Find borders of a truth map

Hi,

I have an array

truth_map =

 [0 0 0 0 0 0 0 0 1 1 ; 
 0 0 0 0 0 0 0 0 1 1 ;
 0 0 0 0 0 0 0 1 1 1 ;
 0 0 0 0 0 0 1 1 1 0 ;
 0 0 0 0 0 1 1 1 1 0 ;
 0 1 1 1 1 0 0 0 0 0 ;
 1 1 1 1 1 0 0 0 0 0 ;
 1 1 1 1 1 0 0 0 0 0 ;
 1 1 1 1 0 0 0 0 0 0 ;
 1 1 1 0 0 0 0 0 0 0 ; ]

Which I want to make look like this:

truth_map =

 [0 0 0 0 0 0 0 0 1 0 ; 
 0 0 0 0 0 0 0 0 1 0 ;
 0 0 0 0 0 0 0 1 0 1 ;
 0 0 0 0 0 0 1 0 1 0 ;
 0 0 0 0 0 1 1 1 1 0 ;
 0 1 1 1 1 0 0 0 0 0 ;
 1 0 0 0 1 0 0 0 0 0 ;
 0 0 0 0 1 0 0 0 0 0 ;
 0 0 0 1 0 0 0 0 0 0 ;
 0 0 1 0 0 0 0 0 0 0 ; ]

This is trivial to do by hand for small arrays but my array is 1000x1000 and the area's are randomly shaped.

What would be a good way to just get an array of the borders?

Thanks, Daniel

Answer 1

[Guillaume]

The way you've defined your edges they correspond to either transition from 0 to 1 or from 1 to 0 when scanning from left to right or top to bottom. This is easily detected by diff:

m =  [0 0 0 0 0 0 0 0 1 1 ; 
      0 0 0 0 0 0 0 0 1 1 ;
      0 0 0 0 0 0 0 1 1 1 ;
      0 0 0 0 0 0 1 1 1 0 ;
      0 0 0 0 0 1 1 1 1 0 ;
      0 1 1 1 1 0 0 0 0 0 ;
      1 1 1 1 1 0 0 0 0 0 ;
      1 1 1 1 1 0 0 0 0 0 ;
      1 1 1 1 0 0 0 0 0 0 ;
      1 1 1 0 0 0 0 0 0 0 ; ]
m2 = [zeros(1, size(m, 2)); diff(m) == 1] | ... 0 to 1 from top to bottom
     [diff(m) == -1; zeros(1, size(m, 2))] | ... 1 to 0 from top to bottom
     [zeros(size(m, 1), 1), diff(m, [], 2) == 1] | ... 0 to 1 from left to right
     [diff(m, [], 2) == -1, zeros(size(m, 1), 1)] % 1 to 0 from left to right

[Daniel Cleveland]

Thanks so much! Exactly what I was looking for

Answer 2

[Walter Roberson]

http://www.mathworks.com/help/images/ref/bwboundaries.html

http://www.mathworks.com/help/images/ref/bwperim.html