In some situations, it is desirable to consider both Alpha inputs to the compositor in determining the weight to be given to each of the two RGB inputs to the compositing operation. This is accomplished by providing two weighting transfer functions

f(Alpha₁,Alpha₂) = WeightingFactor_n

one for each of the two RGB inputs to the compositor.

In the degenerate (and common) case where simple additive compositing (RGB₁*Alpha₁)+(RGB₂* (1-Alpha₁)) is desired, identity weighting transfer functions can be specified to just pass through the Alpha ₁and (1-Alpha ₁) values (or Alpha ₁and Alpha ₂values) to be the weighing factors for the RGB inputs.

For ease of specification of these transfer functions and for maximum flexibility, each of these weighting transfer functions is defined as a look-up table with inputs Alpha₁ and Alpha₂. Each LUT contains 8-bit integer values which are then normalized to the range 0.0 to 1.0 by dividing by 255 (2⁸-1).

See Section FF.2.3.3.2 “Internal Structure of RGB and RGBA Compositor Components” in PS3.4 for details on the use of these weighting transfer functions, and see Section XXX.5 “Compositing and the Use of Weighting Transfer Functions” in PS3.17 for clinical applications.