Post-tessellation Vertex Function and Raytracing: Getting More Detailed Geometries for the Acceleration Structures - swift

Question

I have recently gained some interest in the raytracing API provided by the Metal framework. I understand that you can attach a vertex buffer to a geometry descriptor that Metal will use to create the acceleration structure later (on a MTLPrimitiveAccelerationStructureDescriptor instance for example).
This made me wonder if it were possible to write the output of the tessellator into a separate vertex buffer from the post-tessellation vertex shader and pass that along to the raytracer. I thought that perhaps you could get more detailed geometry and still render without rasterization. For example, I might have the following simple post-tessellation vertex function:
[[patch(triangle, 3)]]
vertex FunctionOutIn tessellation_vertex_triangle(PatchIn patchIn [[stage_in]],
float3 patch_coord [[ position_in_patch ]])
{
// Barycentric coordinates
float u = patch_coord.x;
float v = patch_coord.y;
float w = patch_coord.z;
// Convert to cartesian coordinates
float x = u * patchIn.control_points[0].position.x + v * patchIn.control_points[1].position.x + w * patchIn.control_points[2].position.x;
float y = u * patchIn.control_points[0].position.y + v * patchIn.control_points[1].position.y + w * patchIn.control_points[2].position.y;
// Output
FunctionOutIn vertexOut;
vertexOut.position = float4(x, y, 0.0, 1.0);
vertexOut.color = half4(u, v, w, 1.0h);
return vertexOut;
}
However, the following doesn't compile
// Triangle post-tessellation vertex function
[[patch(triangle, 3)]]
vertex void tessellation_vertex_triangle(device void *outputBuffer [[ buffer(0) ]],
PatchIn patchIn [[stage_in]],
float3 patch_coord [[ position_in_patch ]])
{
// Barycentric coordinates
float u = patch_coord.x;
float v = patch_coord.y;
float w = patch_coord.z;
// Convert to cartesian coordinates
float x = u * patchIn.control_points[0].position.x + v * patchIn.control_points[1].position.x + w * patchIn.control_points[2].position.x;
float y = u * patchIn.control_points[0].position.y + v * patchIn.control_points[1].position.y + w * patchIn.control_points[2].position.y;
// Output
FunctionOutIn vertexOut;
vertexOut.position = float4(x, y, 0.0, 1.0);
vertexOut.color = half4(u, v, w, 1.0h);
}
But I noticed that the function doesn't compile when I don't use the data in the control points as output, like so
[[patch(triangle, 3)]]
vertex FunctionOutIn tessellation_vertex_triangle(PatchIn patchIn [[stage_in]],
float3 patch_coord [[ position_in_patch ]])
{
// Barycentric coordinates
float u = patch_coord.x;
float v = patch_coord.y;
float w = patch_coord.z;
// Convert to cartesian coordinates
float x = u * patchIn.control_points[0].position.x + v * patchIn.control_points[1].position.x + w * patchIn.control_points[2].position.x;
float y = u * patchIn.control_points[0].position.y + v * patchIn.control_points[1].position.y + w * patchIn.control_points[2].position.y;
// Output
FunctionOutIn vertexOut;
// Does not use x or y (and therefore the `patch_control_point<T>`'s values
// are not used as output into the rasterizer)
vertexOut.position = float4(1.0, 1.0, 0.0, 1.0);
vertexOut.color = half4(1.0h, 1.0h, 1.0h, 1.0h);
return vertexOut;
}
I looked at the patch_control_point<T> template that was publicly exposed but didn't see anything enforcing this. What is going on here?
In particular, how would I go about increasing the quality of the geometry into the raytracer? Would I simply have to use more complex assets? Tessellation has its place in the rasterization pipeline, but can it be used elsewhere?