uniform shader image;
uniform float2 imageSize;
uniform float amount;
uniform float angle;

float4 imageEval(float2 uv) {
  uv.y = 1.0 - uv.y;
  float2 coord = uv * imageSize;
  return unpremul(image.eval(coord));
}

half4 main(float2 coord) {
  float2 uv = coord / imageSize;
  uv.y = 1.0 - uv.y;
  float2 offset = amount * float2(sin(angle), cos(angle));
  float2 greenAlpha = unpremul(image.eval(coord)).ga;
  float2 redAlpha = imageEval(uv + offset).ra;
  float2 blueAlpha = imageEval(uv - offset).ba;
  float3 color = float3(redAlpha.x, greenAlpha.x, blueAlpha.x);
  float avgAlpha = (redAlpha.y + greenAlpha.y + blueAlpha.y) / 3.0;
  return float4(color * avgAlpha, avgAlpha);
}