A decal is a special type of graphic 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 fashions:
- Manually in a map editor. This allows the placement of graphical elements to a wall that could not have it otherwise, such as a poster spread across two different wall textures. This reduces the need to define texture variants.
- Automatically by the engine, typically to leave blood stains, bullet holes and scorch marks.
Decals were initially planned to be included in the Doom engine, and even advertised in the press release announcing the game.