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Subject: RE: [Tiff] bug? JPEGQUALITY seems not to be read correctly
Date: Fri, 15 Feb 2008 09:14:32 -0600
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Thread-Topic: [Tiff] bug? JPEGQUALITY seems not to be read correctly
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From: "Grissom, Ed" <ed.grissom@intergraph.com>
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Status:   



> 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.=20

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.=20


--=20
ed grissom
ed.grissom@intergraph.com


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