I don't know about DOOM specifically, but many engines (including ours) will expose different options for what kind of normals you want. That way you can choose between flat normals, "round" normals, normal maps, etc. depending on what kind of particle system you're authoring. I'm not familar with the processes that our FX artists use for baking out normal maps, but I can ask them if you're curious.

There is also the bitsquid presentation on particle lighting which might interest you: http://roxlu.com/downloads/scholar/008.rendering.practical_particle_lighting.pdf

Sorry, I forgot to follow up on this. I talked to our lead FX artist, and at our studio they often just generate normal maps directly from their color maps using a tool like CrazyBump. There are a few cases where they will run a simulation in Maya, in which cases they may have Maya render out both color and normal maps to use for the particle system.

I've been thinking about lighting our particles in this manner as well.
Anyone have idea on a fast way to:

- Allocate a location for the quad in the atlas. I can easily see how this would be done on the CPU. But lets say you want to do it while rendering out the atlas itself. Ideas?

- If you want to have a normal map for the particle you'd probably still have to render out something like the HL2 basis. They don't seem to do this ... it seem like the lighting resolution would be too low. Thoughts?

Thanks!