Decal

A texture with transparency on a floating polygon, so that it appears to be painted directly on the underlying mesh. When placing decals, care must be taken to avoid Z-Fighting. Decals are commonly used for effects such as bullet marks, graffiti, broken edges, and tire tracks.

For hard-edged decals use "alpha test" because this is usually cheaper to render. If the decals must be softer (tire tracks in a racing game) then use alpha blend instead. See also MultiTexture.

Another common use is for blob shadows, which are soft dark circles under a model. Blob shadows are usually faster to render than using shadow-casting lights, and can replicate the look of soft shadowing.

Blob shadow example. Image by Eric Chadwick.

Tutorials

Using Decals for Destroyed Structures - from the CryENGINE Art Asset Creation Guide - Using alpha blended decals for broken concrete edges.

Broken concrete for CE3 (Tutorial) - by Valdemar 'sltrOlsson' Nyhlén - Video tutorials using Zbrush and Maya to generate tiled maps for broken concrete edges.

Fallout 3 – Edges from Simon Schreibt - Investigating decal usage in Fallout 3.

Decals in Star Citizen - By Kristóf 'Obscura' Lovas - Text based tutorial for decals, as they are used in the game.

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