Jaakko Hyvätti wrote:
A' = As + (1 - As) * Ad
R' = As * Rs + (1 - As) * Ad
Example:
As = 0.5
Rs = 0.8
Ad = 0
Rd = 0.2
R' = As * Rs + (1 - As) * Ad = 0.5 * 0.8 + 0.5 * 0.2
Presumably that should be Rd there (and above too)?
= 0.5
This is incorrect, the correct R' is obviously 0.8, because Ad = 0, and Rd
should not affect the resulting colour at all! The solution is to compute
the R' by weighing the Rs and Rd with their alphas relative to resulting
alpha:
R' = As/A' * Rs + (1-As/A1) * Rd
R' = 0.5/0.5 * 0.8 + (1-0.5/0.5) * 0.2
= 1 * 0.8 + 0 * 0.2
= 0.8
Are you suggesting this as an extension which will be forever restricted
to software rendering? I think it would be incredibly hard for a hw
vendor to pick this up. Weighting sounds like an expensive operation,
and maybe worse all graphic chips (well I can only guess...) do the
alpha and color blend simultaneously, which obviously isn't going to work.
Roland
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