your own programs.) + +If you are on a Unix machine you may prefer to read the Unix-style manual +pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1. + + +INTRODUCTION + +These programs implement JPEG image encoding, decoding, and transcoding. +JPEG (pronounced "jay-peg") is a standardized compression method for +full-color and gray-scale images. + + +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] >jpegfile +or + djpeg [switches] [jpegfile] >imagefile +The programs read the specified input file, or standard input if none is +named. They always write to standard output (with trace/error messages to +standard error). These conventions are handy for piping images between +programs. + +On most non-Unix systems, you say: + cjpeg [switches] imagefile jpegfile +or + djpeg [switches] jpegfile imagefile +i.e., both the input and output files are named on the command line. This +style is a little more foolproof, and it loses no functionality if you don't +have pipes. (You can get this style on Unix too, if you prefer, by defining +TWO_FILE_COMMANDLINE when you compile the programs; see install.txt.) + +You can also say: + cjpeg [switches] -outfile jpegfile imagefile +or + djpeg [switches] -outfile imagefile jpegfile +This 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, Targa, and RLE (Utah Raster Toolkit +format). (RLE is supported only if the URT library is available.) +cjpeg recognizes the input image format automatically, with the exception +of 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 as +one letter. Upper and lower case are equivalent (-BMP is the same as -bmp). +British spellings are also accepted (e.g., -greyscale), though for brevity +these 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 + BMP file, because cjpeg isn't bright enough to notice + whether a BMP file uses only shades of gray. By + saying -grayscale, you'll get a smaller JPEG file that + takes less time to process. + + -rgb Create RGB JPEG file. + Using this switch suppresses the conversion from RGB + colorspace input to the default YCbCr JPEG colorspace. + Use this switch in combination with the -block N + switch (see below) for lossless JPEG coding. + + -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). + + -scale M/N Scale the output image by a factor M/N. Currently + supported scale factors are M/N with all N from 1 to + 16, where M is the destination DCT size, which is 8 by + default (see -block N switch 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 of +the reconstructed image: the higher the quality setting, the larger the JPEG +file, and the closer the output image will be to the original input. Normally +you want to use the lowest quality setting (smallest file) that decompresses +into something visually indistinguishable from the original image. For this +purpose the quality setting should be between 50 and 95; the default of 75 is +often about right. If you see defects at -quality 75, then go up 5 or 10 +counts at a time until you are happy with the output image. (The optimal +setting will vary from one image to another.) + +-quality 100 will generate a quantization table of all 1's, minimizing loss +in the quantization step (but there is still information loss in subsampling, +as well as roundoff error). This setting is mainly of interest for +experimental purposes. Quality values above about 95 are NOT recommended for +normal use; the compressed file size goes up dramatically for hardly any gain +in output image quality. + +In the other direction, quality values below 50 will produce very small files +of low image quality. Settings around 5 to 10 might be useful in preparing an +index of a large image library, for example. Try -quality 2 (or so) for some +amusing Cubist effects. (Note: quality values below about 25 generate 2-byte +quantization tables, which are considered optional in the JPEG standard. +cjpeg emits a warning message when you give such a quality value, because some +other JPEG programs may be unable to decode the resulting file. Use -baseline +if you need to ensure compatibility at low quality values.) + +The -quality option has been extended in IJG version 7 for support of separate +quality settings for luminance and chrominance (or in general, for every +provided quantization table slot). This feature is useful for high-quality +applications which cannot accept the damage of color data by coarse +subsampling settings. You can now easily reduce the color data amount more +smoothly with finer control without separate subsampling. The resulting file +is fully compliant with standard JPEG decoders. +Note that the -quality ratings refer to the quantization table slots, and that +the last value is replicated if there are more q-table slots than parameters. +The default q-table slots are 0 for luminance and 1 for chrominance with +default tables as given in the JPEG standard. This is compatible with the old +behaviour in case that only one parameter is given, which is then used for +both luminance and chrominance (slots 0 and 1). More or custom quantization +tables can be set with -qtables and assigned to components with -qslots +parameter (see the "wizard" switches below). +CAUTION: You must explicitly add -sample 1x1 for efficient separate color +quality selection, since the default value used by library is 2x2! + +The -progressive switch creates a "progressive JPEG" file. In this type of +JPEG file, the data is stored in multiple scans of increasing quality. If the +file is being transmitted over a slow communications link, the decoder can use +the first scan to display a low-quality image very quickly, and can then +improve the display with each subsequent scan. The final image is exactly +equivalent to a standard JPEG file of the same quality setting, and the total +file size is about the same --- often a little smaller. + +Switches for advanced users: + + -arithmetic Use arithmetic coding. CAUTION: arithmetic coded JPEG + is not yet widely implemented, so many decoders will + be unable to view an arithmetic coded JPEG file at + all. + + -block N Set DCT block size. All N from 1 to 16 are possible. + Default is 8 (baseline format). + Larger values produce higher compression, + smaller values produce higher quality + (exact DCT stage possible with 1 or 2; with the + default quality of 75 and default Luminance qtable + the DCT+Quantization stage is lossless for N=1). + CAUTION: An implementation of the JPEG SmartScale + extension is required for this feature. SmartScale + enabled JPEG is not yet widely implemented, so many + decoders will be unable to view a SmartScale extended + JPEG file at all. + + -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. + + -nosmooth Don't use high-quality downsampling. + + -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 to +resynchronize after a transmission error. Without restart markers, any damage +to a compressed file will usually ruin the image from the point of the error +to the end of the image; with restart markers, the damage is usually confined +to the portion of the image up to the next restart marker. Of course, the +restart markers occupy extra space. We recommend -restart 1 for images that +will be transmitted across unreliable networks such as Usenet. + +The -smooth option filters the input to eliminate fine-scale noise. This is +often useful when converting dithered images to JPEG: a moderate smoothing +factor of 10 to 50 gets rid of dithering patterns in the input file, resulting +in a smaller JPEG file and a better-looking image. Too large a smoothing +factor will visibly blur the image, however. + +Switches for wizards: + + -baseline Force baseline-compatible quantization tables to be + generated. This clamps quantization values to 8 bits + even at low quality settings. (This switch is poorly + named, since it does not ensure that the output is + actually baseline JPEG. For example, you can use + -baseline and -progressive together.) + + -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 you +don't know what you are doing, DON'T USE THEM. These switches are documented +further in the file wizard.txt. + + +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 + supported scale factors are M/N with all M from 1 to + 16, where N is the source DCT size, which is 8 for + baseline JPEG. If the /N part is omitted, then M + specifies the DCT scaled size to be applied on the + given input. For baseline JPEG this is equivalent to + M/8 scaling, since the source DCT size for baseline + JPEG is 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 Don't use high-quality upsampling. + + -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 for +compressing full-color (24-bit) images. In particular, don't try to convert +cartoons, line drawings, and other images that have only a few distinct +colors. GIF works great on these, JPEG does not. If you want to convert a +GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options +to get a satisfactory conversion. -smooth 10 or so is often helpful. + +Avoid running an image through a series of JPEG compression/decompression +cycles. Image quality loss will accumulate; after ten or so cycles the image +may be noticeably worse than it was after one cycle. It's best to use a +lossless format while manipulating an image, then convert to JPEG format when +you 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 low +quality settings to make very small JPEG files; the percentage improvement +is often a lot more than it is on larger files. (At present, -optimize +mode is always selected when generating progressive JPEG files.) + +GIF input files are no longer supported, to avoid the Unisys LZW patent. +(Conversion of GIF files to JPEG is usually a bad idea anyway.) + + +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 but +much lower quality than the default behavior. "-dither none" may give +acceptable 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 significant +in practice. + +Two-pass color quantization requires a good deal of memory; on MS-DOS machines +it may run out of memory even with -maxmemory 0. In that case you can still +decompress, with some loss of image quality, by specifying -onepass for +one-pass quantization. + +To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files. These +are larger than they should be, but are readable by standard GIF decoders. + + +HINTS FOR BOTH PROGRAMS + +If more space is needed than will fit in the available main memory (as +determined by -maxmemory), temporary files will be used. (MS-DOS versions +will try to get extended or expanded memory first.) The temporary files are +often rather large: in typical cases they occupy three bytes per pixel, for +example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough +free 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 TMP +or TEMP environment variable, or in the current directory if neither of those +exist. Amiga implementations put the temp files in the directory named by +JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free +space. + +The default memory usage limit (-maxmemory) is set when the software is +compiled. If you get an "insufficient memory" error, try specifying a smaller +-maxmemory value, even -maxmemory 0 to use the absolute minimum space. You +may want to recompile with a smaller default value if this happens often. + +On machines that have "environment" variables, you can define the environment +variable JPEGMEM to set the default memory limit. The value is specified as +described for the -maxmemory switch. JPEGMEM overrides the default value +specified when the program was compiled, and itself is overridden by an +explicit -maxmemory switch. + +On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to +use. (Extended or expanded memory is also used if available.) Most +DOS-specific versions of this software do their own memory space estimation +and do not need you to specify -maxmemory. + + +JPEGTRAN + +jpegtran performs various useful transformations of JPEG files. +It can translate the coded representation from one variant of JPEG to another, +for example from baseline JPEG to progressive JPEG or vice versa. It can also +perform some rearrangements of the image data, for example turning an image +from landscape to portrait format by rotation. + +jpegtran works by rearranging the compressed data (DCT coefficients), without +ever fully decoding the image. Therefore, its transformations are lossless: +there is no image degradation at all, which would not be true if you used +djpeg followed by cjpeg to accomplish the same conversion. But by the same +token, jpegtran cannot perform lossy operations such as changing the image +quality. + +jpegtran uses a command line syntax similar to cjpeg or djpeg. +On Unix-like systems, you say: + jpegtran [switches] [inputfile] >outputfile +On most non-Unix systems, you say: + jpegtran [switches] inputfile outputfile +where both the input and output files are JPEG files. + +To specify the coded JPEG representation used in the output file, +jpegtran accepts a subset of the switches recognized by cjpeg: + -optimize Perform optimization of entropy encoding parameters. + -progressive Create progressive JPEG file. + -arithmetic Use arithmetic coding. + -restart N Emit a JPEG restart marker every N MCU rows, or every + N MCU blocks if "B" is attached to the number. + -scans file Use the scan script given in the specified text file. +See the previous discussion of cjpeg for more details about these switches. +If you specify none of these switches, you get a plain baseline-JPEG output +file. The quality setting and so forth are determined by the input file. + +The image can be losslessly transformed by giving one of these switches: + -flip horizontal Mirror image horizontally (left-right). + -flip vertical Mirror image vertically (top-bottom). + -rotate 90 Rotate image 90 degrees clockwise. + -rotate 180 Rotate image 180 degrees. + -rotate 270 Rotate image 270 degrees clockwise (or 90 ccw). + -transpose Transpose image (across UL-to-LR axis). + -transverse Transverse transpose (across UR-to-LL axis). + +The transpose transformation has no restrictions regarding image dimensions. +The other transformations operate rather oddly if the image dimensions are not +a multiple of the iMCU size (usually 8 or 16 pixels), because they can only +transform complete blocks of DCT coefficient data in the desired way. + +jpegtran's default behavior when transforming an odd-size image is designed +to preserve exact reversibility and mathematical consistency of the +transformation set. As stated, transpose is able to flip the entire image +area. Horizontal mirroring leaves any partial iMCU column at the right edge +untouched, but is able to flip all rows of the image. Similarly, vertical +mirroring leaves any partial iMCU row at the bottom edge untouched, but is +able to flip all columns. The other transforms can be built up as sequences +of transpose and flip operations; for consistency, their actions on edge +pixels are defined to be the same as the end result of the corresponding +transpose-and-flip sequence. + +For practical use, you may prefer to discard any untransformable edge pixels +rather than having a strange-looking strip along the right and/or bottom edges +of a transformed image. To do this, add the -trim switch: + -trim Drop non-transformable edge blocks. +Obviously, a transformation with -trim is not reversible, so strictly speaking +jpegtran with this switch is not lossless. Also, the expected mathematical +equivalences between the transformations no longer hold. For example, +"-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by +"-rot 180 -trim" trims both edges. + +If you are only interested in perfect transformation, add the -perfect switch: + -perfect Fails with an error if the transformation is not + perfect. +For example you may want to do + jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg +to do a perfect rotation if available or an approximated one if not. + +We also offer a lossless-crop option, which discards data outside a given +image region but losslessly preserves what is inside. Like the rotate and +flip transforms, lossless crop is restricted by the current JPEG format: the +upper left corner of the selected region must fall on an iMCU boundary. If +this does not hold for the given crop parameters, we silently move the upper +left corner up and/or left to make it so, simultaneously increasing the region +dimensions to keep the lower right crop corner unchanged. (Thus, the output +image covers at least the requested region, but may cover more.) + +The image can be losslessly cropped by giving the switch: + -crop WxH+X+Y Crop to a rectangular subarea of width W, height H + starting at point X,Y. + +Other not-strictly-lossless transformation switches are: + + -grayscale Force grayscale output. +This option discards the chrominance channels if the input image is YCbCr +(ie, a standard color JPEG), resulting in a grayscale JPEG file. The +luminance channel is preserved exactly, so this is a better method of reducing +to grayscale than decompression, conversion, and recompression. This switch +is particularly handy for fixing a monochrome picture that was mistakenly +encoded as a color JPEG. (In such a case, the space savings from getting rid +of the near-empty chroma channels won't be large; but the decoding time for +a grayscale JPEG is substantially less than that for a color JPEG.) + + -scale M/N Scale the output image by a factor M/N. +Currently supported scale factors are M/N with all M from 1 to 16, where N is +the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted, +then M specifies the DCT scaled size to be applied on the given input. For +baseline JPEG this is equivalent to M/8 scaling, since the source DCT size +for baseline JPEG is 8. CAUTION: An implementation of the JPEG SmartScale +extension is required for this feature. SmartScale enabled JPEG is not yet +widely implemented, so many decoders will be unable to view a SmartScale +extended JPEG file at all. + +jpegtran also recognizes these switches that control what to do with "extra" +markers, such as comment blocks: + -copy none Copy no extra markers from source file. This setting + suppresses all comments and other excess baggage + present in the source file. + -copy comments Copy only comment markers. This setting copies + comments from the source file, but discards + any other inessential (for image display) data. + -copy all Copy all extra markers. This setting preserves + miscellaneous markers found in the source file, such + as JFIF thumbnails, Exif data, and Photoshop settings. + In some files these extra markers can be sizable. +The default behavior is -copy comments. (Note: in IJG releases v6 and v6a, +jpegtran always did the equivalent of -copy none.) + +Additional switches recognized by jpegtran are: + -outfile filename + -maxmemory N + -verbose + -debug +These work the same as in cjpeg or djpeg. + + +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, they +are widely used to hold user-supplied text strings. This lets you add +annotations, titles, index terms, etc to your JPEG files, and later retrieve +them as text. COM blocks do not interfere with the image stored in the JPEG +file. The maximum size of a COM block is 64K, but you can have as many of +them as you like in one JPEG file. + +We provide two utility programs to display COM block contents and add COM +blocks to a JPEG file. + +rdjpgcom searches a JPEG file and prints the contents of any COM blocks on +standard output. The command line syntax is + rdjpgcom [-raw] [-verbose] [inputfilename] +The switch "-raw" (or just "-r") causes rdjpgcom to also output non-printable +characters in comments, which are normally escaped for security reasons. +The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG +image 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 some +operating 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 you +can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG +file; it does not modify the input file. DO NOT try to overwrite the input +file by directing wrjpgcom's output back into it; on most systems this will +just destroy your file. + +The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like +systems, it is + wrjpgcom [switches] [inputfilename] +The output file is written to standard output. The input file comes from +the named file, or from standard input if no input file is named. + +On most non-Unix systems, the syntax is + wrjpgcom [switches] inputfilename outputfilename +where 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 text +to add, you can provide it on the command line with -comment. The comment +text must be surrounded with quotes so that it is treated as a single +argument. Longer comments can be read from a text file. + +If you give neither -comment nor -cfile, then wrjpgcom will read the comment +text from standard input. (In this case an input image file name MUST be +supplied, so that the source JPEG file comes from somewhere else.) You can +enter 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 a +file. + +These utility programs do not depend on the IJG JPEG library. In +particular, the source code for rdjpgcom is intended as an illustration of +the minimum amount of code required to parse a JPEG file header correctly. |