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2008.02.06 18:40 "[Tiff] tiff library and exif", by Jamie L. Finch
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2008.02.06 20:03 "[Tiff] Bug in tif_print.c for TIFFTAG_REFERENCEBLACKWHITE", by Edward Lam
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2008.02.15 14:36 "[Tiff] bug? JPEGQUALITY seems not to be read correctly", by John
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2008.02.15 14:57 "RE: [Tiff] bug? JPEGQUALITY seems not to be read correctly", by Ed Grissom
- 2008.02.15 15:02 "Re: [Tiff] bug? JPEGQUALITY seems not to be read correctly", by John
- 2008.02.15 17:51 "RE: [Tiff] bug? JPEGQUALITY seems not to be read correctly", by Bob Friesenhahn
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2008.02.15 14:57 "RE: [Tiff] bug? JPEGQUALITY seems not to be read correctly", by Ed Grissom
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2008.02.15 14:36 "[Tiff] bug? JPEGQUALITY seems not to be read correctly", by John
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2008.02.06 20:03 "[Tiff] Bug in tif_print.c for TIFFTAG_REFERENCEBLACKWHITE", by Edward Lam
2008.02.18 12:29 "Re: [Tiff] bug? JPEGQUALITY seems not to be read correctly", by John
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