Maximize your Metal ray tracing performance

float alpha = texture.sample(sampler, UV).w;
return alpha >= 0.5f;

[[intersection(triangle, raytracing::triangle_data, raytracing::instancing)]]
bool alphaTestIntersection(float2               coordinates    [[barycentric_coord]],
                           unsigned int         primitiveIndex [[primitive_id]],
                           unsigned int         instanceIndex  [[instance_id]],
                           device GlobalData   *globalData     [[buffer(1)]],
                           device InstanceData *instanceData   [[buffer(0)]])
{
    device Material *materials = globalData->materials;
    InstanceData instance = instanceData[instanceIndex];
    float2 UV = calculateSamplingCoords(coordinates,
                                        instance.uvs[primitiveIndex * 3 + 0],
                                        instance.uvs[primitiveIndex * 3 + 1],
                                        instance.uvs[primitiveIndex * 3 + 2]);
    int materialIndex = instance.materialIndices[primitiveIndex];
    float alpha = materials[materialIndex].texture.sample(sam, UV).w;
    return alpha >= 0.5f;
}

struct PrimitiveData
{
    texture2d<float> texture;
    float2 uvs[3];
};

// Alpha testing intersection function
[[intersection(triangle, raytracing::triangle_data, raytracing::instancing)]]
bool alphaTestIntersection(float2               coordinates    [[barycentric_coord]],
                     const device PrimitiveData *primitiveData [[primitive_data]])
{
    PrimitiveData ppd = *primitiveData;
    float2 UV = calculateSamplingCoords(coordinates,
                                        ppd.uvs[0],
                                        ppd.uvs[1],
                                        ppd.uvs[2]);
    float alpha = ppd.texture.sample(sam, UV).w;
    return alpha >= 0.5f;
}

geometryDescriptor.primitiveDataBuffer = primitiveDataBuffer
geometryDescriptor.primitiveDataElementSize = MemoryLayout<PrimitiveData>.size
geometryDescriptor.primitiveDataStride = MemoryLayout<PrimitiveData>.stride
geometryDescriptor.primitiveDataBufferOffset = primitiveDataOffset

// Intersection function argument:
const device void *primitiveData [[primitive_data]]

// Intersection result:
primitiveData = intersection.primitive_data;

// Intersection query:
primitiveData = query.get_candidate_primitive_data();
primitiveData = query.get_committed_primitive_data();

device int *buffer = intersectionFunctionTable.get_buffer<device int *>(index);

visible_function_table<uint(uint)> table =
    intersectionFunctionTable.get_visible_function_table<uint(uint)>(index);

uint result = table[0](parameter);

let icbDescriptor = MTLIndirectCommandBufferDescriptor()

icbDescriptor.supportRayTracing = true

for (index, accelerationStructure) in accelerationStructures.enumerated() {
    encoder.build(accelerationStructure: accelerationStructure,
                  descriptor: descriptors[index],
                  scratchBuffer: scratchBuffers[index % numScratchBuffers],
                  scratchBufferOffset: 0)
}

let geometryDescriptor = MTLAccelerationStructureTriangleGeometryDescriptor()

geometryDescriptor.vertexFormat = .uint1010102Normalized

var scaleTransform =
    MTLPackedFloat4x3(columns: (
        MTLPackedFloat3Make( scale.x,      0.0,      0.0),
        MTLPackedFloat3Make(     0.0,  scale.y,      0.0),
        MTLPackedFloat3Make(     0.0,      0.0,  scale.z),
        MTLPackedFloat3Make(offset.x, offset.y, offset.z))

let transformBuffer = device.makeBuffer(length: MemoryLayout<MTLPackedFloat4x3>.size,
                                        options: .storageModeShared)!

transformBuffer.contents().copyMemory(from: &scaleTransform,
                                      byteCount: MemoryLayout<MTLPackedFloat4x3>.size)

let geometryDescriptor = MTLAccelerationStructureTriangleGeometryDescriptor()

geometryDescriptor.transformationMatrixBuffer = transformBuffer
geometryDescriptor.transformationMatrixBufferOffset = 0

let sphereGeometryDescriptor = MTLAccelerationStructureTriangleGeometryDescriptor()
sphereGeometryDescriptor.vertexBuffer = sphereVertexBuffer
sphereGeometryDescriptor.indexBuffer = sphereIndexBuffer
sphereGeometryDescriptor.transformationMatrixBuffer = sphereTransformBuffer

let redBoxGeometryDescriptor = MTLAccelerationStructureTriangleGeometryDescriptor()
redBoxGeometryDescriptor.vertexBuffer = boxVertexBuffer
redBoxGeometryDescriptor.indexBuffer = boxIndexBuffer
redBoxGeometryDescriptor.transformationMatrixBuffer = redBoxTransformBuffer

let blueBoxGeometryDescriptor = MTLAccelerationStructureTriangleGeometryDescriptor()
blueBoxGeometryDescriptor.vertexBuffer = boxVertexBuffer
blueBoxGeometryDescriptor.indexBuffer = boxIndexBuffer blueBoxGeometryDescriptor.transformationMatrixBuffer = blueBoxTransformBuffer

let primitiveASDescriptor = MTLPrimitiveAccelerationStructureDescriptor()

primitiveASDescriptor.geometryDescriptors =
    [sphereGeometryDescriptor, redBoxGeometryDescriptor, blueBoxGeometryDescriptor]

let heap = device.makeHeap(descriptor: heapDescriptor)!

let accelerationStructure = heap.makeAccelerationStructure(descriptor: descriptor)

let sizeAndAlign = device.heapAccelerationStructureSizeAndAlign(descriptor: descriptor)

let accelerationStructure = heap.makeAccelerationStructure(size: sizeAndAlign.size)