duotone-style shader

shader.metal

#include <metal_stdlib>
using namespace metal;
kernel void duotone(
  texture2d<float, access::write> outTex [[texture(0)]],
  texture2d<float, access::read> inTex [[texture(1)]],
  uint2 gid [[thread_position_in_grid]])
{
    float3 c = inTex.read(gid).rgb;
    float mn = (c.r + c.g + c.b) / 3.0;
    float4 out = float4(0.5, 0.5 + mn, 1.0, 1.0);
    outTex.write(out.rgba, gid);
}