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April 2008
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2008.05.05 12:08 "Re: (no subject)", by Gerben Vos
Andrew Wan wrote: > I think my HP G95 scanner creates uncompressed tiff files. > It's on default setting 150DPI, 24bit color. > A single A4 page scanned is 6,518KB tif file (1275x1743). Yes, that's probably uncompressed. You could check that using tiffinfo or tiffdump. > If I use "tiffcp -c lzw" will this compress using > default/standard settings? Is LZW loseless? Yes. Yes. But for photographic images, JPEG with a good quality factor doesn't lose much, and compresses much better. -c jpeg uses 75, which should be good enough, but if you're feeling paranoid, try -c jpeg:95 . The setting that's really optimal differs per image. Don't use -c jpeg:r unless you know what you're doing; the YCbCr colour space usually works best for JPEG. > What are the optional options in "-c lzw[:opts]"? It's the predictor value. Default is 1 (no predictor). 2 means horizontal differencing, which I would recommend if you're using LZW or Deflate with photographic RGB images. 3 is floating point horizontal differencing; don't use that unless you know what you're doing. Compression stays lossless, but the compression may improve. (PNG, for example, has five predictors and compressors usually have an "auto" setting which tries them all.) http://www.awaresystems.be/imaging/tiff/tifftags/predictor.html Gerben.