Tackling alpha blend in OpenGL for better performance

Question

Since having blends is hitting perfomance of our game, we tried several blending strategies for creating the "illusion" of blending. One of them is drawing a sprite every odd frame, resulting in the sprite being visible half of the time. The effect is quit good. (You'd need a proper frame rate by the way, else your sprite would be noticeably flickering)

Despite that, I would like to know if there are any good insights out there in avoiding blending in order to better the overal performance without compromising (too much) of the visual experience.

Answer 1

Is it the actual blending that's killing your performance? (i.e. video memory bandwidth)

What games commonly do these days to handle lots of alpha blended stuff (think large explosions that cover whole screen): render them into a smaller texture (e.g. 2x2 smaller or 4x4 smaller than screen), and composite them back onto the main screen.

Doing that might require rendering depth buffer of opaque surfaces into that smaller texture as well, to properly handle intersections with opaque geometry. On some platforms (consoles) doing multisampling or depth buffer hackery might make that a very cheap operation; no such luck on regular PC though.

See article from GPU Gems 3 for example: High-Speed, Off-Screen Particles. Christer Ericson's blog post overviews a lot of optimization approaches as well: Optimizing the rendering of a particle system

Answer 2

Excellent article here about rendering particle systems quickly. It covers the smaller off screen buffer technique and suggest quite a few other approaches.

You can read it here

Answer 3

It is not quite clear from your question what kind of application of blending hits your game's performance. Generally blending is blazingly fast. If your problems are particle system related, then what is most likely to kill framerate is the number and size of particles drawn. Particularly lots of close up (and therefore large) particles will require high memory bandwidth and fill rate of the graphics card. I have implemented a particle system myself, and while I can render tons of particles in the distance, I feel the negative impact of e.g. flying through smoke (that will fill the entire screen because the viewer is amidst of it) very much on weaker hardware.