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June 2011
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2011.06.06 16:55 "Re: Questions on TIFF LZW compression", by Bob Friesenhahn
On Sun, 5 Jun 2011, Thomas Richter wrote: > > First, how does LZW compression work for images that are not 8 bits/pixel? > As far as I read the specs, the input to the LZW compression is the > "raw" TIFF stripe buffer, but this leaves a couple of corner > cases open. I see that this type of compression works for any bit depth > that divides eight, or is a multiple of eight, but what > happens for 10 bits per pixel images? Is LZW really applied to the > bit-packed(!) input, or is the LZW algorithm applied to 16 bit > data, where 10 bits are packed (left-justified? right-justified?) into a > 16 bit word? In the former case, LZW will very unlikely give > much compression performance, of course. While I am not an expert at TIFF compression issues, I suspect that you are correct that the predictor has little value (and may cause harm) at depths other than 8 (and perhaps 16) bits since the compressor works on an octet stream. The value of the predictor depends on the nature of the image. When Adobe produced a preliminary application note about Photoshop CS2's added support for 16 and 24 bit floats, they also defined new types of predictors which work better for that sort of data. TIFF is not PNG. PNG has a huge number of filter mechanisms, but TIFF is more of a "working format" so it focuses less on achieving the absolute smallest file size. The ability of TIFF to directly store 10 bit data is already an accomplishment compared with other formats which would be forced to promote to 16-bit and then use compression to make the result smaller. Bob -- Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/