precision mediump float;
uniform vec2 u_blur_radius;
uniform vec2 u_scale_add;
varying vec2 v_offset;   // Relative to the blur center.
varying vec4 v_color;
void main() {
  // Distance from the blur radius.
  // Both v_offset and u_blur_radius are expressed in terms of the
  //   blur sigma.
  vec2 pos = abs(v_offset) - u_blur_radius;
  vec2 pos2 = pos * pos;
  vec2 pos3 = pos2 * pos;
  vec4 posx = vec4(1.0, pos.x, pos2.x, pos3.x);
  vec4 posy = vec4(1.0, pos.y, pos2.y, pos3.y);

  // Approximation of the gaussian integral from [x, +inf).
  // http://stereopsis.com/shadowrect/
  const vec4 klower = vec4(0.4375, -1.125, -0.75, -0.1666666);
  const vec4 kmiddle = vec4(0.5, -0.75, 0.0, 0.3333333);
  const vec4 kupper = vec4(0.5625, -1.125, 0.75, -0.1666666);

  float gaussx = 0.0;
  if (pos.x < -1.5) {
    gaussx = 1.0;
  } else if (pos.x < -0.5) {
    gaussx = dot(posx, klower);
  } else if (pos.x < 0.5) {
    gaussx = dot(posx, kmiddle);
  } else if (pos.x < 1.5) {
    gaussx = dot(posx, kupper);
  }

  float gaussy = 0.0;
  if (pos.y < -1.5) {
    gaussy = 1.0;
  } else if (pos.y < -0.5) {
    gaussy = dot(posy, klower);
  } else if (pos.y < 0.5) {
    gaussy = dot(posy, kmiddle);
  } else if (pos.y < 1.5) {
    gaussy = dot(posy, kupper);
  }

  float alpha_scale = gaussx * gaussy * u_scale_add.x + u_scale_add.y;
  gl_FragColor = v_color * alpha_scale;
}