Unity: Simple “pass through” geometry shader

Question

I would like to implement a geometry shader that does nothing, just for the purpose of learning and to go ahead with that and do something useful with it.

I currently have:

• A compute shader creating "particles".
• A vertex shader doing the transformation
• A fragment shader...

This is the shader file:

StructuredBuffer<float3> particles;

struct fsInput {
    float4 pos : SV_POSITION;
};

fsInput vert(uint id : SV_VertexID)
{
    fsInput fsIn;
    float3 worldPos = particles[id];
    fsIn.pos = mul(UNITY_MATRIX_VP, float4(worldPos, 1.0f));
    return fsIn;
}

//[maxvertexcount(1)]
//void geom(point fsInput p[1], inout TriangleStream<fsInput> output)
//{
//  fsInput fsOut = (fsInput) 0;
//  fsOut.pos = p[0].pos;
//  output.Append(fsOut);
//}

float4 frag(fsInput fsIn) : COLOR
{
    return float4(1.0, 0.2, 0.2, 1);
}

I'm drawing my particles with this call:

Graphics.DrawProcedural(MeshTopology.Points, NUM_PARTICLES);

In my understanding, which is quite limited due to the perfect documentation of geometry shaders, my geometry shader should do nothing. Just forward the points to the fragment shader. However, if I use that shader (comment in those lines), I wont see my particles anymore. Without the geometry shader, I can see every particle.

So the first question is, where is the error? The second question is: Can I, starting with points and using the geometry shader, go from points to a, for example, cube? Or should I do it in the compute shaders? But if so, which MeshTopology should I use to draw the data from the compute shader as Cube, Sphere, etc...?

Thank you for help!

Answer 1

It looks like the problem is that you're using a TriangleStream. You've marked it as maxvertexcount(1) which means that you can't actually draw anything, because a triangle with one vertex isn't a triangle. Try using a PointStream as your output type.

A GeometryShader with a TriangleStream can be used to output two triangles that are facing the screen in order to draw billboards for particles, but if you really want an identity geometry shader, the output type must be the same as the input type.