it works somewhat, but not really

src/metal/Compute.metal

@@ -272,3 +272,47 @@ kernel void computeKernel(
     accumulator.write(result, threadId);
     image.write(pow(max(result * resolver, 0), float4(0.45f)), threadId);
 }
+
+
+// Screen filling quad in normalized device coordinates.
+constant float2 quadVertices[] = {
+    float2(-1, -1),
+    float2(-1,  1),
+    float2( 1,  1),
+    float2(-1, -1),
+    float2( 1,  1),
+    float2( 1, -1)
+};
+
+struct CopyVertexOut {
+    float4 position [[position]];
+    float2 uv;
+};
+
+// Simple vertex shader that passes through NDC quad positions.
+vertex CopyVertexOut copyVertex(unsigned short vid [[vertex_id]]) {
+    float2 position = quadVertices[vid];
+
+    CopyVertexOut out;
+
+    out.position = float4(position, 0, 1);
+    out.uv = position * 0.5f + 0.5f;
+
+    return out;
+}
+
+// Simple fragment shader that copies a texture and applies a simple tonemapping function.
+fragment float4 copyFragment(CopyVertexOut in [[stage_in]],
+                             texture2d<float> tex)
+{
+    constexpr sampler sam(min_filter::nearest, mag_filter::nearest, mip_filter::none);
+
+    float3 color = tex.sample(sam, in.uv).xyz;
+
+    // Apply a simple tonemapping function to reduce the dynamic range of the
+    // input image into a range which the screen can display.
+    color = color / (1.0f + color);
+
+    return float4(color, 1.0f);
+}
+