SceneKit Error: C3DMeshElementSetPrimitives invalid index buffer size - ios

Question

I have got the following error :
[SceneKit] Error: C3DMeshElementSetPrimitives invalid index buffer size
It appears at every frame (many errors !)
Do you know how to solve this issue ?
Thanks

Answer 1

Check that indices array is paring the vertices in the vertices array properly.
Suppose you wanted to draw a square using lines with 4 vertices like this:
v1 ------ v0
| |
| |
v2 ------ v3
The vertices array would be v0, v1, v2, v3.
The lines pairs would be: (v0,v1) , (v1,v2) , (v2,v3) , (v3,v0)
So indices array would be: 0,1,1,2,2,3,3,0
In code, this is an example that draws a node that contains a red circle using lines:
func circlePathNode() -> SCNNode {
var vertices:[SCNVector3] = []
let N = 200
var indices:[Int16] = []
// vertices
for i in 1...N {
let t = (Float(i-1) / Float(N)) * 2 * .pi
let v = SCNVector3(cos(t), sin(t), 0)
vertices.append(v)
}
// indices pair vertices
for i in 1...N-1 {
indices.append(Int16(i-1))
indices.append(Int16(i))
}
// last:
indices.append(Int16(N-1))
indices.append(0)
let verticesSource = SCNGeometrySource(vertices: vertices)
let indexData = Data(bytes: indices, count: indices.count * MemoryLayout<Int16>.size) // 2 for 2 bytes each
let element = SCNGeometryElement(data: indexData,
primitiveType: .line,
primitiveCount: N,
bytesPerIndex: MemoryLayout<Int16>.size)
let geometry = SCNGeometry(sources: [verticesSource],elements: [element])
#if os(iOS)
geometry.firstMaterial?.emission.contents = UIColor.red
#else
geometry.firstMaterial?.emission.contents = NSColor.red
#endif
return SCNNode(geometry: geometry)
}

Answer 2

I had the same error when i accidentally pass index count to primitive count. Make sure that you pass exactly primitive count parameter (not index count) to primitive count parameter of SCNGeometryElement constructor.
Primitive count can be calculated this way:
func calculatePrimintiveCount( indexCount: Int, primitiveType: SCNGeometryPrimitiveType) -> Int {
switch (primitiveType){
case .triangles: return indexCount / 3
case .triangleStrip: return indexCount - 2
case .line: return indexCount / 2
case .point: return indexCount
case .polygon: return indexCount / 4 // not sure
}
}