Here is a solution to the problem: 
# read in matrix
while (<DATA>) {
   next if /^\s*$/;
   push @r, [split];
}

# find the boundaries
for my $r (0..@r-1) {
   for my $c (0..@{$r[0]}-2) {
      $vert[$r][$c] = $r[$r][$c] ne $r[$r][$c+1] ? 1 : 0;
   }
}
for my $r (0..@r-2) {
   for my $c (0..@{$r[0]}-1) {
      $horz[$r][$c] = $r[$r][$c] ne $r[$r+1][$c] ? 1 : 0;
   }
}
for my $r (0..@r-2) {
   for my $c (0..@{$r[0]}-2) {
      $cross[$r][$c] =  $vert[$r][$c] + $vert[$r+1][$c]
                      + $horz[$r][$c] + $horz[$r][$c+1] > 1 ? 1 : 0;
   }
}

# Print it out
for my $r (0..@r-1) {
   for my $c (0..@{$r[0]}-1) {
      print $r[$r][$c];
      print $vert[$r][$c] ? '|' : ' ' if $c < @{$r[0]}-1;
   }
   print "\n";
   next unless $r < @r-1;
   for my $c (0..@{$r[0]}-1) {
      print $horz[$r][$c] ? '-' : ' ' ;
      print $cross[$r][$c]  ? '+' : ' ' if $c < @{$r[0]}-1;
   }
   print "\n";
}
__DATA__
P P B R B

P R B B B

B R R R B

B R R R B

B G G B B

[download]
I have simplified the printing to just put a cross at all intersections, you can easily generalize to vertical and horizontal connections, etc. I'll leave the conversion to Fortran 1-based matrices to you :-)

Update: If efficiency is a concern, the three passes to find the boundary can be combined into one with a little extra logic. For large matrices, this change may blow the cache fewer times.

-Mark

In reply to Re: Map grid to boundary conversion algorithm by kvale
in thread Map grid to boundary conversion algorithm by Willard B. Trophy

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