Access GPU Particle vec4 seed in Custom Effect

Questions
1

Hi, I am colorizing my particles with a simple fragment shader and would like to do a HSL jitter that is a little different for each particle. Is there a way to access either the seed or the id or the particle number in my fragment shader for this purpose? I saw that the GPU Vertex Shader has a vec4 seed attribute, would I be able to somehow pass that along to my custom effect fragment shader?

2

You can use gl_VertexID with a division to compute the particle number. Any vertex attribute can be forward to the fragment shader.

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3

Ah, that sounds promising ā€“ what division do you mean exactly? And I was hoping it would be easy to forward vertex attribute to the fragment shader but I donā€™t see that listed in the Custom Effect documentation:

4

Looking at existing code, it looks like it will need to forward vertex id from vertex shader to fragment. We are in a code freeze but that can be done after the 8.0 release.
Am I missing something here @Evgeni_Popov ?

A division is needed to convert from a vertex id to a particle id. vertex 0ā€¦3 for particle 0. vertex 4ā€¦7 for particle 1 and so on.

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5

Ah darn, I seeā€¦ ok if there is no per-particle attribute I can use to uniquely colorize per-particle in the fragment shader maybe Iā€™ll switch back to CPU particles. Thanks for the info!

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6

As we use instancing, you could use gl_instanceID to get a unique value per particle. However, Iā€™m not sure this value will stay the same during the lifespan of a particle, because particles are recycled, so they may not always have the same position in the vertex streamā€¦

7

Ooh thatā€™s better than nothing! At least that is something I can use if I am doing a single-frame fixed-number particle burst (I have been using emitCount in my GPU systems instead of emitRate).

I also had an idea this morning that I havenā€™t tried yet ā€“ I believe there is way to tell the GPU particles to use a random cell of a spritesheet as their texture. If that is true, then I can make a spritesheet with like 100 cells and then use the particleā€™s uv coordinate as my seed.

It wonā€™t be completely unique, but unique modulo 100 which is probably good enough to get the shading variation I am looking for with my HSV jitter custom effect.

Thanks!

EDIT: Isnā€™t gl_instanceID only available in the vertex shader? For Particle System Custom Effect I think you can only pass in a fragment shader:

8

Yes, gl_instanceID is only available in the vertex shader, you will have to pass it from the vertex to the fragment.

This PR will let you pass the vertex shader name:

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