309 lines
14 KiB
Plaintext
309 lines
14 KiB
Plaintext
USAGE instructions for the Independent JPEG Group's JPEG software
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=================================================================
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INTRODUCTION
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This distribution contains software to implement JPEG image compression and
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decompression. JPEG (pronounced "jay-peg") is a standardized compression
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method for full-color and gray-scale images. JPEG is designed to handle
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"real-world" scenes, for example scanned photographs. Cartoons, line
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drawings, and other non-realistic images are not JPEG's strong suit; on this
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sort of material you may get poor image quality and/or little compression.
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JPEG is lossy, meaning that the output image is not necessarily identical to
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the input image. Hence you should not use JPEG if you have to have identical
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output bits. However, on typical real-world images, very good compression
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levels can be obtained with no visible change, and amazingly high compression
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is possible if you can tolerate a low-quality image. You can trade off image
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quality against file size by adjusting the compressor's "quality" setting.
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This file describes usage of the standard programs "cjpeg" and "djpeg" that
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can be built directly from the distributed C code. See the README file for
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hints on incorporating the JPEG software into other programs.
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If you are on a Unix machine you may prefer to read the Unix-style manual
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pages in files cjpeg.1 and djpeg.1. But also see the HINTS section below,
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which is not present in either manual page.
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NOTE: the switch syntax has been redesigned since the v3 release of
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cjpeg/djpeg. Switch names are now words instead of single letters.
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GENERAL USAGE
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We provide two programs, cjpeg to compress an image file into JPEG format,
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and djpeg to decompress a JPEG file back into a conventional image format.
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On Unix-like systems, you say:
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cjpeg [switches] [imagefile] >jpegfile
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or
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djpeg [switches] [jpegfile] >imagefile
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The programs read the specified input file, or standard input if none is
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named. They always write to standard output (with trace/error messages to
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standard error). These conventions are handy for piping images between
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programs.
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On most non-Unix systems, you say:
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cjpeg [switches] imagefile jpegfile
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or
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djpeg [switches] jpegfile imagefile
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i.e., both the input and output files are named on the command line. This
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style is a little more foolproof, and it loses no functionality if you don't
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have pipes. (You can get this style on Unix too, if you prefer, by defining
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TWO_FILE_COMMANDLINE when you compile the programs; see SETUP.)
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You can also say:
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cjpeg [switches] -outfile jpegfile imagefile
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or
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djpeg [switches] -outfile imagefile jpegfile
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This syntax works on all systems, so it is useful for scripts.
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The currently supported image file formats are: PPM (PBMPLUS color format),
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PGM (PBMPLUS gray-scale format), GIF, Targa, and RLE (Utah Raster Toolkit
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format). (RLE is supported only if the URT library is available.)
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cjpeg recognizes the input image format automatically, with the exception
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of some Targa-format files. You have to tell djpeg which format to generate.
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The only JPEG file format currently supported is the JFIF format. Support for
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the TIFF 6.0 JPEG format will probably be added at some future date.
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All switch names may be abbreviated; for example, -grayscale may be written
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-gray or -gr. Most of the "basic" switches can be abbreviated to as little as
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one letter. Upper and lower case are equivalent (-GIF is the same as -gif).
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British spellings are also accepted (e.g., -greyscale), though for brevity
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these are not mentioned below.
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CJPEG DETAILS
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The basic command line switches for cjpeg are:
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-quality N Scale quantization tables to adjust image quality.
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Quality is 0 (worst) to 100 (best); default is 75.
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(See below for more info.)
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-grayscale Create monochrome JPEG file from color input.
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Be sure to use this switch when compressing a grayscale
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GIF file, because cjpeg isn't bright enough to notice
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whether a GIF file uses only shades of gray. By
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saying -grayscale, you'll get a smaller JPEG file that
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takes less time to process.
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-optimize Perform optimization of entropy encoding parameters.
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Without this, default encoding parameters are used.
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-optimize usually makes the JPEG file a little smaller,
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but cjpeg runs somewhat slower and needs much more
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memory. Image quality and speed of decompression are
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unaffected by -optimize.
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-targa Input file is Targa format. Targa files that contain
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an "identification" field will not be automatically
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recognized by cjpeg; for such files you must specify
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-targa to make cjpeg treat the input as Targa format.
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The -quality switch lets you trade off compressed file size against quality of
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the reconstructed image: the higher the quality setting, the larger the JPEG
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file, and the closer the output image will be to the original input. Normally
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you want to use the lowest quality setting (smallest file) that decompresses
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into something visually indistinguishable from the original image. For this
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purpose the quality setting should be between 50 and 95; the default of 75 is
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often about right. If you see defects at -quality 75, then go up 5 or 10
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counts at a time until you are happy with the output image. (The optimal
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setting will vary from one image to another.)
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-quality 100 will generate a quantization table of all 1's, eliminating loss
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in the quantization step (but there is still information loss in subsampling,
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as well as roundoff error). This setting is mainly of interest for
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experimental purposes. Quality values above about 95 are NOT recommended for
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normal use; the compressed file size goes up dramatically for hardly any gain
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in output image quality.
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In the other direction, quality values below 50 will produce very small files
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of low image quality. Settings around 5 to 10 might be useful in preparing an
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index of a large image library, for example. Try -quality 2 (or so) for some
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amusing Cubist effects. (Note: quality values below about 25 generate 2-byte
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quantization tables, which are considered optional in the JPEG standard.
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cjpeg emits a warning message when you give such a quality value, because
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some commercial JPEG programs may be unable to decode the resulting file.)
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Switches for advanced users:
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-maxmemory N Set limit for amount of memory to use in processing
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large images. Value is in thousands of bytes, or
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millions of bytes if "M" is attached to the number.
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For example, -max 4m selects 4000000 bytes. If more
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space is needed, temporary files will be used.
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-restart N Emit a JPEG restart marker every N MCU rows, or every
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N MCU blocks if "B" is attached to the number.
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-restart 0 (the default) means no restart markers.
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-smooth N Smooth the input image to eliminate dithering noise.
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N, ranging from 1 to 100, indicates the strength of
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smoothing. 0 (the default) means no smoothing.
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-verbose Enable debug printout. More -v's give more printout.
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or -debug Also, version information is printed at startup.
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The -restart option inserts extra markers that allow a JPEG decoder to
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resynchronize after a transmission error. Without restart markers, any damage
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to a compressed file will usually ruin the image from the point of the error
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to the end of the image; with restart markers, the damage is usually confined
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to the portion of the image up to the next restart marker. Of course, the
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restart markers occupy extra space. We recommend -restart 1 for images that
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will be transmitted across unreliable networks such as Usenet.
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The -smooth option filters the input to eliminate fine-scale noise. This is
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often useful when converting GIF files to JPEG: a moderate smoothing factor of
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10 to 50 gets rid of dithering patterns in the input file, resulting in a
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smaller JPEG file and a better-looking image. Too large a smoothing factor
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will visibly blur the image, however.
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Switches for wizards:
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-arithmetic Use arithmetic coding rather than Huffman coding.
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(Not currently supported for legal reasons.)
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-nointerleave Generate noninterleaved JPEG file (not yet supported).
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-qtables file Use the quantization tables given in the specified
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file. The file should contain one to four tables
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(64 values each) as plain text. Comments preceded by
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'#' may be included in the file. The tables are
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implicitly numbered 0,1,etc. If -quality N is also
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specified, the values in the file are scaled according
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to cjpeg's quality scaling curve.
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-sample HxV[,...] Set JPEG sampling factors. If you specify
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fewer H/V pairs than there are components, the
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remaining components are set to 1x1 sampling. The
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default setting is equivalent to "-sample 2x2".
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The "wizard" switches are intended for experimentation with JPEG. If you
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don't know what you are doing, DON'T USE THEM. You can easily produce files
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with worse image quality and/or poorer compression than you'll get from the
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default settings. Furthermore, these switches should not be used when making
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files intended for general use, because not all JPEG implementations will
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support unusual JPEG parameter settings.
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DJPEG DETAILS
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The basic command line switches for djpeg are:
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-colors N Reduce image to at most N colors. This reduces the
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or -quantize N number of colors used in the output image, so that it
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can be displayed on a colormapped display or stored in
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a colormapped file format. For example, if you have
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an 8-bit display, you'd need to reduce to 256 or fewer
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colors. (-colors is the recommended name, -quantize
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is provided only for backwards compatibility.)
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-gif Select GIF output format. Since GIF does not support
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more than 256 colors, -colors 256 is assumed (unless
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you specify a smaller number of colors).
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-pnm Select PBMPLUS (PPM/PGM) output format (this is the
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default format). PGM is emitted if the JPEG file is
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gray-scale or if -grayscale is specified; otherwise
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PPM is emitted.
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-rle Select RLE output format. (Requires URT library.)
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-targa Select Targa output format. Gray-scale format is
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emitted if the JPEG file is gray-scale or if
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-grayscale is specified; otherwise, colormapped format
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is emitted if -colors is specified; otherwise, 24-bit
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full-color format is emitted.
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Switches for advanced users:
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-blocksmooth Perform cross-block smoothing. This is slow, quite
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memory-intensive, and only seems to improve the image
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at very low quality settings (-quality 10 to 20 or so).
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At normal quality settings it may make things worse.
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-grayscale Force gray-scale output even if JPEG file is color.
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Useful for viewing on monochrome displays.
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-maxmemory N Set limit for amount of memory to use in processing
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large images. Value is in thousands of bytes, or
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millions of bytes if "M" is attached to the number.
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For example, -max 4m selects 4000000 bytes. If more
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space is needed, temporary files will be used.
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-nodither Do not use dithering in color quantization.
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By default, Floyd-Steinberg dithering is applied when
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quantizing colors, but on some images dithering may
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result in objectionable "graininess". If that
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happens, you can turn off dithering with -nodither.
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-nodither is ignored unless you also say -colors N.
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-onepass Use one-pass instead of two-pass color quantization.
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The one-pass method is faster and needs less memory,
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but it produces a lower-quality image. -onepass is
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ignored unless you also say -colors N. Also,
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the one-pass method is always used for gray-scale
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output (the two-pass method is no improvement then).
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-verbose Enable debug printout. More -v's give more printout.
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or -debug Also, version information is printed at startup.
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HINTS
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Color GIF files are not the ideal input for JPEG; JPEG is really intended for
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compressing full-color (24-bit) images. In particular, don't try to convert
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cartoons, line drawings, and other images that have only a few distinct
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colors. GIF works great on these, JPEG does not. If you want to convert a
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GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
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to get a satisfactory conversion. -smooth 10 or so is often helpful.
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Avoid running an image through a series of JPEG compression/decompression
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cycles. Image quality loss will accumulate; after ten or so cycles the image
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may be noticeably worse than it was after one cycle. It's best to use a
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lossless format while manipulating an image, then convert to JPEG format when
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you are ready to file the image away.
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The -optimize option to cjpeg is worth using when you are making a "final"
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version for posting or archiving. It's also a win when you are using low
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quality settings to make very small JPEG files; the percentage improvement
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is often a lot more than it is on larger files.
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When making images to be posted on Usenet, we recommend using cjpeg's option
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-restart 1. This option limits the damage done to a compressed image by
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netnews transmission errors.
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The default memory usage limit (-maxmemory) is set when the software is
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compiled. If you get an "insufficient memory" error, try specifying a smaller
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-maxmemory value, even -maxmemory 0 to use the absolute minimum space. You
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may want to recompile with a smaller default value if this happens often.
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On machines that have "environment" variables, you can define the environment
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variable JPEGMEM to set the default memory limit. The value is specified as
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described for the -maxmemory switch. JPEGMEM overrides the default value
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specified when the program was compiled, and itself is overridden by an
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explicit -maxmemory switch.
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On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
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use. (Extended or expanded memory is also used if available.) Most
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DOS-specific versions of this software do their own memory space estimation
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and do not need -maxmemory.
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djpeg with two-pass color quantization requires a good deal of memory; on
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MS-DOS machines it may run out of memory even with -maxmemory 0. In that case
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you can still decompress, with some loss of image quality, by specifying
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-onepass for one-pass quantization.
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If more space is needed than will fit in the available main memory (as
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determined by -maxmemory), temporary files will be used. (MS-DOS versions
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will try to get extended or expanded memory first.) The temporary files are
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often rather large: in typical cases they occupy three bytes per pixel, for
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example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough
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free disk space, leave out -optimize (for cjpeg) or specify -onepass (for
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djpeg). On MS-DOS, the temporary files are created in the directory named by
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the TMP or TEMP environment variable, or in the current directory if neither
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of those exist. Amiga implementations put the temp files in the directory
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named by JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with
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adequate free space.
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