I finally have a decal implementation working in the graphics component of my engine project. This really was a simple implementation and I've had this on my to-do list for a while, but it just got buried by a ton of other things I wanted to get out of the way first. Anyway, here's a screenshot using some art I made some time ago (click to enlarge):
Decals have access to all the features of the engine's material system and are completely managed on the GPU, so there's no need to involve the physics engine to calculate decal positions for example.
Both 2D decals and volume decals are supported.
The scene in the screenshot I posted is still a work in progress, hence the absence of shadows, indirect/ambient lighting, anti-aliasing and the crude texturing. I'll probably post a journal entry when it's completed.