private function getHeaderSize()
    {
        // reset stream pointer to 0
        fseek($this->_stream, 0);
        // check for Aldus placeable metafile header
        // L: vorzeichenloser Long-Typ (immer 32 Bit, Byte-Folge maschinenabhängig)
        // magic:
        $magic = fread($this->_stream, 4);
        $headerSize = 18; // WMF header
        if ($magic == "\xd7\xcd\xc6\x9a") {
            $this->_headerStartBytesFound = true;
            $headerSize += 22; // Aldus header
        }
        return $headerSize;
    }