//

void runAlphaTest(float alpha, float alphaThreshold)
{
  if (alphaThreshold > 0.0) {
    // http://alex-charlton.com/posts/Dithering_on_the_GPU/
    // https://forums.khronos.org/showthread.php/5091-screen-door-transparency
    mat4 thresholdMatrix = mat4(
      1.0  / 17.0,  9.0 / 17.0,  3.0 / 17.0, 11.0 / 17.0,
      13.0 / 17.0,  5.0 / 17.0, 15.0 / 17.0,  7.0 / 17.0,
      4.0  / 17.0, 12.0 / 17.0,  2.0 / 17.0, 10.0 / 17.0,
      16.0 / 17.0,  8.0 / 17.0, 14.0 / 17.0,  6.0 / 17.0
    );

    alpha = clamp(alpha - 0.5 * thresholdMatrix[int(mod(gl_FragCoord.x, 4.0))][int(mod(gl_FragCoord.y, 4.0))], 0.0, 1.0);

    if (alpha < alphaThreshold)
      discard;
  }
}