uniform shader image;
uniform float size;
uniform float amount;
uniform float darkness;
half4 main(float2 coord) {

  float4 sum = float4(0.0);

  float weights[4];
  weights[0] = 0.1531;
  weights[1] = 0.12245;
  weights[2] = 0.0918;
  weights[3] = 0.051;

  float4 baseColor = unpremul(image.eval(coord));
  for (int i = 1; i <= 4; i++) {
    float weight = weights[i - 1];
    float offset = size * float(i);
    sum += weight * (unpremul(image.eval(coord + float2(offset,  0))) - darkness);
    sum += weight * (unpremul(image.eval(coord + float2(-offset, 0))) - darkness);
    sum += weight * (unpremul(image.eval(coord + float2(0,  offset))) - darkness);
    sum += weight * (unpremul(image.eval(coord + float2(0, -offset))) - darkness);
  }
  sum += 2.0 * 0.1633 * (baseColor - darkness);

  // Additive Blend
  float4 result = baseColor + max(sum, 0.0) * amount;
  result = clamp(result, float4(0.0), float4(1.0));
  result.rgb *= result.a;
  return result;
}