SwiftUI Shader Simple Lens

by Annett Schwarze | 2026-01-30

With SwiftUI it is easy to add Metal shaders and with this example a simple lens effect is created. The simple lens shader is implemented in a `.metal` file. The function name is defined as `simpleLens`. To use it in SwiftUI, the modifier `.visualEffect` is used. `simpleLens` is automatically available as part of the `ShaderLibrary` type.

import SwiftUI

struct SampleShaderLensView: View {
    @State private var loc = CGPoint.zero

    var body: some View {
        ZStack {
            Image("mtfuji")
                .resizable()
                .scaledToFill()
            Text("富士山\nMount Fuji")
                .font(.largeTitle)
                .bold()
        }
            .drawingGroup()
            .visualEffect { content, proxy in
                content
                    .layerEffect(
                        ShaderLibrary.simpleLens(
                            .float2(proxy.size.applying(CGAffineTransform(scaleX: 0.3, y: 0.3))),
                            .float2(loc),
                            .float(0.15),
                            .float(1.5)
                        ),
                        maxSampleOffset: .zero
                    )
            }
            .gesture(
                DragGesture(minimumDistance: 0)
                    .onChanged { loc = $0.location }
            )
    }

}

#Preview {
    SampleShaderLensView()
}
    

#include 
#include 
using namespace metal;

[[ stitchable ]] half4 simpleLens(
    float2 position,
    SwiftUI::Layer layer,
    float2 size,
    float2 pos,
    float maxDistance,
    float zoomFactor
) {
    half2 norm = half2(position / size);
    half2 center = half2(pos / size);
    half2 d = norm - center;
    half ar = size.x / size.y;
    half distance = (d.x * d.x) + (d.y * d.y) / ar / ar;
    half zm = 1.0h;
    if (distance < maxDistance) {
        zm /= zoomFactor;
    }
    half2 p = d * zm + center;
    return layer.sample(float2(p) * size);
}