Passing Metal buffer to SceneKit shader - scenekit

Question

I'd like to use a Metal compute shader to calculate some positions that are then fed into a Metal shader. Sounds straight forward, but I'm having trouble getting my MTLBuffer data into the Metal based SCNProgram.
The compute kernel is as follows, in this contrived example it's taking in three 3D vectors (in both buffers).
kernel void doSimple(const device float3 *inVector [[ buffer(0) ]],
device float3 *outVector [[ buffer(1) ]],
uint id [[ thread_position_in_grid ]]) {
float yDisplacement = 0;
. . . .
outVector[id] = float3(
inVector[id].x,
inVector[id].y + yDisplacement,
inVector[id].z);
}
This kernel function is run each frame in the - renderer:willRenderScene:atTime: method of my SCNSceneRendererDelegate. There's two buffers, and they get switched after each frame.
Buffers are created as follows;
func setupBuffers() {
positions = [vector_float3(0,0,0), vector_float3(1,0,0), vector_float3(2,0,0)]
let bufferSize = sizeof(vector_float3) * positions.count
//copy same data into two different buffers for initialisation
buffer1 = device.newBufferWithBytes(&positions, length: bufferSize, options: .OptionCPUCacheModeDefault)
buffer2 = device.newBufferWithBytes(&positions, length: bufferSize, options: .OptionCPUCacheModeDefault)
}
And the compute shader is run using the following (in the willRenderScene func);
let computeCommandBuffer = commandQueue.commandBuffer()
let computeCommandEncoder = computeCommandBuffer.computeCommandEncoder()
computeCommandEncoder.setComputePipelineState(pipelineState)
computeCommandEncoder.setBuffer(buffer1, offset: 0, atIndex: 0)
computeCommandEncoder.setBuffer(buffer2, offset: 0, atIndex: 1)
computeCommandEncoder.dispatchThreadgroups(numThreadgroups, threadsPerThreadgroup: threadsPerGroup)
computeCommandEncoder.endEncoding()
computeCommandBuffer.commit()
computeCommandBuffer.waitUntilCompleted()
let bufferSize = positions.count*sizeof(vector_float3)
var data = NSData(bytesNoCopy: buffer2.contents(), length: bufferSize, freeWhenDone: false)
var resultArray = [vector_float3](count: positions.count, repeatedValue: vector_float3(0,0,0))
data.getBytes(&resultArray, length:bufferSize)
for outPos in resultArray {
print(outPos.x, ", ", outPos.y, ", ", outPos.z)
}
This works, and I can see my compute shader is updating the y coordinate for each vector in the array.
This scene consists of three spheres evenly spaced. The vertex shader simply takes the position calculated in the compute shader and adds it to each vertex position (well the y component anyway). I give each sphere an index, the vertex shader uses this index to pull the appropriate position out of my computed array.
The Metal vertex function is shown below, it's referenced by a SCNProgram and set to the material of each sphere.
vertex SimpleVertex simpleVertex(SimpleVertexInput in [[ stage_in ]],
constant SCNSceneBuffer& scn_frame [[buffer(0)]],
constant MyNodeBuffer& scn_node [[buffer(1)]],
constant MyPositions &myPos [[buffer(2)]],
constant uint &index [[buffer(3)]]
)
{
SimpleVertex vert;
float3 posOffset = myPos.positions[index];
float3 pos = float3(in.position.x,
in.position.y + posOffset.y,
in.position.z);
vert.position = scn_node.modelViewProjectionTransform * float4(pos,1.0);
return vert;
}
MyPositions is a simple struct containing an array of float3s.
struct MyPositions
{
float3 positions[3];
};
I have no problem passing data to the vertex shader using the setValue method of each sphere's material as shown below (also done in the willRenderScene method). Everything works as expected (the three spheres move upwards).
var i0:UInt32 = 0
let index0 = NSData(bytes: &i0, length: sizeof(UInt32))
sphere1Mat.setValue(index0, forKey: "index")
sphere1Mat.setValue(data, forKey: "myPos")
BUT this requires the data be copied from the GPU to CPU to GPU and is really something I'd rather avoid. So my question is... How do I pass a MTLBuffer to a SCNProgram?
Have tried the following in willRenderScene but get nothing but EXEC_BAD...
let renderCommandEncoder = renderer.currentRenderCommandEncoder!
renderCommandEncoder.setVertexBuffer(buffer2, offset: 0, atIndex: 2)
renderCommandEncoder.endEncoding()
Complete example is over on GitHub.
Thanks for reading, been struggling with this one. Workaround is to use a MTLTexture in place of a MTLBuffer as I've been able to pass these into an SCNProgram via the diffuse mat prop.

Answer 1

just switch the bindings of the buffers from step to step.
step1
computeCommandEncoder.setBuffer(buffer1, offset: 0, atIndex: 0)
computeCommandEncoder.setBuffer(buffer2, offset: 0, atIndex: 1)
step2
computeCommandEncoder.setBuffer(buffer1, offset: 0, atIndex: 1)
computeCommandEncoder.setBuffer(buffer2, offset: 0, atIndex: 0)
step3
computeCommandEncoder.setBuffer(buffer1, offset: 0, atIndex: 0)
computeCommandEncoder.setBuffer(buffer2, offset: 0, atIndex: 1)
and so on ...
the out buffer becomes the new in buffer and vice versa ...