William Edwards, Coder

When playing with game engine ideas, I struggled with the fact that my laptops all had very very bad integrated chips by Intel.  Yeap, those ones.

I wanted a nice cartoony fluffy clouds and bobbly trees, but all those spheres were taking just too many polygons.

The solution I prototyped was to bill-board spheres… complete with depth.

The billboard image was actually a depth-map.  I was using a special value to indicate no texel and do a discard in the fragment shader.

To get nice edges on the spheres I did it in two passes; first pass, draw all opaque texels inside the circle of the billboard, setting depth; second pass, draw all semi-transparent texels on the edges of the spheres without setting depth (i.e. its at billboard depth).

I found that I could draw almost a thousand such spheres at a good enough framerate on the worst cards.  On more average ATI and NVidia cards the framerate was nicely in the hundreds.  So this is a very efficient technique.

Shadows can be done by sampling the depth buffer in a second pass and using a 1D texture that is the cross-section of the sphere.  I didn’t prototype that bit; its an exercise for the dedicated reader ;)