Grafika\Gd\Editor::_blendOverlay PHP Method

    private function _blendOverlay($canvas, $gd1, $gd2, $loopStartY, $loopEndY, $loopStartX, $loopEndX, $offsetX, $offsetY, $opacity)
    {
        for ($y = $loopStartY; $y <= $loopEndY; $y++) {
            for ($x = $loopStartX; $x <= $loopEndX; $x++) {
                $canvasX = $x + $offsetX;
                $canvasY = $y + $offsetY;
                $argb1 = imagecolorat($gd1, $canvasX, $canvasY);
                $r1 = $argb1 >> 16 & 0xff;
                $g1 = $argb1 >> 8 & 0xff;
                $b1 = $argb1 & 0xff;
                $argb2 = imagecolorat($gd2, $x, $y);
                $a2 = $argb2 >> 24 & 0x7f;
                // 127 in hex. These are binary operations.
                $r2 = $argb2 >> 16 & 0xff;
                $g2 = $argb2 >> 8 & 0xff;
                $b2 = $argb2 & 0xff;
                $r1 /= 255;
                $r2 /= 255;
                if ($r1 < 0.5) {
                    $r3 = 2 * ($r1 * $r2);
                } else {
                    $r3 = 1 - 2 * (1 - $r1) * (1 - $r2);
                }
                $g1 /= 255;
                $g2 /= 255;
                if ($g1 < 0.5) {
                    $g3 = 2 * ($g1 * $g2);
                } else {
                    $g3 = 1 - 2 * (1 - $g1) * (1 - $g2);
                }
                $b1 /= 255;
                $b2 /= 255;
                if ($b1 < 0.5) {
                    $b3 = 2 * ($b1 * $b2);
                } else {
                    $b3 = 1 - 2 * (1 - $b1) * (1 - $b2);
                }
                $reverse = 127 - $a2;
                $reverse = round($reverse * $opacity);
                $argb3 = imagecolorallocatealpha($canvas, $r3 * 255, $g3 * 255, $b3 * 255, 127 - $reverse);
                imagesetpixel($canvas, $canvasX, $canvasY, $argb3);
            }
        }
        return $canvas;
    }