User:Lgd

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

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