Shader3DMesh.swift

enum Axis {
    case x
    case y
    case z
    
    var simd: SIMD3<Float> {
        switch self {
        case .x: return .init(1, 0, 0)
        case .y: return .init(0, 1, 0)
        case .z: return .init(0, 0, 1)
        }
    }
}

constants.modelMatrix = matrix_identity_float4x4
    * Matrix.translation(direction: position)
    * Matrix.rotation(angle: .degrees(rotation.x), axis: Axis.x.simd)
    * Matrix.rotation(angle: .degrees(rotation.y), axis: Axis.y.simd)
    * Matrix.rotation(angle: .degrees(rotation.z), axis: Axis.z.simd)
    * Matrix.scale(axis: scale)
    
struct Matrix {
    static func translation(direction d: SIMD3<Float>) -> matrix_float4x4 {
        return .init(columns: (
            .init(  1,   0,   0,  0),
            .init(  0,   1,   0,  0),
            .init(  0,   0,   1,  0),
            .init(d.x, d.y, d.z,  1)
        ))
    }

    static func scale(axis a: SIMD3<Float>) -> matrix_float4x4 {
        return .init(columns: (
            .init(a.x,   0,   0,  0),
            .init(  0, a.y,   0,  0),
            .init(  0,   0, a.z,  0),
            .init(  0,   0,   0,  1)
        ))
    }

    static func rotation(angle: Angle, axis: SIMD3<Float>) -> matrix_float4x4 {
        .init(simd_quatf(angle: Float(angle.radians), axis: axis))
    }
}