Design Nomenclature / Digital Model Making
Alpha Compositing
Also called compositing, the over operator, Porter-Duff, blending
Combining images by coverage. The over operator lays one atop another as c = a + b(1−α), which is associative but not commutative, and it underlies every layer stack there has ever been. It assumes alpha means coverage, so premultiplied values compose directly and straight ones need dividing out first.
In practice
Porter and Duff described twelve operators in 1984 and over is the only one most people ever name. Compositing in gamma-encoded space is the other half of why edges go dark: the arithmetic assumes linear light and nothing warns you that it isn't.