Overlay blends Multiply and Screen conditionally on the base layer: darks get darker, lights get lighter, midtones are unaffected

Overlay: f(a,b) = 2ab if a<0.5, else 1-2(1-a)(1-b). When the base layer (a) is below 0.5 (dark), it applies Multiply-like darkening. When a ≥ 0.5 (light), it applies Screen-like lightening. When a = 0.5 (mid-grey), the top layer has no effect. Applied to the same image twice, Overlay creates an S-curve that increases contrast. Unlike Multiply and Screen, Overlay is NOT commutative: swapping base and blend layers gives a different result (that mode is called Hard Light).

Examples

Overlay a 50% grey on any image: no change. Overlay a bright (0.9) colour on a dark base (0.2): 2×0.9×0.2=0.36 — moderate darkening. Overlay the same bright colour on a light base (0.8): 1-2(0.1)(0.2)=0.96 — strong brightening.

Assessment

Explain why Overlay creates an S-curve when applied to a copy of itself. Then explain why Overlay is useful for adding contrast to a midtone-heavy image without blowing out highlights or crushing shadows.