Draw Indexed Primitives (Metal Part 9)

Source Code

See Project in GitHub 👩‍💻

References

• Metal Render Pipeline tutorial series by Rick Twohy
• Index Buffer

---

Table of Content

• Mesh Indices
• Index Buffer
• Draw Indexed Primitive

---

Mesh Indices

The Mesh now also sets an array of indices to go along the array of vertices.

This allows to eliminate duplicates and optimize memory utilization.

Image Source 🔗

1class Mesh {
2    public var vertices: [Vertex]! = []
3    public var indices: [UInt32]! = []
4
5    init(){
6        createMesh()
7    }
8
9    func createMesh() {}
10}

1class QuadMesh : Mesh{
2    override func createMesh() {
3        vertices = [
4            Vertex(position: float3( 0.5, 0.5,0), color: float4(1,0,0,1)), //Top Right
5            Vertex(position: float3(-0.5, 0.5,0), color: float4(0,1,0,1)), //Top Left
6            Vertex(position: float3(-0.5,-0.5,0), color: float4(0,0,1,1)), //Bottom Left
7            Vertex(position: float3( 0.5,-0.5,0), color: float4(1,0,1,1))  //Bottom Right
8        ]
9
10        indices = [
11            0,1,2,
12            0,2,3
13        ]
14    }
15}

---

Index Buffer

The Mesh Renderer will create another MTLBuffer to send these indexes to the GPU.

1class MeshRenderer : Component, Renderable {
2
3    ...
4
5    private var _vertexBuffer: MTLBuffer!
6    private var _indexBuffer: MTLBuffer!
7
8    ...
9
10    init(mesh: Mesh) {
11        _mesh = mesh
12
13        _vertexBuffer = Engine.device.makeBuffer(bytes: _mesh.vertices, length: Vertex.stride * _mesh.vertices.count, options: [])
14
15        if(_mesh.indices.count > 0){
16            _indexBuffer = Engine.device.makeBuffer(bytes: _mesh.indices, length: UInt32.stride * _mesh.indices.count, options: [])
17        }
18    }
19}

---

Draw Indexed Primitive

The Mesh Renderer will draw the primitives by sending the indices as instructions on how to interpret the vertices to draw the triangles.

1func doRender(renderCommandEncoder: MTLRenderCommandEncoder) {
2
3    ...
4
5    if(_mesh.indices.count > 0){
6        renderCommandEncoder.drawIndexedPrimitives(
7            type: MTLPrimitiveType.triangle,
8            indexCount: _mesh.indices.count,
9            indexType: MTLIndexType.uint32,
10            indexBuffer: _indexBuffer,
11            indexBufferOffset: 0,
12            instanceCount: 1 // for now, might change in the future
13        )
14    } else {
15        renderCommandEncoder.drawPrimitives(
16            type: MTLPrimitiveType.triangle,
17            vertexStart: 0,
18            vertexCount: _mesh.vertices.count
19        )
20    }
21}