User:Lgd

The first assumption is that in the case where the background is opaque (i.e. $α_{b} = 1$ ), the over operator represents the convex combination of $a$ and $b$ : $C_{o} = α_{a} C_{a} + (1 - α_{a}) C_{b}$

The second assumption is that the operator must respect the associative rule: $(a ⊙ b) ⊙ c = a ⊙ (b ⊙ c)$