I have a lot of billboarded 3D spheres (they have a heightmap so work properly with depth and penetration; although each is made of just a quad (with slight distortion), they look and feel and are true 3D spheres): http://www.youtube.com/watch?v=88idbFlvQV0
The choice of using billboards is for throughput on low-end hardware.
How can I effectively cast shadows from these? It occurs to me that using proper shadow-volumes will take a lot of fill-rate and complexity. I am fill-rate sensitive.
I was wondering if there is any way to billboard the shadows cast by my billboards?