はじめに

CIFilterを使って画像の色数を制限してみます
サンプルアプリ

実装

import SwiftUI

struct ContentView: View {
    private let image = UIImage(named: "sample")!

    @State private var uiImage: UIImage?
    
    @State private var colorCount: Int = 0

    var body: some View {
        ScrollView {
            VStack {
                if let uiImage {
                    Image(uiImage: uiImage)
                        .resizable()
                        .scaledToFit()
                } else {
                    Image(uiImage: image)
                        .resizable()
                        .scaledToFit()
                }

                HStack(spacing: 15) {
                    Button {
                        applyColorPosterizeFilter()
                    } label: {
                        Text("適用")
                    }

                    Button {
                        uiImage = nil
                    } label: {
                        Text("リセット")
                    }
                }
                
                Stepper(value: $colorCount) {
                    Text("色数: \(colorCount)")
                }
            }
            .padding(.horizontal, 20)
        }
    }

    func applyColorPosterizeFilter() {
        let context = CIContext()
        let inputImage = CIImage(image: image)!

        guard let colorPosterizeFilter = CIFilter(name: "CIColorPosterize", parameters: [
            kCIInputImageKey: inputImage,
            "inputLevels": colorCount
        ]) else { return }
        guard let outputImage = colorPosterizeFilter.outputImage else { return }
        guard let cgImage = context.createCGImage(outputImage, from: outputImage.extent) else { return }
        self.uiImage = UIImage(cgImage: cgImage)
    }
}
おわり

CIFilterはまだまだたくさんあるので一通り触ってみたいです