The source image in an ARGB format may have its RGB component already premultiplied
with the Alpha (AR’G’B’) where:
R’ = A*R
G’ = A*G
B’ = A*B
The processing in that case is as follows:
- Color component of premultiplied layer are multiplied with the global alpha
if global_A is not equal to zero
- Color component of the composed underlying layer are multiplied with
(1-A*Global_A)
Note: L1 (Layer-1) cannot be pre-multiplied if the
overlay output is driving the display since the layer (or the background color)
defines the bottom most layer. In this case, L1 alpha is 1 always.