Over Operator

For CB and CF, which are not premultiplied:
- C’ = aB(1-aF)*CB + aF*CF
- a = aB(1-aF) + aF

For C’B and C’F, which are premultiplied:
- C’ = (1-aB)*C’F + C’B
- a = aB(1-aF) + aF

Assumption:

coverages of B and F are uncorrelated

for each pixel

For CB and CF, which are not premultiplied: