Find local maxima of a matrix and the neighboring elements

Hi! I have a 2D matrix A and I used

     B=imregionalmax(A)

Now the matrix B contains 1 for all identified maximas. But I want to include also the surrounding elements and get the position (i,j) of the elements. How do I do this?

Thanks a lot!!

Answer 1

[Guillaume]

• Include the surrounding elements: imdilate your regionalmax binary image by the relevant structuring element. If you just want the orthogonal neighbours:

C = imdilate(B, [0 1 0;1 1 1;0 1 0])  %or imdilate(B, strel('diamond', 1))

If you also want the diagonal neighbours:

C = imdilate(B, ones(3)   %or imdilate(B, strel('square', 3))

• Get the position (i,j) of the elements:

[row, column] = find(C)

[Antonio Sereira]

Thanks,this is it!

Answer 2

[KSSV]

If (i,j) is the position and you want it's surrounding element positions, you can use:

[in,jn] = meshgrid(i-1:i+1,j-1:j+1) ;

You have to take care of edges. And in above in, jn includes (i,j) too.

Alternatively you can use knnsearch also.

[Antonio Sereira]

Hi! The problem is that imregionalmax(A) is not giving the positions (i,j) of the local maxima. Is there any other function to get the loacal maxima positions (i,j) of a 2D matrix? Then I could use meshgrid function. I tried already findpeaks() but it is not working for 2D arrays and a summing of all rows is not useful because I do not get the elemnts surrounding the local maxima. Therefore I sticked to imregionalmax().

[KSSV]

YOu can use max

[Antonio Sereira]

Hi, I used

[M,I]=max(B)

with

B=imregionalmax(A)

Now I is a row vector 1x60 (B was 100x60) where i correspond to the column of B and the number in cell i correspond to the j value of B. As I said before, B contains logical values, i.e. 1 for local maxima and zero else. Now I know the indices (i,j) of the local maxima. I want to create a new Matrix C (100x60) with logical numbers but the zeros surrounding 1 in B should be turned to 1, too.

How do I do this?

[KSSV]

[val,idx] = max(A(:)) ;
[i,j] = ind2sub(size(A),idx) ;