function getFontDecender( $size = false )
    {
        // note that this will most likely return a negative value
        if ( !$this->numFonts )
        {
            $this->selectFont( './fonts/Helvetica' );
        }
        $h = $this->fonts[$this->currentFont]['FontBBox'][1];
        if ( $size === false )
        {
            $size = $this->fontSize();
        }
        return $size*$h/1000;
    }