Draw a line with SceneKit using Swift on iOS: line drawn but not visible?

Question

Follow-up to my previous question in Objective-C here (specifically, I'm trying to translate the answer I provided with Objective-C into Swift):

Drawing a line between two points using SceneKit

Problem: SceneKit statistics show that line is being drawn, but it doesn't appear on screen.

Minimal example:

• Start a new Game project in Xcode using Swift and SceneKit.
• Remove boilerplate:
  • let scene = SCNScene() instead of SCNScene(named: "art.scnassets/ship.dae")!
  • comment out let ship = ... and ship.runAction(...)
• Change scnView.backgroundColor = UIColor.blackColor() to scnView.backgroundColor = UIColor.grayColor() (I originally thought maybe the lines were just black on a black background, so changed the background colour accordingly)

• Add the following code:

  var positions: [SCNVector3] = [SCNVector3Make(0.0,0.0,0.0),SCNVector3Make(10.0,10.0,10.0)]
  
  // using Swift's Int gives an error with unsupported "SceneKit: error, C3DRendererContextBindMeshElement unsupported byte per index (8)"
  // when constructing element: SCNGeometryElement below, so switched to CInt
  var indicies: [CInt] = [0,1]
  
  // for whatever reason, the following doesn't work on iOS:
  // let vertexSource = SCNGeometrySource(vertices: &positions, count: positions.count)
  // see https://stackoverflow.com/questions/26890739/how-to-solve-scenekit-double-not-supported-error
  // so using more complex SCNGeometrySource() initializer instead
  
  let vertexData = NSData(bytes: &positions, length: positions.count)
  let vertexSource = SCNGeometrySource(data: vertexData, semantic: SCNGeometrySourceSemanticVertex,
  vectorCount: positions.count, floatComponents: true, componentsPerVector: 3,
  bytesPerComponent: sizeof(Float), dataOffset: 0, dataStride: sizeof(SCNVector3))
  
  let indexData = NSData(bytes: &indicies, length: indicies.count)
  let element = SCNGeometryElement(data: indexData, primitiveType: SCNGeometryPrimitiveType.Line,
  primitiveCount: indicies.count, bytesPerIndex: sizeof(CInt))
  
  let lines = SCNGeometry(sources: [vertexSource], elements: [element])
  
  let cellNode = SCNNode(geometry: lines)
  scene.rootNode.addChildNode(cellNode)
  

As mentioned, the SceneKit statistics suggest the line is being drawn, but it doesn't seem to appear, and there are no compilation or runtime errors, which makes it a little tricky to track down.

Edit: Using the GPU 'Analyse' tool in the debug navigator gave the following errors:

Draw call exceeded element array buffer bounds

Draw call exceeded array buffer bounds

in glDrawElements(GL_LINES, 4, GL_UNSIGNED_INT, NULL) which suggests there's something wrong with my SCNGeometryElement construction?

Answer 1

I haven't had a chance to debug the code live, but most likely these lines are your problem:

let vertexData = NSData(bytes: &positions, length: positions.count)
// ...
let indexData = NSData(bytes: &indicies, length: indicies.count)

The length of your data is the number of entries times the size (in bytes) of each entry. Each element in positions is a SCNVector3, which is three Floats on iOS, so each position is 3 * sizeof(Float) or 12 bytes. Each element in indicies (sic) is a CInt, which I believe is a 32-bit integer. So your data buffers are much shorter than your SCNGeometrySource and SCNGeometryElement constructors claim they should be, which means that at render time SceneKit is asking OpenGL to take a long walk draw off a short pier buffer.

Generally, if you're constructing an NSData from an array, you want the length parameter to be the array count times the sizeof the array's element type.