A SpriteKit game with zero image or audio files: how we built Street Legends

Street Legends is our fast five-a-side street football game for iPhone, with a league to climb, a nine-player squad and kits to unlock. Open the app bundle and you will not find a single PNG or audio file. Every visual and every sound is generated in Swift at runtime.

#Why build a game this way?

• Tiny download: no texture atlases or audio packs, so the app installs in seconds
• Sharp at every size: shapes are drawn for the exact screen, from the smallest iPhone to an iPad
• Infinite variations: a new kit is a few colours, not a new set of images
• Fast iteration: change a number and the whole game updates, no export step

The trade-off is that you cannot hand a file to an illustrator. It suits stylized, geometric art, which is exactly the look we wanted.

#Drawing sprites with Core Graphics

Drawing complex shapes with SKShapeNode every frame is expensive. Instead, we draw once into an image with UIGraphicsImageRenderer and turn it into a texture:

import SpriteKit
import UIKit

func ballTexture(diameter: CGFloat) -> SKTexture {
    let size = CGSize(width: diameter, height: diameter)
    let image = UIGraphicsImageRenderer(size: size).image { context in
        let rect = CGRect(origin: .zero, size: size)
        UIColor.white.setFill()
        context.cgContext.fillEllipse(in: rect)

        UIColor.black.setFill()
        let patch = rect.insetBy(dx: diameter * 0.34, dy: diameter * 0.34)
        context.cgContext.fillEllipse(in: patch)
    }
    return SKTexture(image: image)
}

let ball = SKSpriteNode(texture: ballTexture(diameter: 28))

Textures are cached by their parameters, so each one is drawn once and reused by every node that needs it. Kits work the same way: a jersey shape filled with the team's colours and a number drawn on top.

#Synthesizing sound with AVAudioEngine

Sound effects are short waveforms with an envelope. AVAudioSourceNode lets you write samples directly:

import AVFoundation

final class Synth {
    private let engine = AVAudioEngine()
    private var phase = 0.0
    private var frequency = 0.0
    private var amplitude = 0.0

    init() {
        let sampleRate = engine.outputNode.inputFormat(forBus: 0).sampleRate
        let source = AVAudioSourceNode { [unowned self] _, _, frameCount, bufferList -> OSStatus in
            let buffers = UnsafeMutableAudioBufferListPointer(bufferList)
            for frame in 0..<Int(frameCount) {
                let sample = Float(sin(phase) * amplitude)
                phase += 2 * .pi * frequency / sampleRate
                if phase > 2 * .pi { phase -= 2 * .pi }
                amplitude *= 0.9993 // quick decay, like a plucked sound
                for buffer in buffers {
                    buffer.mData?.assumingMemoryBound(to: Float.self)[frame] = sample
                }
            }
            return noErr
        }
        let format = AVAudioFormat(standardFormatWithSampleRate: sampleRate, channels: 1)
        engine.attach(source)
        engine.connect(source, to: engine.mainMixerNode, format: format)
        try? engine.start()
    }

    func kick() {
        frequency = 90
        amplitude = 0.7
    }
}

A kick is a low tone that drops quickly. A whistle is a high tone with a little vibrato. Crowd noise is filtered random noise that swells after a goal. Combining a handful of these generators covers every sound in the game.

#Keeping it fast

• Draw textures once, at load time, never inside update(_:)
• Reuse nodes for repeated effects instead of creating new ones
• Use SpriteKit physics bodies with simple shapes, circles for players and the ball
• Profile on the oldest device you support, not on the newest

#When procedural art is the right call

It shines for arcade games, puzzle games and stylized worlds where geometry is the aesthetic. For painterly or character-heavy art, an artist and a texture atlas are still the better tool; our match-3 game Paw & Home is a good example of that side.

Want a game built? Tell us about it. We build in SpriteKit, SceneKit and Unity.