> Then, when polygon B is drawn, the green is mixed in according to the 
> pixel's current alpha value. The alpha of the new polygon is then added to 
> the existing alpha. When all polygons have been drawn, the final alpha 
> value is used to mix in the background.

The "background" is not some fixed bitmap though, the "background" is just 
whatever happens to be behind the triangle you are drawing, probably another 
triangle, which has another triangle behind it, etc.  Your method requires 
you to store a "background" framebuffer for every group of joined triangles 
that are to be rendered.  Anyway, the graphics card doesn't know which 
triangles are meant to be joined, it just chugs through a huge list of 
triangles, how is it going to know when to pause and do the alpha mixing 
stage?

> Well, no. You'd want to smooth it somehow, without slowing the computation 
> to the speed of molasses...

Yeh you need something like an isosurface, where the "strength" is somehow 
related to the distance to neighbouring particles.  Is relatively easy to 
both raytrace this directly or create a mesh from.

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