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January 2008
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2008.01.10 22:01 "Re: LibTiff..Reading partial strips", by Bob Friesenhahn
On Thu, 10 Jan 2008, Nikhil Shirahatti wrote: > I havent tried it..but the cautionary note that " It is not always possible > to do so due to decompression constraints" scares me. I also read on the > archive that "that *strip* *chopping* only works for uncompressed IFDs.". > How common(or uncommon) is this? When would one use compressed IFD's? > The tiff's we get can be compressed or uncompressed formats and we have to > support them all. :) It is quite likely that other folks here know a lot more about this than I do so hopefully they will chime in. I don't know what a compressed IFD might be since and IFD is just a structure. My impression is that libtiff decompresses into a buffer and then uses part (or all) of that buffer. Some compression algorithms/options either require decompressing a huge amount of data (e.g. the whole strip), or else using simultaneous streaming decompression at the same time that data is consumed by the rest of the library. If libtiff decompresses in blocks and does not stream (as I suspect) then sometimes it will need to decompress the entire strip before it can be used. G4 FAX compression with a strip-per-page is likely a good example of that. Bob ====================================== Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/