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|TIFF and LibTiff Mailing List Archive|
LibTiff Mailing List
2006.09.17 10:39 "Re: alpha in Grayscale or Palette", by Graeme Gill
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.