Eu deveria ver 2 triângulos amarelos, mas não vejo nada.

class Terrain {

    private class func createGeometry () -> SCNGeometry {

        let sources = [
            SCNGeometrySource(vertices:[
                SCNVector3(x: -1.0, y: -1.0, z:  0.0),
                SCNVector3(x: -1.0, y:  1.0, z:  0.0),
                SCNVector3(x:  1.0, y:  1.0, z:  0.0),
                SCNVector3(x:  1.0, y: -1.0, z:  0.0)], count:4),
            SCNGeometrySource(normals:[
                SCNVector3(x:  0.0, y:  0.0, z: -1.0),
                SCNVector3(x:  0.0, y:  0.0, z: -1.0),
                SCNVector3(x:  0.0, y:  0.0, z: -1.0),
                SCNVector3(x:  0.0, y:  0.0, z: -1.0)], count:4)
        ]

        let elements = [
            SCNGeometryElement(indices: [0, 2, 3, 0, 1, 2], primitiveType: .Triangles)
        ]

        let geo = SCNGeometry(sources:sources, elements:elements)

        let mat = SCNMaterial()
        mat.diffuse.contents = UIColor.yellowColor()
        mat.doubleSided = true
        geo.materials = [mat]

        return geo
    }

    class func createNode () -> SCNNode {

        let node = SCNNode(geometry: createGeometry())
        node.name = "Terrain"
        node.position = SCNVector3()
        return node
    }
}

Eu o uso da seguinte maneira:

let terrain = Terrain.createNode()
sceneView.scene?.rootNode.addChildNode(terrain)


let camera = SCNCamera()
camera.zFar = 10000
self.camera = SCNNode()
self.camera.camera = camera
self.camera.position = SCNVector3(x: -20, y: 15, z: 30)
let constraint = SCNLookAtConstraint(target: terrain)
constraint.gimbalLockEnabled = true
self.camera.constraints = [constraint]

sceneView.scene?.rootNode.addChildNode(self.camera)

Eu recebo outros nós com geometria não personalizada que vejo. O que há de errado?