*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; }