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## GPU particles trajectoriy and external effects

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### #1mrheisenberg  Members

Posted 16 February 2013 - 10:22 PM

I was looking at the UE4 pdf for ideas on how to get trajectories and external effects(winds,explosions) to affect my particles(which I currently run on a compute shader) http://www.unrealengine.com/files/misc/The_Technology_Behind_the_Elemental_Demo_16x9_(2).pdf In the Particles section they say they use "Particle Curves to modulate particle attributes* (1D lookup)",so I guess they somehow store the trajectory in a 1D texture?How would one go about encoding the trajectory data in a texture?The second thing I didn't quite get is about the Vector Fields "Additional forces from Vector Fields* (3D lookup)" - I'm completely lost here,are they storing all vector fields in the current scene in some volume texture and then in the compute shader for each particle checking for interactions with all "Vector Fields" in the volume?They also mention "Affect all particle systems globally or a single system",so there must be some indexing going on.

### #2David Neubelt  Members

Posted 17 February 2013 - 02:12 AM

I was looking at the UE4 pdf for ideas on how to get trajectories and external effects(winds,explosions) to affect my particles(which I currently run on a compute shader) http://www.unrealengine.com/files/misc/The_Technology_Behind_the_Elemental_Demo_16x9_(2).pdf In the Particles section they say they use "Particle Curves to modulate particle attributes* (1D lookup)",so I guess they somehow store the trajectory in a 1D texture?How would one go about encoding the trajectory data in a texture?The second thing I didn't quite get is about the Vector Fields "Additional forces from Vector Fields* (3D lookup)" - I'm completely lost here,are they storing all vector fields in the current scene in some volume texture and then in the compute shader for each particle checking for interactions with all "Vector Fields" in the volume?They also mention "Affect all particle systems globally or a single system",so there must be some indexing going on.

If you need external forces to affect your particles then just pass it as a uniform to your compute shader. If you need localized effects then you can have an array of N effectors that you pass to the shader. If you want to have complex and localized external forces then you have to store it in a 3d grid.

The curves they are talking about is to allow for time varying attributes (e.g. color over life of the particle, or speed over life of the particle).

Finally, the vector fields are vector fields that are divergent free (there are no set of vectors that will make the particle get stuck at a point). Most 3d packages that support particle simulation use an approximation called curl-noise which maintains the divergent free property without having to resort to a 3d fluid simulation. However, they also support more complicated simulations as well that can export these fields. These vector fields can either be generated directly in your engine or imported from popular 3d packages.

No matter what source you use it just controls the motion of the particle at a point in space. In particular, it seems that UE4 allows it to control either the velocity or acceleration.

-= Dave

Edited by David Neubelt, 17 February 2013 - 02:15 AM.

Graphics Programmer - Ready At Dawn Studios

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