Volumetric particles

Question

I'm toying with the idea of volumetric particles. By 'volumetric' I don't mean actually 3D model per particle - usually it's more expensive and harder to blend with other particles. What I mean is 2D particles that will look as close as possible to be volumetric. Right now what I/we have tried is particles with additional local Z texture (spherical for example), and we conduct the alpha transparency according to the combination of the alpha value and the closeness by Z which is improved by the fact that particle does not have a single planar Z. I think a cool add would be interaction with lighting (and shadows as well), but here the question is how will the lighting formula look like (taking transparency into account, let's assume that we are talking about smoke and dust/clouds and not additive blend) - any suggestions would be welcomed.

I also though about adding normal so I can actually squeeze all in two textures:

• Diffuse & Alpha texture.
• Normal & 256 level precision Z channel texture.

I ask this question to see what other directions can be thought of and to get your ideas regarding the proper lighting equation that might be used.

Answer 1

It sounds like you are asking for information on techniques for the simulation of participating media: "Participating media may absorb, emit and/or scatter light. The simplest participating medium only absorbs light. That means that light passing through the medium is attenuated depending on the density of the medium."

Here are some links to some example images and to Frisvad, Christensen, Jensen's the SIGGRAPH 2007 paper (including the PDF).

Answer 2

A nice paper on using spherical billboards to represent volumetric effects:

http://www.iit.bme.hu/~szirmay/firesmoke_link.htm

Doesn't handle particpating media, though.

Answer 3

See Volume Rendering and Voxel.