ABSTRACT
Decals are often implemented as textured quads that are placed on top of the scene geometry. While this implementation works well enough in many cases, it can also provide some challenges. Using decals as textured quads can cause Z-fighting problems. The underlying geometry may not be flat, causing the decal to cut into the geometry below it. The decal may also overhang an edge, completely ruining its effect. Dealing with this problem often involves clipping the decal to the geometry or discarding it entirely upon detecting the issue. Alternatively, very complex code is needed to properly wrap the decal around arbitrary meshes, and access to vertex data is required. On a PC this could mean that system-memory copies of geometry are needed to maintain good performance. Furthermore, disturbing discontinuities can occur, as in the typical case of shooting a rocket into a corner and finding that only one of the walls got a decal or that the decals do not match up across the corner. This article proposes a technique that overcomes all of these challenges by projecting a decal volume onto the underlying scene geometry, using the depth buffer.
