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c++particle-systemgeometry-shader

Should Particle systems be updated entirely in the geometry shader

Should Particle systems be updated entirely in the geometry shader or should the geometry shader be passed updated data for positions and life ect. At the moment i update everything in the geometry but iam not sure if this is the best idea incase some of the data is needed in the C++.

Solution

  • It's possible to almost everything in shaders ( especially if you're going for SM4+ ). I don't recommend going for anything over SM3 if you want any sort of market penetration. I still regret we didn't provide a SM2 fallback for our latest game, because quite a few people still use old crappy SM2 cards.

    More on to the question. You can use RTT and never to do a round trip back to the main memory ( this is slow as hell, minimize transfer from graphics memory to main memory ), but the down side is that you need to use some rather elaborate tricks to compute AABBs ( which you'll want on the CPU side of things ) if you go pure GPU.

    Instead we do everything which requires changing the state of a particle on the CPU side. We then have a tight memory representation of that data which gets updated to GPU. The vertex shader is rather meaty ( but that's totally fine, do as much as you possibly can in the vertex shader! ), it extracts this compressed representation of a particle, transforms it, and passes the uncompressed data on to the pixel shader. An important observation here is that you can, and should, split per vertex & per particle data. This implies using instancing ( which is just a way of saying: use frequency dividers ). We represent particle rotation with a normal + rotation about that normal.

    Another reason for doing the state changes of a particle CPU side is that it's a heck of a lot easier to composite behavior CPU side. Any at least half decent particle system needs quite a bit of knobs to turn to be able to create interesting particle effects.

    EDIT: And if you have anything resembling Particle::Update that can't be inlined you've failed, minimize per particle function calls, especially virtual ones, and keep the memory representation of a particle tightly packed!