Combine the hole and its parent boundary extracted from bwboundaries

I have a binary image and I used the bwboundaries to extracted several boundaries from the image.

However, bwboundaries not only gave me hole boundaries, but also their parents.

For example, the output from this code:

B = bwboundaries(mask, 'holes');

returns multiple cells and each cell correspond to a collection of boundary points for an object.

Cell 1 may be a parent object and Cell 2 may be a hole object reside within the parent object. What I want is to combine these two cells so that the output become a new object and is equivalent to the parent object substracted by the hole.

Right now, if I output the boundary and visualize using R, the result be like:

You can see the holes will be overlapped by their parent polygons.

Of course I can do it manually but I have hundreds of images to process and it is very time consuming to manually process them all.

Is there any computational way for batch processing? Thank you.

Answer 1

[Uday Pradhan]

Hello,

It is my understanding that you would like to remove the child-hole objects from the output of "bwboundaries". For this, you can utilise the below mentioned script which I adapted from the documentation page of this function (link):

[B,L,N,A] = bwboundaries(BW); 
figure; imshow(BW); hold on; 
% Loop through object boundaries  
for k = 1:N 
    % Boundary k is the parent of a hole if the k-th column 
    % of the adjacency matrix A contains a non-zero element 
    if (nnz(A(:,k)) > 0) 
        % Loop through the children of boundary k and remove them
        for l = find(A(:,k))' 
             B{l} = {};
        end 
    end 
end

Boundaries before removal of holes:

After removal of child-holes:

NOTE: Using bwboundaries with 'noholes' option also gives the same result as above. You might try that as well. I hope this helps!

[HAOYANG MI]

Hi Uday,

Thank you so much for the help. However, what I want is to still remain the hole but do not make the hole and parent two separate object. Is it possible?

[Uday Pradhan]

Hello, if I understand correctly, you can take the parent cell array and find its children through the same technique given in the code above and then combine the cell arrays. See: https://www.mathworks.com/help/matlab/matlab_prog/combine-cell-arrays.html

[HAOYANG MI]

Problem solved, thank you.