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@@ -0,0 +1,292 @@ +.TH CJPEG 1 "20 March 1998" +.SH NAME +cjpeg \- compress an image file to a JPEG file +.SH SYNOPSIS +.B cjpeg +[ +.I options +] +[ +.I filename +] +.LP +.SH DESCRIPTION +.LP +.B cjpeg +compresses the named image file, or the standard input if no file is +named, and produces a JPEG/JFIF file on the standard output. +The currently supported input 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.) +.SH OPTIONS +All switch names may be abbreviated; for example, +.B \-grayscale +may be written +.B \-gray +or +.BR \-gr . +Most of the "basic" switches can be abbreviated to as little as one letter. +Upper and lower case are equivalent (thus +.B \-BMP +is the same as +.BR \-bmp ). +British spellings are also accepted (e.g., +.BR \-greyscale ), +though for brevity these are not mentioned below. +.PP +The basic switches are: +.TP +.BI \-quality " N" +Scale quantization tables to adjust image quality. Quality is 0 (worst) to +100 (best); default is 75. (See below for more info.) +.TP +.B \-grayscale +Create monochrome JPEG file from color input. Be sure to use this switch when +compressing a grayscale BMP file, because +.B cjpeg +isn't bright enough to notice whether a BMP file uses only shades of gray. +By saying +.BR \-grayscale , +you'll get a smaller JPEG file that takes less time to process. +.TP +.B \-optimize +Perform optimization of entropy encoding parameters. Without this, default +encoding parameters are used. +.B \-optimize +usually makes the JPEG file a little smaller, but +.B cjpeg +runs somewhat slower and needs much more memory. Image quality and speed of +decompression are unaffected by +.BR \-optimize . +.TP +.B \-progressive +Create progressive JPEG file (see below). +.TP +.B \-targa +Input file is Targa format. Targa files that contain an "identification" +field will not be automatically recognized by +.BR cjpeg ; +for such files you must specify +.B \-targa +to make +.B cjpeg +treat the input as Targa format. +For most Targa files, you won't need this switch. +.PP +The +.B \-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 +.B \-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.) +.PP +.B \-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 +.B not +recommended for normal use; the compressed file size goes up dramatically for +hardly any gain in output image quality. +.PP +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 +.B \-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. +.B 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 +.B \-baseline +if you need to ensure compatibility at low quality values.) +.PP +The +.B \-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. +.B Caution: +progressive JPEG is not yet widely implemented, so many decoders will be +unable to view a progressive JPEG file at all. +.PP +Switches for advanced users: +.TP +.B \-dct int +Use integer DCT method (default). +.TP +.B \-dct fast +Use fast integer DCT (less accurate). +.TP +.B \-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. +.TP +.BI \-restart " N" +Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is +attached to the number. +.B \-restart 0 +(the default) means no restart markers. +.TP +.BI \-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. +.TP +.BI \-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, +.B \-max 4m +selects 4000000 bytes. If more space is needed, temporary files will be used. +.TP +.BI \-outfile " name" +Send output image to the named file, not to standard output. +.TP +.B \-verbose +Enable debug printout. More +.BR \-v 's +give more output. Also, version information is printed at startup. +.TP +.B \-debug +Same as +.BR \-verbose . +.PP +The +.B \-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 +.B \-restart 1 +for images that will be transmitted across unreliable networks such as Usenet. +.PP +The +.B \-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. +.PP +Switches for wizards: +.TP +.B \-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 +.B \-baseline +and +.B \-progressive +together.) +.TP +.BI \-qtables " file" +Use the quantization tables given in the specified text file. +.TP +.BI \-qslots " N[,...]" +Select which quantization table to use for each color component. +.TP +.BI \-sample " HxV[,...]" +Set JPEG sampling factors for each color component. +.TP +.BI \-scans " file" +Use the scan script given in the specified text file. +.PP +The "wizard" switches are intended for experimentation with JPEG. If you +don't know what you are doing, \fBdon't use them\fR. These switches are +documented further in the file wizard.doc. +.SH EXAMPLES +.LP +This example compresses the PPM file foo.ppm with a quality factor of +60 and saves the output as foo.jpg: +.IP +.B cjpeg \-quality +.I 60 foo.ppm +.B > +.I foo.jpg +.SH HINTS +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 +.BR cjpeg 's +.B \-quality +and +.B \-smooth +options to get a satisfactory conversion. +.B \-smooth 10 +or so is often helpful. +.PP +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. +.PP +The +.B \-optimize +option to +.B 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, +.B \-optimize +mode is always selected when generating progressive JPEG files.) +.SH ENVIRONMENT +.TP +.B JPEGMEM +If this environment variable is set, its value is the default memory limit. +The value is specified as described for the +.B \-maxmemory +switch. +.B JPEGMEM +overrides the default value specified when the program was compiled, and +itself is overridden by an explicit +.BR \-maxmemory . +.SH SEE ALSO +.BR djpeg (1), +.BR jpegtran (1), +.BR rdjpgcom (1), +.BR wrjpgcom (1) +.br +.BR ppm (5), +.BR pgm (5) +.br +Wallace, Gregory K. "The JPEG Still Picture Compression Standard", +Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44. +.SH AUTHOR +Independent JPEG Group +.SH BUGS +Arithmetic coding is not supported for legal reasons. +.PP +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.) +.PP +Not all variants of BMP and Targa file formats are supported. +.PP +The +.B \-targa +switch is not a bug, it's a feature. (It would be a bug if the Targa format +designers had not been clueless.) +.PP +Still not as fast as we'd like. |