Matrix in matlab? - matlab

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

Answer 2

If the numbers don't need to be preserved, you can simply call bwlabel on your original image:
newImage = bwlabel(originalImage>0);
EDIT
Here's another version. It checks each connected component to see whether there's another connected component touching it. If yes, that connected component is relabelled.
%# nCC: number of connected components
nCC = max(originalImage(:));
for cc = 1:nCC
%# check whether the component exists
myCC = originalImage==cc;
if any(any(myCC))
%# create a mask to check for neighbors
%# by creating a border of 1 pixel
%# around the original label
msk = imdilate(myCC,true(3)) & ~myCC;
%# read all the pixel values under the mask
neighbours = originalImage(msk);
%# we're not interested in zeros, remove them
neighbours = neighbours(neighbours > 0);
if ~isempty(neighbours)
%# set the label of all neighbours to cc
originalImage( ismember(originalImage,neighbours) ) = cc;
end
end
end