If I select an element in a matrix, say MAT(2,2), how to find how many elements of the same value of MAT(2,2) in the matrix are "connected" to it.

For example, Mat=[1 2 3 4 2;2 2 2 2 0;2 0 4 2 1;1 3 4 2 8] and there're 7 elements (namely MAT(1,2), MAT(2,1), MAT(2,3), MAT(2,4), MAT(3,1), MAT(3,4), MAT(4,4)). I found that the function "bwareaopen" can be used to remove connected pixels whose size is smaller than the given threshold; I thought it must have used a similar trick, but I failed to figure it out.

Answer 1

[Massimo Zanetti]

You need bwconncomp

https://it.mathworks.com/help/images/ref/bwconncomp.html

For example:

Mat=[1 2 3 4 2;2 2 2 2 0;2 0 4 2 1;1 3 4 2 8]
%checks for the elements of Mat that equals Mat(2,2)
CC = bwconncomp(Mat==Mat(2,2));
connComponents = labelmatrix(CC)

since Mat(2,2)=2 you get

Mat =

       1     2     3     4     2
       2     2     2     2     0
       2     0     4     2     1
       1     3     4     2     8

connComponents =

      0    1    0    0    1
      1    1    1    1    0
      1    0    0    1    0
      0    0    0    1    0

Answer 2

[Andrei Bobrov]

[~,out] = bwlabel(Mat(2,2)==Mat)