A decal is a special type of graphics that is applied atop another surface, such as a wall. For technical reasons related to the design of the Doom rendering engine, applying decals to floors and ceilings in Doom is harder to implement than applying them to walls.
Decals can be used in two different fashion:
- Manually in a map editor. This allows to place graphical elements to a wall that could not have it otherwise, such as for example a poster spread across two different wall textures. This allows to reduce the need to define texture variants.
- Automatically by the engine, typically to leave blood stains, bullet holes and scorch marks.