protected function modify($image, $dst_x, $dst_y, $src_x, $src_y, $dst_w, $dst_h, $src_w, $src_h)
    {
        // create new image
        $modified = imagecreatetruecolor($dst_w, $dst_h);

        // get current image
        $resource = $image->getCore();

        // preserve transparency
        $transIndex = imagecolortransparent($resource);

        if ($transIndex != -1) {
            $rgba = imagecolorsforindex($modified, $transIndex);
            $transColor = imagecolorallocatealpha($modified, $rgba['red'], $rgba['green'], $rgba['blue'], 127);
            imagefill($modified, 0, 0, $transColor);
            imagecolortransparent($modified, $transColor);
        } else {
            imagealphablending($modified, false);
            imagesavealpha($modified, true);
        }

        // copy content from resource
        $result = imagecopyresampled(
            $modified,
            $resource,
            $dst_x,
            $dst_y,
            $src_x,
            $src_y,
            $dst_w,
            $dst_h,
            $src_w,
            $src_h
        );

        // set new content as recource
        $image->setCore($modified);

        return $result;
    }