diff options
Diffstat (limited to 'libjpegtwrp/cjpeg.1')
| -rw-r--r-- | libjpegtwrp/cjpeg.1 | 292 |
1 files changed, 0 insertions, 292 deletions
diff --git a/libjpegtwrp/cjpeg.1 b/libjpegtwrp/cjpeg.1 deleted file mode 100644 index d175a961c..000000000 --- a/libjpegtwrp/cjpeg.1 +++ /dev/null @@ -1,292 +0,0 @@ -.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. |