Grafika\Gd\Editor::_blendScreen PHP Method

    private function _blendScreen($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;
                $r3 = 255 - (255 - $r1) * (255 - $r2) / 255;
                $g3 = 255 - (255 - $g1) * (255 - $g2) / 255;
                $b3 = 255 - (255 - $b1) * (255 - $b2) / 255;
                $reverse = 127 - $a2;
                $reverse = round($reverse * $opacity);
                $argb3 = imagecolorallocatealpha($canvas, $r3, $g3, $b3, 127 - $reverse);
                imagesetpixel($canvas, $canvasX, $canvasY, $argb3);
            }
        }
        return $canvas;
    }