# Particle system using a compute shader

Hi there,

Here is a small example of a particle system using a compute shader.
Uploaded for ‎Georgios Cherouvim‎ to demonstrate.

There are not much comments in the code, my sincere apologies.
If you have any questions, let me know

Cheers,
Tim

EDIT
New version including comments & billboard renderer here:
computeParticles.tox (7.04 KB)

Hello good day, very good contribution.
What programming language is created?
Will you have online tutorials so you can practice and create this type of systems through codes?

Thank you

Hi there,
The compute shader that calculates the positions and directions of all particles is written in GLSL. I haven’t made any tutorials about it (yet). I might do so in the near future.

To start with writing compute shaders I suggest to check the following urls:
derivative.ca/wiki099/index … a_GLSL_TOP

Cheers,
tim

Thank you! This is a clear and helpful example. I noticed that the resolution of the shader can be optimized a little further by finding a resolution that covers the desired number of particles but isn’t necessarily square. Suppose you want 8 particles, and each particle needs 4 pixels. Right now you’re saying that’s 8*4=32 total pixels. Ceil(sqrt(32))=6 pixels, so a 6x6 texture would cover the 8 desired particles. More efficiently, the compute shader could be 4x8. It’s pretty minor but why not if it works without messing up anything else (I hope…). Thanks again for sharing!
computeParticles_2.tox (8.07 KB)

@davidbraun, great!! Didn’t think about that, but this makes it also easier to communicate the amount of particles to other shaders. Was solving this the whole time by sending the real amount of particles using an uniform.

Thank you : D

Hey @timgerritsen, thanks again for sharing that example a while back. I learned a lot and here is what I’ve done so far.

derivative.ca/Forum/viewtop … 22&t=11384

Since those are 2d Particles, I reduce the data size from 4 to 3, but I am still a little unclear of how the dispatchSize and the layout(local_size) do and how they work together. Am I right to assume that it has something to do with how the texture is divided to be multi-threaded on the graphics card?

hey @ch3,

When building a compute shader, you are in control on how many parallel ‘processes’ are
ran (dispatched). One dispatched ‘process’ will then run your shader code ‘local_size’-times.
So instead of that it’s bound to the resolution of your output image (with a normal pixel
shader), it now can be different.

For example, if you have a dispatched size of 32x32 and a local size of 16x16, it means your
shader code will be run 1024 * 256 = 262144 times. If you would put a pixel at the
coordinates of gl_GlobalInvocationID, it will only fill an area of 512x512 pixels, even though
your TOP could have a resolution of 1024x1024.

So what I did with this particle effect shader is to set the dispatched * localsize to the
amount of particles. Then in the code it adds multiple pixels (4 * 3 floats) to the output
buffer. So I set the output resolution to 4 times larger than the dispatch size * localsize.

The reason this is split, is because of the memory management as far as I understand.
Within the local group you can access local shared memory. So there is a relatively fast way
to ‘communicate’ with your other instances of that localgroup.
I haven’t played with this much yet, so not completely sure how this works

Hope I make a bit of sense, if not check out this page:
And try to play with the build-in input variables By plotting them to the output buffer and
see what they do:

gl_NumWorkGroups, gl_WorkGroupSize, gl_WorkGroupID,
gl_LocalInvocationID, gl_GlobalInvocationID, gl_LocalInvocationIndex.

For example to get the uv coordinate of the current instance:
vec2 uv = gl_GlobalInvocationID / vec2(gl_NumWorkGroups * gl_WorkGroupSize);

Cheers,
tim

Here is my version:
now you can define the shape of the floor by a TOP.

1 Like

Great Simone! you inspired me to rebuild the example and using instancing instead of billboards.
Uploaded as a community post here: