Gavin Wiggins

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Color view with compute kernel

Written on September 8, 2023

Here is a basic example of using Metal to change the color of a view using a compute function (kernel). The pixels in the view are represented in Metal as a grid. The sliders change the RGB color values for each pixel in the grid.

The main view is shown below and contains the MetalView() where the color is drawn.

// ContentView.swift

import SwiftUI
import MetalKit

struct ContentView: View {

    @State private var mtkView = MTKView()
    @State private var renderer: Renderer?
    @State private var color = SIMD4<Float>(0.8, 0.2, 0.7, 1)

    var body: some View {
        VStack {
            MetalView(mtkView: mtkView)
                .onAppear {
                    renderer = Renderer(metalView: mtkView)
                    renderer?.color = color
                    renderer?.initializeBuffer()
                }

            Slider(value: $color.x, in: 0...1) {
                Text("Red \(color.x, specifier: "%.2f")")
            }
            .onChange(of: color.x) { newValue in
                renderer?.color.x = newValue
                renderer?.initializeBuffer()
            }

            Slider(value: $color.y, in: 0...1) {
                Text("Green \(color.y, specifier: "%.2f")")
            }
            .onChange(of: color.y) { newValue in
                renderer?.color.y = newValue
                renderer?.initializeBuffer()
            }

            Slider(value: $color.z, in: 0...1) {
                Text("Blue \(color.z, specifier: "%.2f")")
            }
            .onChange(of: color.z) { newValue in
                renderer?.color.z = newValue
                renderer?.initializeBuffer()
            }

            Text("Adjust the sliders to change color")
        }
        .padding()
        .frame(minWidth: 400, minHeight: 300)
    }
}

The MetalView represents a MTKView as shown below. Notice the MTKView must be wrapped with a NSViewRepresentable for SwiftUI.

// MetalView.swift

import SwiftUI
import MetalKit

struct MetalView: NSViewRepresentable {

    let mtkView: MTKView

    func makeNSView(context: Context) -> some NSView {
        return mtkView
    }

    func updateNSView(_ nsView: NSViewType, context: Context) { }
}

The Renderer object sets up the Metal device, pipeline, and buffers for drawing to the view. It also passes the color variable to the appropriate Metal buffer for assigning the RGB value to each pixel in the Metal view.

// Renderer.swift

import MetalKit

class Renderer: NSObject {

    static var device: MTLDevice!
    static var commandQueue: MTLCommandQueue!

    var colorState: MTLComputePipelineState!
    var colorBuffer: MTLBuffer!

    var color = SIMD4<Float>()

    init(metalView: MTKView) {

        super.init()

        guard let device = MTLCreateSystemDefaultDevice(),
              let commandQueue = device.makeCommandQueue()
        else {
            fatalError("GPU not available")
        }

        Renderer.device = device
        Renderer.commandQueue = commandQueue

        let library = device.makeDefaultLibrary()
        let colorScreen = library?.makeFunction(name: "colorScreen")

        do {
            colorState = try device.makeComputePipelineState(function: colorScreen!)
        } catch let error as NSError {
            print(error)
        }

        metalView.device = device
        metalView.framebufferOnly = false
        metalView.delegate = self

        initializeBuffer()
    }

    func initializeBuffer() {
        let size = MemoryLayout<SIMD4<Float>>.size
        colorBuffer = Renderer.device.makeBuffer(bytes: &color, length: size)
    }
}

extension Renderer: MTKViewDelegate {
    func mtkView(_ view: MTKView, drawableSizeWillChange size: CGSize) { }

    func draw(in view: MTKView) {
        guard let drawable = view.currentDrawable else { return }

        // Command buffer
        let commandBuffer = Renderer.commandQueue.makeCommandBuffer()

        // Command encoder
        let commandEncoder = commandBuffer?.makeComputeCommandEncoder()
        commandEncoder?.setComputePipelineState(colorState)
        commandEncoder?.setTexture(drawable.texture, index: 0)
        commandEncoder?.setBuffer(colorBuffer, offset: 0, index: 0)

        // Threads
        let w = colorState.threadExecutionWidth
        let h = colorState.maxTotalThreadsPerThreadgroup / w
        let threadsPerThreadGroup = MTLSize(width: w, height: h, depth: 1)
        let threadsPerGrid = MTLSize(width: drawable.texture.width, height: drawable.texture.height, depth: 1)
        commandEncoder?.dispatchThreads(threadsPerGrid, threadsPerThreadgroup: threadsPerThreadGroup)

        // Complete encoding
        commandEncoder?.endEncoding()

        // Commit buffer
        commandBuffer?.present(drawable)
        commandBuffer?.commit()
    }
}

Lastly, the kernel shader function outputs the color for each pixel in the Metal view.

// Shader.metal

#include <metal_stdlib>
using namespace metal;

kernel void colorScreen (texture2d<half, access::write> output [[texture(0)]],
                         device float4 *color [[buffer(0)]],
                         uint2 id [[thread_position_in_grid]])
{
    output.write(half4(*color), id);
}