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References: <7157931292F8D64D9747FBE4640C992F01C31FC5@redbaron.int.infograph.com><Pine.SOC.4.60.0511030930230.7597@blade.simplesystems.org><001c01c5e3d0$05d4c4e0$4dc2f451@dfgrrxsknr4e>
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Subject: Re: [Tiff] Writing bilevel and transparent TIFFs efficiently
Date: Mon, 7 Nov 2005 22:09:48 +0100
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Status:   

> > > It is possible to use an associated alpha channel with palette
> > > (PHOTOMETRIC_PALETTE) images,
> >
> > Is it? I'm not entirely convinced.
>
> I did not see anything in the specification which specifically forbids
> it.  However, it is much easer to read such files than to write them
> since writing them requires computing a palette which is conjoined
> with the alpha channel.  A very ugly thing indeed.

I think I agree with you, but I'm not sure...

What makes me uncertain is the bitdepth issue. It somehow seems 'wrong'
when the bitdepth should fit both a palette, which is essentially an
enumeration, and an alpha channel, which is essentially a discrete
value. Not that it's impossible, of course, but it seems arbitrary and
somehow 'wrong'. Of course, this problem exist only in the TIFF subset
that needs equal bits for all channels.

Add to that the problem you mention, that the writer has to dither
palette+alpha, which may not be a logical problem, but it sure is such
an operational problem that it almost borders the realm of the
logical...

If the rule is what we came to accept, that anything logical and
unambigious goes, then this is... not sure, it may be the gray zone, not
white nor the black, or it may be white or black, I just dare not
conclude.


Joris Van Damme
info@awaresystems.be
http://www.awaresystems.be/
Download your free TIFF tag viewer for windows here:
http://www.awaresystems.be/imaging/tiff/astifftagviewer.html

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