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Message-ID: <450D25C4.1010005@argyllcms.com>
Date: Sun, 17 Sep 2006 20:39:00 +1000
From: Graeme Gill <graeme2@argyllcms.com>
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To: tiff@lists.maptools.org
Subject: Re: [Tiff] is there alpha
	component	presentin	GrayscaleorPalettecolorimage
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Status:   

Joris wrote:
> By applying the arbitrary weight 1, you can effectively hide the fact
> that you're applying arbitrary weights, and that's what you seem to be
> doing.

Why do you think that's an arbitrary weight ? Even the Alpha
component can be related back to a visible L*a*b* distance,
by the effect it has on the color, giving us the 4th dimension
co-ordinate. If you wanted to do lots of work, you could
even compute this by taking the color against a range of
opaque backgrounds, and using some statistical extraction
of the delta E (ie. median, largest etc.), that represents the
visible result of the Alpha value.

In any case, there's nothing inherently wrong with arbitrary weights
:- they're what you adjust to get an algorithm working in real
world situations.

> As to convertion to Lab, OK, I'm all for that. So now we do indeed have
> a 4D space. To make my point, let's encode L, a and b as their natural
> float value, and alpha as a float value ranging from 0 to 1. So, you're
> saying the difference between L*a*b*alpha (0,0,0,1) (which is
> non-transparent black) and (1,0,0,1) (which is non-transparent
> almost-black) is equal to the difference between (0,0,0,1) (again
> non-transparent black) and (0,0,0,0) (which is total transparency).

Hmm. Why would you do that ? The notional range of L* is 0 to 100,
and a* and b* are often taken to range from -128 to +128 (which
is somewhat arbitrary, but that's the nature of the colorspace
and the real world), so logically the Alpha should be scaled to
be 0 to 100 (at least as a starting point). If, in the light of
the result, fewer table values were to be given to variations
in Alpha, then the scale could be lowered somewhat.

> See, we apply weights (full transparent range is equally important as a
> difference of 1 in the L range). This is obvious if we pick very bad
> weights as we did here. So what are good weights? Full transparent range
> is equally important as a difference of 100 in the L range? Or is it
> twice as important as a difference of 100 in the L range? Why?

Do you expect everything to be handed to you on a plate ? :-)
Experiment and find out of course ! - then write it up, and publish
it in a suitable journal. That's what research is all about.

Graeme Gill.
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