NFePHP\Extras\PdfNFePHP::_parsepng PHP Method

    function _parsepng($file)
    {
        //Extract info from a PNG file
        $f = fopen($file, 'rb');
        if (!$f) {
            $this->Error('Can\'t open image file: ' . $file);
        }
        //Check signature
        if ($this->_readstream($f, 8) != chr(137) . 'PNG' . chr(13) . chr(10) . chr(26) . chr(10)) {
            $this->Error('Not a PNG file: ' . $file);
        }
        //Read header chunk
        $this->_readstream($f, 4);
        if ($this->_readstream($f, 4) != 'IHDR') {
            $this->Error('Incorrect PNG file: ' . $file);
        }
        $w = $this->_readint($f);
        $h = $this->_readint($f);
        $bpc = ord($this->_readstream($f, 1));
        if ($bpc > 8) {
            $this->Error('16-bit depth not supported: ' . $file);
        }
        $ct = ord($this->_readstream($f, 1));
        if ($ct == 0) {
            $colspace = 'DeviceGray';
        } elseif ($ct == 2) {
            $colspace = 'DeviceRGB';
        } elseif ($ct == 3) {
            $colspace = 'Indexed';
        } else {
            fclose($f);
            // the only changes are
            return 'alpha';
            // made in those 2 lines
        }
        if (ord($this->_readstream($f, 1)) != 0) {
            $this->Error('Unknown compression method: ' . $file);
        }
        if (ord($this->_readstream($f, 1)) != 0) {
            $this->Error('Unknown filter method: ' . $file);
        }
        if (ord($this->_readstream($f, 1)) != 0) {
            $this->Error('Interlacing not supported: ' . $file);
        }
        $this->_readstream($f, 4);
        $parms = '/DecodeParms <</Predictor 15 /Colors ' . ($ct == 2 ? 3 : 1) . ' /BitsPerComponent ' . $bpc . ' /Columns ' . $w . '>>';
        //Scan chunks looking for palette, transparency and image data
        $pal = '';
        $trns = '';
        $data = '';
        do {
            $n = $this->_readint($f);
            $type = $this->_readstream($f, 4);
            if ($type == 'PLTE') {
                //Read palette
                $pal = $this->_readstream($f, $n);
                $this->_readstream($f, 4);
            } elseif ($type == 'tRNS') {
                //Read transparency info
                $t = $this->_readstream($f, $n);
                if ($ct == 0) {
                    $trns = array(ord(substr($t, 1, 1)));
                } elseif ($ct == 2) {
                    $trns = array(ord(substr($t, 1, 1)), ord(substr($t, 3, 1)), ord(substr($t, 5, 1)));
                } else {
                    $pos = strpos($t, chr(0));
                    if ($pos !== false) {
                        $trns = array($pos);
                    }
                }
                $this->_readstream($f, 4);
            } elseif ($type == 'IDAT') {
                //Read image data block
                $data .= $this->_readstream($f, $n);
                $this->_readstream($f, 4);
            } elseif ($type == 'IEND') {
                break;
            } else {
                $this->_readstream($f, $n + 4);
            }
        } while ($n);
        if ($colspace == 'Indexed' && empty($pal)) {
            $this->Error('Missing palette in ' . $file);
        }
        fclose($f);
        return array('w' => $w, 'h' => $h, 'cs' => $colspace, 'bpc' => $bpc, 'f' => 'FlateDecode', 'parms' => $parms, 'pal' => $pal, 'trns' => $trns, 'data' => $data);
    }