But it cannot be applied to all color models. For example, Lab color would produce results that were surprising.

An alternative model is that at every point in each element to be combined for transparency there is an associated color and opacity between 0 and 1. For each color channel, you might work with this model: if a channel with intensity and opacity overlays a channel with intensity and opacity the result will be a channel with intensity equal to , and opacity . Each channel must be