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Diffstat (limited to 'jpeg/usage.txt')
| -rw-r--r-- | jpeg/usage.txt | 57 |
1 files changed, 38 insertions, 19 deletions
diff --git a/jpeg/usage.txt b/jpeg/usage.txt index d06e3fc27..b20484bb2 100644 --- a/jpeg/usage.txt +++ b/jpeg/usage.txt @@ -47,7 +47,7 @@ or 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 grayscale format), BMP, Targa, and RLE (Utah Raster Toolkit +PGM (PBMPLUS grayscale format), BMP, GIF, Targa, and RLE (Utah Raster Toolkit format). (RLE is supported only if the URT library is available, which it isn't on most non-Unix systems.) cjpeg recognizes the input image format automatically, with the exception of some Targa-format files. You have to @@ -73,10 +73,10 @@ The basic command line switches for cjpeg are: -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. + BMP or GIF file, because cjpeg isn't bright enough to + notice whether a BMP or GIF file uses only shades of + gray. By saying -grayscale, you'll get a smaller + JPEG file that takes less time to process. -rgb Create RGB JPEG file. Using this switch suppresses the conversion from RGB @@ -317,10 +317,17 @@ The basic command line switches for djpeg are: is specified, or if the JPEG file is grayscale; 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. + -gif Select GIF output format (LZW compressed). + 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. + + -gif0 Select GIF output format (uncompressed). + 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 @@ -411,10 +418,6 @@ 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 -(now expired). -(Conversion of GIF files to JPEG is usually a bad idea anyway.) - HINTS FOR DJPEG @@ -440,10 +443,6 @@ 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 (now expired), djpeg produces uncompressed GIF -files. These are larger than they should be, but are readable by standard GIF -decoders. - HINTS FOR BOTH PROGRAMS @@ -571,13 +570,33 @@ The image can be losslessly cropped by giving the switch: -crop WxH+X+Y Crop to a rectangular subarea of width W, height H starting at point X,Y. +Crop extension: The width or height parameters can be made larger than the +source image. In this case the extra area is filled in with zero (neutral +gray). A larger width parameter has two more options: Attaching an 'f' +character ("flatten") to the width number will fill in the extra area with +the DC of the adjacent block, instead of gray out. Attaching an 'r' +character ("reflect") to the width number will fill in the extra area with +repeated reflections of the source region, instead of gray out. + A complementary lossless-wipe option is provided to discard (gray out) data inside a given image region while losslessly preserving what is outside: -wipe WxH+X+Y Wipe (gray out) a rectangular subarea of width W, height H starting at point X,Y. -Attaching an 'f' character ("flatten") to the width number will fill -the region with the average of adjacent blocks, instead of gray out. +Attaching an 'f' character ("flatten") to the width number will fill the +region with the average of adjacent blocks, instead of gray out. In case +the wipe region and outside area form two horizontally adjacent rectangles, +attaching an 'r' character ("reflect") to the width number will fill the +region with repeated reflections of the outside area, instead of gray out. + +Another option is lossless-drop, which replaces data at a given image +position by another image: + -drop +X+Y filename Drop another image + +Both source images must have the same subsampling values. It is best if +they also have the same quantization, otherwise quantization adaption occurs. +The trim option can be used with the drop option to requantize the drop file +to the source file. Other not-strictly-lossless transformation switches are: |