SCNGeometry with polygon as primitiveType

Question

Trying to figure out how I create a SCNGeometry with polygon as primitiveType, My goal is to add polygon shaped node as a child of a sphere node, and make it look like MKPol

Answer 1

You have two problems:

1. SceneKit (and Metal) only support 32 bit integers as indices (source). So the type of your indices array needs to be [Int32].

2. SceneKit needs two pieces of information for polygons: The number of points in the polygon and the index of the point in the vertex array. From Apple's documentation on SCNGeometryPrimitiveTypePolygon (which only exists in Objective-C):

The element’s data property holds two sequences of values.

• The first sequence has a number of values equal to the geometry element’s primitiveCount value. Each value in this sequence specifies the number of vertices in a polygon primitive. For example, if the first sequence is [5, 3], the geometry element contains a pentagon followed by a triangle.
• The rest of the data is a sequence of vertex indices. Each entry in the first sequence specifies a corresponding number of entries in the second sequence. For example, if the first sequence includes the values [5, 3], the second sequence includes five indices for the pentagon, followed by three indices for the triangle.

You need to change your index array to:

let indices: [Int32] = [7, /* We have a polygon with seven points */,
                        0,1,2,3,4,5,6 /* The seven indices for our polygon */
                       ]

Then, set the primitiveCount to 1 (we have one polygon to draw) and change the size of the buffer:

let indexData = Data(bytes: indices, 
                     count: indices.count * MemoryLayout<Int32>.size)

// Now without runtime error
let element = SCNGeometryElement(data: indexData, 
                                 primitiveType: .polygon,
                                 primitiveCount: 1, 
                                 bytesPerIndex: MemoryLayout<Int32>.size)