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February 2008
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2008.02.18 12:29 "Re: bug? JPEGQUALITY seems not to be read correctly", by <jcupitt@gmail.com>
On 18/02/2008, Gerben Vos <Gerben@zylab.com> wrote: > Some rather sophisticated code is available on the Internet to reconstruct > the quality value from the tables in a JPEG. Another approach might be to make TIFFTAG_JPEGQUALITY into a real tag. This could be a simpler solution, and almost as good. I noticed that TTN2 (the revised JPEG-in-tiff note) obsoleted tags 512 -- 521. Perhaps one of these could be reused? RESTART interval, perhaps (it's a TIFF_SHORT): #define TIFFTAG_JPEGRESTARTINTERVAL 515 /* !restart interval length */ So the procedure on new-style jpeg load and save might be: save -- note the value of TIFFTAG_JPEGQUALITY in tag 515 load -- if present (and in the range 25 - 100, perhaps), use tag 515 to set the default value for TIFFTAG_JPEGQUALITY Old-style jpeg-in-tiff would be unaltered. Of course Q settings are not transferrable between jpeg implementations, but it sounds like the Q guessers are not exact either. It seems to me that saving the Q setting as a hint would not be any worse in practice than adding a Q guesser and would be far simpler (just a few lines of code). On the downside of course having tags that change meaning depending on context is very, very ugly :-( The same pseudo-tag problem will affect other compressors I suppose, though JPEG is perhaps the most extreme one (huge changes in filesize with different values). John