recalculating normals

normals.swift

// I'm rendering a subdivided quad of resolution x resolution vertices
let vertex = Vertex(position: position, normal: float3(0,0,0), texture: float2(0,0))
vertices.append(vertex)
// vertices are stored in a vertices array, same for indices

// for every vertex but the last row and column we want to create two triangles
if x != resolution-1 && y != resolution-1 {
    self.indices.append(contentsOf: [
        UInt16(i), UInt16(i + resolution), UInt16(i + resolution + 1),  // triangle 1
        UInt16(i), UInt16(i + resolution + 1), UInt16(i + 1)            // triangle 2
    ])
}

// recalculate normals

// access indices in pairs of three
for i in stride(from: 0, to: indices.count, by: 3) {

    // grab the actual vertex index
    let i0 = Int(indices[i])
    let i1 = Int(indices[i+1])
    let i2 = Int(indices[i+2])
    
    // grab the vertices of the triangle
    let v0 = vertices[i0]
    let v1 = vertices[i1]
    let v2 = vertices[i2]
    
    // create two vectors from triangle sides
    let u = v1.position - v0.position
    let v = v2.position - v0.position
    
    // calculate normal
    let n = normalize(cross(v, u))
  
    // add normal to all vertices of triangle
    vertices[i0].normal += n
    vertices[i1].normal += n
    vertices[i2].normal += n
}

// normalize all vertices after summing all the normals
for i in 0..<vertices.count {
    vertices[i].normal = normalize(vertices[i].normal)
}