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CJPEG !!!CJPEG NAME SYNOPSIS DESCRIPTION OPTIONS EXAMPLES HINTS ENVIRONMENT SEE ALSO AUTHOR BUGS ---- !!NAME cjpeg - compress an image file to a JPEG file !!SYNOPSIS __cjpeg__ [[ ''options'' ] [[ ''filename'' ] !!DESCRIPTION __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.) !!OPTIONS All switch names may be abbreviated; for example, __-grayscale__ may be written __-gray__ or __-gr__. Most of the __-BMP__ is the same as __-bmp__). British spellings are also accepted (e.g., __-greyscale__), though for brevity these are not mentioned below. The basic switches 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 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 __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 -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 -max 4m__ selects 4000000 bytes. If more space is needed, temporary files will be used. __-outfile__ ''name'' Send output image to the named file, not to standard output. __-verbose__ Enable debug printout. More __-v__'s give more output. Also, version information is printed at startup. __-debug__ Same as __-verbose__. 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 don't use them__. These switches are documented further in the file wizard.doc. !!EXAMPLES This example compresses the PPM file foo.ppm with a quality factor of 60 and saves the output as foo.jpg: __cjpeg -quality__ ''60 foo.ppm'' ____ ''foo.jpg'' !!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 __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 __-optimize__ mode is always selected when generating progressive JPEG files.) !!ENVIRONMENT __JPEGMEM__ If this environment variable is set, its value is 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__. !!SEE ALSO djpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)__ ppm__(5), pgm(5) Wallace, Gregory K. !!AUTHOR Independent JPEG Group !!BUGS Arithmetic coding is not supported for legal reasons. 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.) Not all variants of BMP and Targa file formats are supported. The __-targa__ switch is not a bug, it's a feature. (It would be a bug if the Targa format designers had not been clueless.) Still not as fast as we'd like. ----
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