Decal

A decal is a texture stamped onto geometry after the fact, letting a game add transient or localized detail without baking it into the mesh or its texture. Projected decals cast a small box onto whatever surfaces it intersects and sample the decal texture there; they wrap around corners and follow the terrain. Shoot a wall and a bullet-hole decal appears exactly where the ray hit.

Early engines used decals as extra transparent polygons floating just above the surface, which caused z-fighting flicker (hence the depth bias). Deferred rendering made screen-space decals cheap by writing into the G-buffer. Decals keep detail dynamic and memory light, complementing the static look of lightmaps.