How to provide custom data to SCNProgram? - ios

Question

I have array with SCNVector3, one for each vertex.
var terrainArray = [SCNVector3]()
I need to provide this data per vertex in my fragment shader. Something like this:
struct TerrainVertexInput
{
float3 position [[attribute(SCNVertexSemanticPosition)]];
float4 color [[attribute(SCNVertexSemanticColor)]];
};
struct TerrainVertexOutput
{
float4 position [[position]];
float3 terrain;
float4 color;
};
vertex TerrainVertexOutput terrainVertex(TerrainVertexInput in [[stage_in]],
constant SCNSceneBuffer& scn_frame [[buffer(0)]],
constant MyNodeBuffer& scn_node [[buffer(1)]],
constant float3 terrain [[buffer(2)]])
{
TerrainVertexOutput v;
v.position = scn_node.modelViewProjectionTransform * float4(in.position, 1.0);
v.terrain = terrain;
v.color = in.color;
return v;
}
As I understand I need to create Data object with array data and provide it to program with setValue(_:forKey:) but I'm not sure if vertex function will get right element for vertex.
How to do this right?

Answer 1

You're on the right track, but you don't want to use SCNVector3 for data you're passing into a Metal shader. SceneKit's vector types have components of type CGFloat, the size of which is platform-dependent.
Instead, your data should use one of the simd vector types. In Swift and Metal, that means float3 or float4. Note that float3 actually occupies 16 bytes of space; there's a dummy element at the end for alignment purposes. If you want to pack your data tightly, using exactly 3 floats per vertex, you can type your buffer in Metal as packed_float3 and write 3 contiguous floats into your data buffer for each vertex. There is no three-element packed float vector type in Swift.
There are many ways to copy an array of SCNVector3 into a suitably-typed data buffer. Here's one:
// Allocate enough memory to store three floats per vertex, ensuring we free it later
let terrainBuffer = UnsafeMutableBufferPointer<Float>.allocate(capacity: terrainArray.count * 3)
defer {
terrainBuffer.deallocate()
}
// Copy each element of each vector into the buffer
terrainArray.enumerated().forEach { i, v in
terrainBuffer[i * 3 + 0] = Float(v.x)
terrainBuffer[i * 3 + 1] = Float(v.y)
terrainBuffer[i * 3 + 2] = Float(v.z)
}
// Copy the buffer data into a Data object, as expected by SceneKit
let terrainData = Data(buffer: terrainBuffer)
You can then use setValue(:forKey:) on your geometry or material:
material.setValue(terrainData, forKey: "terrain")
Rather than taking a single float3 as a parameter in your vertex function, instead take a pointer to packed_float3 and index into it according to the vertex ID:
vertex TerrainVertexOutput terrainVertex(TerrainVertexInput in [[stage_in]],
constant SCNSceneBuffer& scn_frame [[buffer(0)]],
constant MyNodeBuffer& scn_node [[buffer(1)]],
constant packed_float3 *terrain [[buffer(2)]],
uint vid [[vertex_id]]) {
// ...
v.terrain = terrain[vid];
// ...
}
This assumes an exact correspondence between vertices in your geometry and terrain data points. Rather than using the vertex ID directly, you can of course do whatever sort of fancy indexing you want to look up the terrain data for a given vertex.