Swift Talk # 184

Retro Rampage: Pushing Pixels

with special guest Nick Lockwood

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Special guest Nick Lockwood guides us through his first tutorial for recreating a retro game using techniques from the 90s.

00:06 Nick Lockwood has written a great tutorial series in which he recreates Wolfenstein 3D in Swift. He'll join us for some episodes to walk through some parts of it.

00:25 Nick grew up with Wolfenstein, and it was one of the games that inspired him to start programming. He's always wanted to recreate it using the original look and techniques, and Swift, with its balance of high-level and low-level APIs, finally offered an opportunity to realize this goal.

00:54 For this series, the idea is to not use any abstractions, such as Core Graphics, to draw our graphics, but rather to build our own drawing primitives from the ground up. The reason for this is — aside from wanting to avoid heavy abstractions on top of our own code — that we theoretically end up with a project that can run on any platform that supports Swift, including Linux and perhaps even Windows.

01:35 As a disclaimer: this series won't teach us a lot about modern game development. Rather, this is how games might have been built in the early 90s. Modern games make heavy use of graphics cards and GPUs, whereas our project will run solely on the CPU, which means it'll quickly drain a phone battery.

The Goal

02:05 Aside a few small modifications, we'll be pretty closely following selected parts of Nick's tutorials. Reading and coding along with the extensive tutorials is quite a bit of work, so we thought it would be interesting to see some parts of it on video as well. The full series, called Retro Rampage, can be found on GitHub.

03:07 Let's first take a look at what the final result would be after having completed the entire series. The game opens with an enemy monster immediately coming at us. After we shoot that first enemy, we walk around through the maze and open sliding doors until we're killed by an enemy.

03:42 Our version of the game won't be as complete as this demo, but we'll still learn about some interesting aspects, like drawing the scene in 3D.

03:57 We start out with a more-or-less empty project with only a bit of UIKit code that sets up an image view:

class ViewController: UIViewController {
    let imageView = UIImageView()

    override func viewDidLoad() {
        super.viewDidLoad()
        setUpImageView()
    }
}
extension ViewController {
    func setUpImageView() {
        view.addSubview(imageView)
        imageView.translatesAutoresizingMaskIntoConstraints = false
        imageView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        imageView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        imageView.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
        imageView.heightAnchor.constraint(equalTo: view.heightAnchor).isActive = true
        imageView.contentMode = .scaleAspectFit
        imageView.backgroundColor = .black
    }
}

04:14 And that's basically all the UIKit code we'll write. The idea is to let UIKit put an image on the screen, and after that, we have complete control over what goes into that image, and we'll update this image at 60 frames per second.

Color

04:46 Typically, when we put an image on the screen in iOS, we're using a UIImage, which sits on top of CGImage. But we want to peel away the UIKit layers and set up some basic building blocks. The most primitive type we can get started with is a type that represents a color.

05:32 UIColor gives us an abstraction over a color using four floating-point values, which are used to represent red, green, blue, and alpha. This is already more than what goes on under the hood, and we want our type to look more like the raw data, which means we will store each of the four components in 8 bits:

struct Color {
    var r, g, b, a: UInt8
}

06:42 Then we can write some static properties that let us easily create specific color values:

extension Color {
    static let clear = Color(r: 0, g: 0, b: 0, a: 0)
    static let black = Color(r: 0, g: 0, b: 0)
    static let white = Color(r: 255, g: 255, b: 255)
    static let gray = Color(r: 192, g: 192, b: 192)
    static let red = Color(r: 255, g: 0, b: 0)
    static let green = Color(r: 0, g: 255, b: 0)
    static let blue = Color(r: 0, g: 0, b: 255)
}

07:21 By setting a default value of 255 for the alpha component, the auto-generated initializer also uses this default value, so we can create an opaque color by omitting the alpha parameter:

struct Color {
    var r, g, b: UInt8
    var a: UInt8 = 255
}

Bitmap

08:06 Now that we have Color, we can move on to defining a bitmap type. Images are typically stored as a continuous array of color values. Since we want to copy our image type into the image view as efficiently as possible, we model our bitmap type in a way that is very close to what its memory storage looks like:

struct Bitmap {
    var pixels: [Color]
}

09:01 An array of pixels alone doesn't tell us what the dimensions of the image are: we need to specify either the width or the height. We choose to store the image's width, and we provide the height as a computed property:

struct Bitmap {
    let width: Int
    var pixels: [Color]
    
    var height: Int {
        pixels.count / width
    }
}

09:33 Even though the computer is happy to work with a linear array of pixels, it doesn't provide a very convenient interface for us as developers. If we want to set a certain pixel's color, it would be much handier if we could refer to that pixel using two-dimensional coordinates:

struct Bitmap {
    // ...
    subscript(x: Int, y: Int) -> Color {
        get { pixels[y * width + x] }
        set { pixels[y * width + x] = newValue }
    }
}

11:04 In order to conveniently create a Bitmap of a single color, we write an initializer that creates an array of pixels from a given width, height, and color value:

struct Bitmap {
    // ...
    init(width: Int, height: Int, color: Color) {
        self.width = width
        pixels = Array(repeating: color, count: width * height)
    }
}

12:11 Now we can create a Bitmap in viewDidLoad:

let bitmap = Bitmap(width: 8, height: 8, color: .white)

13:11 And in order to turn the bitmap into an image that we can pass to the image view, we paste in a UIImage initializer:

extension UIImage {
    convenience init?(bitmap: Bitmap) {
        let alphaInfo = CGImageAlphaInfo.premultipliedLast
        let bytesPerPixel = MemoryLayout<Color>.stride
        let bytesPerRow = bitmap.width * bytesPerPixel

        guard let providerRef = CGDataProvider(data: Data(bytes: bitmap.pixels, count: bitmap.height * bytesPerRow) as CFData) else {
            return nil
        }

        guard let cgImage = CGImage(
            width: bitmap.width,
            height: bitmap.height,
            bitsPerComponent: 8,
            bitsPerPixel: bytesPerPixel * 8,
            bytesPerRow: bytesPerRow,
            space: CGColorSpaceCreateDeviceRGB(),
            bitmapInfo: CGBitmapInfo(rawValue: alphaInfo.rawValue),
            provider: providerRef,
            decode: nil,
            shouldInterpolate: true,
            intent: .defaultIntent
        ) else {
            return nil
        }

        self.init(cgImage: cgImage)
    }
}

13:45 Let's roughly go over these lines. First, the alphaInfo variable tells the CGImage we're creating that the alpha component is stored in the last (i.e. the least significant) bits of each pixel's color value and that the red, green, and blue components are already multiplied by the alpha component.

15:41 We define the number of bytes per pixel by using the memory size of the Color type — we know this is four bytes, but it's best practice to get this value from the stride constant. Next, the number of bytes per row is equal to the bytes per pixel multiplied by the bitmap's width.

16:16 Then we create a CGDataProvider that takes the array of pixels and turns it into raw data for the CGImage.

16:38 Finally, we use all the above as parameters to create a CGImage, from which we subsequently create a UIImage. Doing so took quite a few lines, and we don't see this type of code every day, but when we walk though it line by line, we can see that the code doesn't hide many surprises, and we're simply following the documentation.

17:10 Now let's use the initializer to get our image on the screen. We make the bitmap's first pixel blue, and we pass the bitmap to the image view:

class ViewController: UIViewController {
    let imageView = UIImageView()

    override func viewDidLoad() {
        super.viewDidLoad()
        setUpImageView()
        
        var bitmap = Bitmap(width: 8, height: 8, color: .white)
        bitmap[0, 0] = Color.blue
        
        imageView.image = UIImage(bitmap: bitmap)
    }
}

18:02 We see the blue pixel in the top-left corner, but it's blurred because the image is stretched out to fit the screen. This look is the result of UIKit's default behavior, which tries to make lower-res images look better by applying some filters. In this case, we want to actually see the individual pixels for a retro look, so we tell the image view to use the nearest neighbor algorithm as its magnification filter. This makes the blue pixel show up as a sharp rectangle:

extension ViewController {
    func setUpImageView() {
        // ...
        imageView.layer.magnificationFilter = .nearest
    }
}

Update Loop

19:20 As this episode's last step, we will set up the basic mechanism of updating the game state in order to make something move onscreen. Currently, we're creating and displaying our image in viewDidLoad only once. But we actually want to set up a render loop.

19:58 Under the hood, iOS is already using a run loop to constantly refresh the screen, and we want to tie into this process. We could use a Timer, but the recommended approach is to use a CADisplayLink instead. CADisplayLink is basically a timer that is synchronized to the refresh rate of the screen, and it prevents a jittery effect that could occur when the game engine updates and screen updates get out of sync.

20:22 So, how often we get called by the display link depends on the refresh rate of the screen. This is usually 60 frames per second, although some of the newer iPads might even reach 120 fps. The display link lets us specify a multiplier in order to be called every single frame, or every two frames, etc. But we want to let it go as fast as it can, so we'll use the default speed.

20:48 CADisplayLink hasn't been modernized like Timer's API, so we can't use a closure, but we have to specify a target and selector. For this, we create an @objc method that will be called with the display link:

class ViewController: UIViewController {
    let imageView = UIImageView()

    override func viewDidLoad() {
        super.viewDidLoad()
        setUpImageView()
        
        let displayLink = CADisplayLink(target: self, selector: #selector(update))
        
    }
    
    @objc func update(_ displayLink: CADisplayLink) {
        
    }
}

21:24 And, unlike Timer, we have to manually add the display link to the main run loop:

displayLink.add(to: .main, forMode: .common)

21:33 Unless we need to pause or remove the display link, we don't need to keep a reference to the display link; adding it to the run loop makes sure it's kept alive.

22:12 The second parameter, the run loop mode, is used to handle performance issues such as pausing a timer when the user is interacting with the screen. Because we want our updating mechanism to be as fast and as constant as possible, we pass in the .common mode to let the run loop update the display link under all circumstances.

22:37 Now we can move the bitmap into our update method:

class ViewController: UIViewController {
    // ...
    
    @objc func update(_ displayLink: CADisplayLink) {
        var bitmap = bitmap(width: 8, height: 8, color: .white)
        bitmap[0, 0] = Color.blue

        imageView.image = UIImage(bitmap: bitmap)
    }
}

22:54 This results in our image being updated at 60 frames per second. To actually see any of this, we can insert a quick hack to move the blue pixel one position to the right after every second:

class ViewController: UIViewController {
    // ...
    
    @objc func update(_ displayLink: CADisplayLink) {
        var bitmap = bitmap(width: 8, height: 8, color: .white)
        let x = Int(displayLink.timestamp) % 8
        bitmap[x, 0] = Color.blue

        imageView.image = UIImage(bitmap: bitmap)
    }
}

Architecture

23:54 We're mixing a lot of responsibilities in the view controller, and it makes sense to move some (or actually most) logic out into separate types.

24:20 Games ask for a different architecture than more generic apps. We want to separate our game logic from our view logic. Within the "view" domain, we can further differentiate between the actual view onscreen and the visual elements we're drawing within our image. We want to pull the drawing code out of the view controller, and later, we'll also want to abstract our model that defines game entities such as the player, the floor, walls, etc.

25:25 As a first step in that direction, we pull the creation of the bitmap out to a new type called Renderer:

struct Renderer {
    var bitmap = Bitmap(width: 8, height: 8, color: .white)
    
    mutating func draw(x: Int) {
        bitmap[x, 0] = Color.blue
    }
}

27:08 In our view controller's update method, we create a renderer, pass the x coordinate in, and pass the renderer's bitmap to the image view:

class ViewController: UIViewController {
    // ...
    
    @objc func update(_ displayLink: CADisplayLink) {
        let x = Int(displayLink.timestamp) % 8
        var renderer = Renderer()
        renderer.draw(x: x)
        imageView.image = UIImage(bitmap: renderer.bitmap)
    }
}

27:44 In this episode, we've covered the basics of rendering. Next time, we'll continue with what it is we'll be rendering, i.e. laying out the floorplan and visualizing the player.

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