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February 2008
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2008.02.15 15:14 "Re: bug? JPEGQUALITY seems not to be read correctly", by Ed Grissom
> On 15/02/2008, Grissom, Ed <ed.grissom@intergraph.com> wrote: > > There is no place in the header for a "quality" setting. This is > > not something that is in the JPEG spec or the TIFF spec. What is > > in both specs is a quality table (q-table). This q-table is > > generated from the q-setting. > > > > It is difficult to recreate the q-setting that was used to create > > a q-table. > > Ah OK, thanks. But tiff could have a tag in the header which hinted at > the Q setting which was used to make the table, couldn't it? It would > make the operation of things like tiffcp more predictable. Certainly it could. And I agree that it would make much more sense to users. We just don't have such a tag defined yet. In my own JPEG code (non-LibTIFF) have had good success faking a q-setting by looking at one certain value in the table. It has a default value of 100, and by seeing how it is changed in the actual table, I can usually compute a reasonably close version of the q-setting that was used to create the table. Of course, if a custom q-table is used, I am way off the mark, but if it is a scaled version of the default table, it works pretty well. -- ed grissom ed.grissom@intergraph.com