finding index inside a region

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suppose I have a matrix I.
I =
4 4 4 4 4 4 4
4 1 1 1 1 3 0
4 1 3 3 1 3 0
4 1 3 3 1 3 0
4 1 1 1 1 3 0
4 4 4 4 4 4 4
idx is the index of 1's in the matrix including all 3's inside 1;
I need to find the index of those 3s those are inside 1s.
I tried with the following code.Perhaps this is not the correct one.
idx1=find(I(idx)==3);
How can I get that? can any body help?
Thanks

Accepted Answer

Image Analyst
Image Analyst on 26 Jul 2011
This will work to get ONLY the 3's inside, not any old number.
binaryImage = I==1
filledImage = imfill(binaryImage, 'holes')
maskedImage = I .* filledImage
justThe3s = ismember(maskedImage, 3)
[rows cols] = find(justThe3s)
You could use either the logical indexes "justThe3s" or the rows and cols variables, whichever you mean by "indexes."
  5 Comments
Nick Ward
Nick Ward on 23 Apr 2021
Thanks for the reply.
I think this is what I'm looking for although I get these errors. Any ideas?
Image Analyst
Image Analyst on 23 Apr 2021
@Nick Ward, attach your image/matrix, and m-file in a new question after reading this:

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More Answers (1)

Andrei Bobrov
Andrei Bobrov on 26 Jul 2011
BW = I==1;
[~,L] = bwboundaries(BW);
idx = find(L==2)
  1 Comment
Image Analyst
Image Analyst on 26 Jul 2011
This will get any collection of numbers inside a contiguous perimeter of 1's, not just the 3's. For example it would also find 4's, 42's, and 18's - whatever happened to be inside. Mohammad, see my answer if you want something that is more "tunable" to just one single number, if that is what you want.

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