# Combine the hole and its parent boundary extracted from bwboundaries

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HAOYANG MI on 21 May 2021
Commented: HAOYANG MI on 24 May 2021
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.

Uday Pradhan on 24 May 2021
Edited: Uday Pradhan on 24 May 2021
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 on 24 May 2021
Problem solved, thank you.

R2020b

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