You are fast! :D
Ok I think I can do this for the other operations.
Just to share another example for the blending mode "Screen",
whose formula is: f(a,b) = (a+b) - (a*b).
R = (1-alphaB)*A + alphaA*alphaB*Blend(B/alphaB,A/alphaA) +
(1-alphaA)*B
Let's deal with Blend first:
Blend(B/alphaB,A/alphaA) = Screen(B/alphaB,A/alphaA) =
B/alphaB + A/alphaA - (B/alphaB) * (A/alphaA) =
(B*alphaA + A*alphaB - B*A) / (alphaA * alphaB)
And finally:
R = (1-alphaB)*A + (B*alphaA + A*alphaB - B*A) + (1-alphaA)*B
which can be further simplified to:
A + B - A*B
That was wicked!
I will write the full formulas as comments for the functions
but inside I will use reduced math for faster calculations.
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