   A cool demo and my confusion
  Hi,I have finished some code to simulate the droplets flow on the glass. My work is: 1. Lattice my scene, and calculate the droplets' trace. 2. Using metaball to simulate the droplets. 3. Environment mapping simulate the ray tracing.I have finish 1 and 2. But it's inefficient. Because the metaball has a very hight CPU impact.So, I find a cool demo, you can see at:http://www.gamedev.net/community/forums/mod/journal/journal.asp?jn=263350&reply_id=2938860The rain effect makes me very confused.  First, I think the "droplet buffer" is the bump mapping texture, and it contains the normals. But,  the droplets can move randomly. He says he render tens to hundreds of droplets in a "droplet buffer", and initially filled with the color (128, 128, 255).  So there are only the (128, 128, 255) in the "droplet buffer", have no bump information. How does he generate droplets ( each with a position and a velocity ), updated in the CPU?Hope you clear my confusion.PS: My english is poor, and now I am learning it. If have syntax error, please indicate to me. Thanks.Ares   