A
decal, similar to its namesake, is a special kind of texture that is overlaid or applied on top of another
texture, i.e. a textured face on a
brush. This can be things like stenciled signs, letters, and alphabet, blood stains, oil spills, drip stains, etc.
The majority of decals are authored to be in index-alpha mode, meaning it consists of a gradient of 255 colours from black to white that determines its opacity, and a final colour entry that determines the colour of the overall decal. This imparts realistic staining effect where the colour can be partly see-through to the base texture below.
Decals are applied to surfaces by placing
infodecal
entities into the level, on the surface of the brushwork where you want the decal to appear.
Only decal textures from
decals.wad wad file can be used with
infodecal
. Editing this file is fine when creating a mod – just add your own custom decals to it – but is off-limits when making custom maps for an existing game/mod (e.g. Half-Life Deathmatch or Counter-Strike maps.)
See also:
Tutorial: Decals: All You Need To Know.