Question

I am working on Connected Components labeling, and my matrix is:

 1     1     0     2     2     2     0     3
 1     1     0     2     0     2     0     3
 1     1     1     1     0     0     0     3
 0     0     0     0     0     0     0     3
 4     4     4     4     0     5     0     3
 0     0     0     4     0     5     0     3
 6     6     0     4     0     0     0     3
 6     6     0     4     0     7     7     7

and now, i want to do the second scan over it, for this i have made following code:

for i=1:1:r
    for j=1:1:c
        if (bw(i,j)>=1) 

            if (i-1>0 & i+1<=r) 
                % if ( bw(i,j)~= bw(i-1,j) | bw(i,j)~= bw(i+1,j))
                if ( (bw(i,j)~= bw(i-1,j) & bw(i-1,j)>0))
                    bw(i,j)= min (bw(i-1,j),bw(i,j))
                elseif ((bw(i,j)~= bw(i+1,j) & bw(i+1,j)>0))
                    bw(i,j) = min(bw(i+1,j),bw(i,j));
                end
            end

            if (j-1>0 & j+1<=c) 
                if ( (bw(i,j)~= bw(i,j-1) & bw(i,j-1)>0))
                    bw(i,j) = min (bw(i,j-1),bw(i,j));
                elseif((bw(i,j)~= bw(i,j+1) & bw(i,j+1)>0))
                    bw(i,j) = min (bw(i,j+1),bw(i,j))  ; 
                end
            end

        end
    end
end
disp(bw);

but the problem is, when i run this code, i get the following output

 1     1     0     2     2     2     0     3
 1     1     0     1     0     2     0     3
 1     1     1     1     0     0     0     3
 0     0     0     0     0     0     0     3
 4     4     4     4     0     5     0     3
 0     0     0     4     0     5     0     3
 6     6     0     4     0     0     0     3
 6     6     0     4     0     7     7     7

only one value changes (2nd row, 4 col) in my result, whereas, I want:

     1     1     0     1     1     1     0     3
     1     1     0     1     0     1     0     3
     1     1     1     1     0     0     0     3
     0     0     0     0     0     0     0     3
     4     4     4     4     0     5     0     3
     0     0     0     4     0     5     0     3
     6     6     0     4     0     0     0     3
     6     6     0     4     0     3     3     3

could somebody please help? where am i making a mistake?

Answer 1

Nice solution by Jonas. As I was a bit into coding when I saw it added, I thought I’d show you my solution as well. Hopefully it is a bit more similar to your original code.

One of the mistakes you made was assuming this connection could be done in just one pass. Generaly it can not, as detections of some of the “snake elements” are dependent on the way you iterate throught the matrix. Becuase of this, I added an outer while loop.

 bw =[  1     1     0     2     2     2     0     3;
        1     1     0     2     0     2     0     3;
        1     1     1     1     0     0     0     3;
        0     0     0     0     0     0     0     3;
        4     4     4     4     0     5     0     3;
        0     0     0     4     0     5     0     3;
        6     6     0     4     0     0     0     3;
        6     6     0     4     0     7     7     7 ];


 %Set up matrix   
 [r,c] = size(bw);   

 %Zero pad border
 bwZ = [zeros(1,c+2);[zeros(r,1) bw zeros(r,1)];zeros(1,c+2)];

 %Iterate over all elements within zero padded border
 done=0;%Done flag 
 cc=1;  %Infinite loop protection
 while not(done) && cc<r*c
     done=1;cc=cc+1;
     for i=2:r+1
         for j=2:c+1
            %Point should be evaluated
            p = bwZ(i,j);
            if p >= 1
                %Pick out elements around active elements
                if bwZ(i-1,j)==0;ue=inf;else ue = bwZ(i-1,j); end;%Up element
                if bwZ(i+1,j)==0;de=inf;else de = bwZ(i+1,j); end;%Down element
                if bwZ(i,j-1)==0;le=inf;else le = bwZ(i,j-1); end;%Left element
                if bwZ(i,j+1)==0;re=inf;else re = bwZ(i,j+1); end;%Right element

                bwZ(i,j) = min([ue de le re]);
                %Set flag, if something has changed 
               if bwZ(i,j) ~= p
                    done = 0;
               end
            end
         end
     end
 end

 %Remove zero padding
 bw = bwZ(2:end-1,2:end-1)

Output:

bw =

 1     1     0     1     1     1     0     3
 1     1     0     1     0     1     0     3
 1     1     1     1     0     0     0     3
 0     0     0     0     0     0     0     3
 4     4     4     4     0     5     0     3
 0     0     0     4     0     5     0     3
 6     6     0     4     0     0     0     3
 6     6     0     4     0     3     3     3