shaderoverlay

Shaderoverlay refers to a technique in computer graphics where a shader blends an overlay graphic or color with a base image or scene to produce a final image. In practice it is usually implemented as a fragment (pixel) shader that samples a base texture or render target and an overlay texture or uniform color, then combines the two using a blending operation.

Common uses include post-processing effects such as light leaks, film grain, damage overlays in games, weather or glare effects, and UI overlays. Overlay textures are often tiled or tinted, and masks can control where the overlay appears or how strong it is.

Implementation details involve inputs such as the base color (texture) and the overlay (texture or uniform). Blending modes include multiply, screen, overlay, additive, or custom equations. Alpha handling matters: overlay may have its own alpha, and premultiplied alpha issues can arise. Color space considerations are important: perform blending in linear space for accuracy, then convert to sRGB for display. Performance considerations include that overlays computed in a single pass are generally fast, while more complex overlays might require multi-pass or separate render targets. Ensuring resolution independence and proper filtering of the overlay texture is also important.

In modern engines, shader overlays are integrated as post-processing materials or screen-space effects. Unity uses image effects or the post-processing stack; Unreal uses post-process materials; Godot uses canvas_item or viewport shaders. They can be authored in GLSL, HLSL, Metal, or Vulkan shading languages, depending on platform.

Variants include procedural overlays generated by noise, gradients, or per-pixel calculations; overlays driven by gameplay state; and separable overlays that tint or highlight parts of the scene.