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How to display the two biggest blobs in terms of area?

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sanyukta
sanyukta on 22 Jul 2019
Commented: Image Analyst on 23 Jul 2019
I have a binary image 'i' which I have attached here. From this image, I want to find the two biggest blobs in terms of area. When I tried this code, I am able to find the biggest blob which I have attached and the next biggest blob is removed.
labeledImage=bwlabel(i);
measurements=regionprops(labeledImage,'Area');
allAreas=[measurements.Area];
[biggestArea, indexOfBiggest]=sort(allAreas,'descend')
biggestBlob=ismember(labeledImage,indexOfBiggest(1));
biggestBlob=biggestBlob>0;
figure,imshow(biggestBlob,[]);
I tried to use the bwareafilt() but MATLAB R2013a does not support it. Kindly help me to display the next biggest blob.
Thanking you.

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Accepted Answer

David K.
David K. on 22 Jul 2019
In the line
biggestBlob=ismember(labeledImage,indexOfBiggest(1));
you are finding the biggest by looking at the first index in a sorted array. So to get the second biggest you do
secondBlob=ismember(labeledImage,indexOfBiggest(2));
Then you can combine them simply by doing
twoBiggest = biggestBlob | secondBlob;
This is because both blobs are binary - 1s and 0s - so by doing an OR operation it makes every white part in both images seen in both images.

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Image Analyst
Image Analyst on 23 Jul 2019
I think
twoBiggest = ismember(labeledImage, indexOfBiggest(1:2));
would be simpler. It gets you both of the two biggest blobs all in one line of code.

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More Answers (1)

Image Analyst
Image Analyst on 22 Jul 2019
It'w very bad practice to call your image i (the imaginary variable). Call it binaryImage, mask, or BW instead. You might want to call imclearborder() to get rid of that outer blob, unless you want it.
mask = imclearborder(mask);
I don't believe you need to use
biggestBlob=biggestBlob>0;
because ismember() will return a logical image already.
Other than that, David's solution should work.

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sanyukta
sanyukta on 23 Jul 2019
Thank you so much for this valuable suggestion.

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