finding index inside a region

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

Answer 1

[Image Analyst]

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."

[Mohammad Golam Kibria]

Thanks

[Nick Ward]

Hi Image Analyst, how could I also specify the size of contiguous values above of >3 ? Say, a minimum region of 4x4 >3 within a larger martrix?

Thanks

[Image Analyst]

@Nick Ward, what do you mean by "Specify the size"? How about

binaryMap = largerMatrix > 3 % All elements > 3
% Get only those that are in contiguous regions of 16 or more
minAllowableSize = 16
bigBlobs = regionprops(binaryMap, minAllowableSize)

[Nick Ward]

Thanks for the reply.

I think this is what I'm looking for although I get these errors. Any ideas?

[Image Analyst]

@Nick Ward, attach your image/matrix, and m-file in a new question after reading this:

TUTORIAL: How to ask a question (on Answers) and get a fast answer

Answer 2

[Andrei Bobrov]

BW = I==1;
[~,L] = bwboundaries(BW);
idx = find(L==2)

[Image Analyst]

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.