How can I change an element in a matrix with the median of itself and its surrounding elements.

Let's say I have a 10x10 matrix. For the element in the second row and second column, I want the median of the 3x3 matrix that surrounds that element, and I want the number replaced by the median. I also want this process to continue across the matrix rows and down the columns. I have the nested for loops set up to do have the calculations move in the direction they need to move, but I do not know how to specify the median function to take the localized 3x3 matrix of a larger array.

Answer 1

[Walter Roberson]

https://www.mathworks.com/help/images/ref/medfilt2.html

medfilt2() is already part of the Image Processing Toolbox.

You can also implement in terms of nlfilter() https://www.mathworks.com/help/images/ref/nlfilter.html which is also part of the toolbox.

If you need to code it yourself, then just use nested for loops and extract M(row-1:row+1, col-1:col+1) and take the median of that.

Answer 2

[per isakson]

"but I do not know how to specify the median function to take the localized 3x3 matrix of a larger array"

Combine these two examples. Calculate the median of a matrix

>> m3x3 = reshape( randperm(9), 3,3 )   % sample data
m3x3 =
     7     5     4
     2     6     1
     8     9     3
>> median( m3x3(:) )
ans =
     5

and find submatrix

>> m10x10 = reshape( (1:100), 10,10 );  % sample data
>> r = 7; c = 3;                        % sample data, row and column
>> m3x3 = m10x10( r-1:r+1, c-1:c+1 )
m3x3 =
    16    26    36
    17    27    37
    18    28    38