diff options
Diffstat (limited to '')
| -rw-r--r-- | libjpegtwrp/usage.doc | 562 |
1 files changed, 0 insertions, 562 deletions
diff --git a/libjpegtwrp/usage.doc b/libjpegtwrp/usage.doc deleted file mode 100644 index 8c4970af0..000000000 --- a/libjpegtwrp/usage.doc +++ /dev/null @@ -1,562 +0,0 @@ -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 compression and decompression. JPEG -(pronounced "jay-peg") is a standardized compression method for full-color -and gray-scale images. JPEG is designed to handle "real-world" scenes, -for example scanned photographs. Cartoons, line drawings, and other -non-realistic images are not JPEG's strong suit; on that sort of material -you may get poor image quality and/or little compression. - -JPEG is lossy, meaning that the output image is not necessarily identical to -the input image. Hence you should not use JPEG if you have to have identical -output bits. However, on typical real-world images, very good compression -levels can be obtained with no visible change, and amazingly high compression -is possible if you can tolerate a low-quality image. You can trade off image -quality 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] >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.doc.) - -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. - - -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 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 -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. CAUTION: progressive -JPEG is not yet widely implemented, so many decoders will be unable to view a -progressive 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 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.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 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. -Use a Unisys-licensed program if you need to read a GIF file. (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. - -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. - -Another not-strictly-lossless transformation switch is: - -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.) - -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 data. - -copy all Copy all extra markers. This setting preserves - miscellaneous markers found in the source file, such - as JFIF thumbnails 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 [-verbose] [inputfilename] -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. |