2005.11.02 16:58 "[Tiff] Writing bilevel and transparent TIFFs efficiently", by Rupali Holmes

2005.11.07 21:09 "Re: [Tiff] Writing bilevel and transparent TIFFs efficiently", by Joris Van Damme

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
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