OpenGL sprites and point size limitation
- by Srdan
I'm developing a simple particle system that should be able to perform on mobile devices (iOS, Andorid). My plan was to use GL_POINT_SPRITE/GL_PROGRAM_POINT_SIZE method because of it's efficiency (GL_POINTS are enough), but after some experimenting, I found myself in a trouble. Sprite size is limited (to usually 64 pixels). I'm calculating size using this formula
gl_PointSize = in_point_size * some_factor / distance_to_camera
to make particle sizes proportional to distance to camera. But at some point, when camera is close enough, problem with size limitation emerges and whole system starts looking unrealistic. Is there a way to avoid this problem?
If no, what's alternative? I was thinking of manually generating billboard quad for each particle. Now, I have some questions about that approach.
I guess minimum geometry data would be four vertices per particle and index array to make quads from these vertices (with GL_TRIANGLE_STRIP). Additionally, for each vertex I need a color and texture coordinate. I would put all that in an interleaved vertex array. But as you can see, there is much redundancy. All vertices of same particle share same color value, and four texture coordinates are same for all particles. Because of how glDrawArrays/Elements works, I see no way to optimise this. Do you know of a better approach on how to organise per-particle data?
Should I use buffers or vertex arrays, or there is no difference because each time I have to update all particles' data.
About particles simulation... Where to do it? On CPU or on a vertex processors? Something tells me that mobile's CPU would do it faster than it's vertex unit (at least today in 2012 :).
So, any advice on how to make a simple and efficient particle system without particle size limitation, for mobile device, would be appreciated.
(animation of camera passing through particles should be realistic)
