Get VertexSize of Instance in VertexShader

Question

I'm using Unitys DrawProceduralIndirect function to directly render the content of a ComputeShader.

I setup the argumentBuffer for this function as follows:

    int[] csArgs = new int[] { ptsOnLine, 2, 0, 0};
    argsBuffer = new ComputeBuffer(4, 4, ComputeBufferType.IndirectArguments);

So every instance consists of ptsOnLine (lets say 10) vertices.

My vertexShader then looks like this:

fsInput vert(uint id : SV_VertexID, uint instanceId : SV_InstanceId)
{
    fsInput fsIn;
    float3 worldPos = linePoints[id + (instanceId * 10)];
    fsIn.pos = mul(UNITY_MATRIX_VP, float4(worldPos, 1.0f));
    return fsIn;
}

The question is simple. My problem is: (instanceId * 10). Is there a built in variable telling me the length of one instance? I couldn't find anything. I mean, its in the argument buffer so it shouldn't be too much of a problem? I could forward this value from the compute buffer but thats ugly. :)

Help is much appreciated!

Answer 1

The "built in" variable is contained in the args buffer.

From the unity docs "Buffer with arguments, bufferWithArgs, has to have four integer numbers at given argsOffset offset: vertex count per instance, instance count, start vertex location, start instance location."

So make sure you pass that argsBuffer to your shader and then use the first index.