NFePHP\Extras\PdfNFePHP::ImagePngWithAlpha PHP Method

    function ImagePngWithAlpha($file, $x, $y, $w = 0, $h = 0, $link = '')
    {
        $tmp_alpha = tempnam('.', 'mska');
        $this->tmpFiles[] = $tmp_alpha;
        $tmp_plain = tempnam('.', 'mskp');
        $this->tmpFiles[] = $tmp_plain;
        list($wpx, $hpx) = getimagesize($file);
        $img = imagecreatefrompng($file);
        $alpha_img = imagecreate($wpx, $hpx);
        // generate gray scale pallete
        for ($c = 0; $c < 256; $c++) {
            ImageColorAllocate($alpha_img, $c, $c, $c);
        }
        // extract alpha channel
        $xpx = 0;
        while ($xpx < $wpx) {
            $ypx = 0;
            while ($ypx < $hpx) {
                $color_index = imagecolorat($img, $xpx, $ypx);
                $col = imagecolorsforindex($img, $color_index);
                imagesetpixel($alpha_img, $xpx, $ypx, $this->_gamma((127 - $col['alpha']) * 255 / 127));
                ++$ypx;
            }
            ++$xpx;
        }
        imagepng($alpha_img, $tmp_alpha);
        imagedestroy($alpha_img);
        // extract image without alpha channel
        $plain_img = imagecreatetruecolor($wpx, $hpx);
        imagecopy($plain_img, $img, 0, 0, 0, 0, $wpx, $hpx);
        imagepng($plain_img, $tmp_plain);
        imagedestroy($plain_img);
        //first embed mask image (w, h, x, will be ignored)
        $maskImg = $this->Image($tmp_alpha, 0, 0, 0, 0, 'PNG', '', true);
        //embed image, masked with previously embedded mask
        $this->Image($tmp_plain, $x, $y, $w, $h, 'PNG', $link, false, $maskImg);
    }