*color_a = sum_a;
    // now un-premultiply the alpha
    if (sum_a > 0.0) {
      *color_r = sum_r / sum_a;
      *color_g = sum_g / sum_a;
      *color_b = sum_b / sum_a;
    } else {
      // it is all transparent, so don't care about the colors
      // (let's make them ugly so bugs will be visible)
      *color_r = 0.0;
      *color_g = 1.0;
      *color_b = 0.0;
    }