So I implemented the overlay system described previously. On the up side, I'm now rendering the water effect at about 35 FPS as opposed to my previous ~27 FPS. Although the native code was refactored somewhat I'm pretty certain the gain comes from using a separate thread. While the GPU is rendering the scene I'm busy preparing the next frame of water. Sweet!
The bitter part is that as soon as I add another layer on top of the water, performance drops drastically (~10 FPS for my test case). There's no way I can render fog on top of water with this system. It's too slow to do all this image compositing on the CPU. I need to leverage the GPU...
I've decided that the technology demo will have to ship without the fog effect. When I come back to this problem I'll experiment with making a good looking fog effect using alpha mapped OpenGL textures.
