uniform shader image;
uniform float2 imageSize;
uniform float offset;
uniform float darkness;
half4 main(float2 coord) {
  float4 texel = unpremul(image.eval(coord));
  float2 uv = coord / imageSize;
  uv = (uv - float2(0.5)) * float2(offset);
  float4 color = float4(mix(texel.rgb, float3(1.0 - darkness), dot(uv, uv)), texel.a);
  color.rgb *= color.a;
  return color;
}