What is a OverlayShader

A combination of a SurfaceShader and MaskShader that blends with the layer below by itself.

An OverlayShader is used in an OverlayMaterial. It has its own setup macros and layer variables. See OverlayShader reference.

An OverlayShader is a combination of a SurfaceShader and a MaskShader. It handles its own blending and can't be used as a Blend input in a Mask.

An OverlayShader can read material properties from the layer below using the LAYER_BELOW_* layer variables. You create the base material, blend it with the layer below using a mask, then output the result using the LAYER_OUT_* layer variables. See the OverlayShader reference for the full list, or Writing OverlayShaders for an example.