// Each #kernel tells which function to compile; you can have many kernels
#pragma kernel MeshToVoxel
#pragma kernel Zero

// Create a RenderTexture with enableRandomWrite flag and set it
// with cs.SetTexture
uint tris;
int numSamples;
half scale;
half3 offset;
uint voxelSide;

StructuredBuffer<half3> VertexBuffer;
StructuredBuffer<int> IndexBuffer;
RWTexture3D<float4> Voxels;

// from https://beta.observablehq.com/@jrus/plastic-sequence
half2 plastic(half index) {
	return half2(0.5 + frac(0.7548776662466927 * index), 0.5 + frac(0.5698402909980532 * index));
}


// sample num, triangle basis vectors
half3 triangleSample(int n, half3 A, half3 B) {
	half2 s = plastic(half(n));
	s = s.x + s.y > 1.0 ? 1.0 - s : s;
	return s.x * A + s.y * B;
}

[numthreads(8, 8, 8)]
void Zero(uint3 id : SV_DispatchThreadID) {
	if (any(id < 0) || any(id > voxelSide)) return;
	Voxels[id] = half4(0.0f, 0.0f, 0.0f, 0.0f);
}

[numthreads(512,1,1)]
void MeshToVoxel(uint3 id : SV_DispatchThreadID)
{
	uint triID = id.x * 3;
	if (triID >= tris * 3) return;
	half3 a = VertexBuffer[IndexBuffer[triID + 0]] * scale + offset;
	half3 b = VertexBuffer[IndexBuffer[triID + 1]] * scale + offset;
	half3 c = VertexBuffer[IndexBuffer[triID + 2]] * scale + offset;
	half3 AB = b - a;
	half3 AC = c - a;
	half side = half(voxelSide);

	for (int i = 0; i < numSamples; i++) {
		half2 s = half2(frac(0.7548776662466927 * i), frac(0.5698402909980532 * i));
		s = s.x + s.y > 1.0 ? 1.0 - s : s;
		half3 pointOnTri = a + s.x * AB + s.y * AC;
		half3 scaled = pointOnTri * side;
		uint3 voxelIdx = uint3(floor(scaled));
		if (!(any(voxelIdx < 0) || any(voxelIdx >= voxelSide))) {
			half distFromCenter = 1.0 - length(frac(scaled) - half3(0.5, 0.5, 0.5));
			Voxels[voxelIdx] = half4(distFromCenter, distFromCenter, distFromCenter, distFromCenter);
		}
	}
}