for my $y ( 0 .. $HEIGHT-1 ) {
    my $s = substr( $$str, $y * $WIDTH, $WIDTH );
    while( $s =~ m[(([^\0])+)]g ) {
        my $c = ord($1); #, $-[0], $+[0];
        $b[ $c ][ LEFT   ] = $-[0]   if $-[0]   < $b[ $c ][ LEFT   ];
        $b[ $c ][ RIGHT  ] = $+[0]-1 if $+[0]-1 > $b[ $c ][ RIGHT  ];
        $b[ $c ][ TOP    ] = $y      if      $y < $b[ $c ][ TOP    ];
        $b[ $c ][ BOTTOM ] = $y      if      $y > $b[ $c ][ BOTTOM ];
    }
}