I have to select mode from matrix with sliding window of 3X3. How to do that?

I have to create a new matrix with mode of 3x3 sliding window, I have written code, but it's not working.

[m, n] = size(LULC_01km);

rows = ceil(m/3);
coln = ceil(n/3);

LULC_3_km = NaN(rows,coln);

for i = 2:rows-1
    for j = 2:coln-1
        
        LULC_3_km(i,j) = mode(LULC_01km((i-1:i+1), (j-1:1+j)), 'all');
        
    end
end

[Adam Danz]

"It's not working" doesn't give us much info. What part isn't working?

• Is the moving window not sliding as expected?
• Are the outputs not stored in the expected indices?
• Do you have doubts about the output values?

Answer 1

[Adam Danz]

n*m sliding mode; slides by intervals of 1

This computes the sliding mode within a 2D window that slides by 1 unit horizontally then vertically.

• LULC_01km - your n*m matrix
• winSz - define window size
• LULC_3_km - output containing modes

LULC_01km = randi(5,9,12) % n*m matrix 

LULC_01km = 9×12

4 3 5 3 2 1 4 3 4 1 4 2 3 5 1 1 2 4 2 5 3 5 3 2 1 4 5 2 1 3 3 3 2 1 3 4 5 2 2 3 1 2 2 1 3 4 1 5 5 2 3 1 5 3 3 3 4 2 3 5 5 1 1 3 2 5 5 1 3 3 5 3 3 2 5 1 2 1 5 2 2 2 2 3 5 1 5 1 4 2 4 5 3 4 2 3 3 2 3 5 5 4 5 5 1 3 1 5

winSz = [3,3];   % window size [width (x), height (y)]

% Compute starting coordinate of sliding windows
[m, n] = size(LULC_01km);
winX0 = 1:n-winSz(1)+1;   % starting index of x-values for each window
winY0 = 1:m-winSz(2)+1;   % starting index of y-values for each window

% Loop through windows to compute mode
LULC_3_km = NaN(numel(winY0),numel(winX0));
xWin = 0:winSz(1)-1;
yWin = 0:winSz(2)-1; 
for i = 1:size(LULC_3_km,1)
    for j = 1:size(LULC_3_km,2)
        LULC_3_km(i,j) = mode(LULC_01km(winY0(i)+yWin, winX0(j)+xWin),'all');
    end
end

% Show results
LULC_3_km

LULC_3_km = 7×10

5 5 1 1 2 3 3 3 3 1 5 2 1 1 2 2 3 3 3 1 2 2 1 1 3 3 3 3 3 1 2 1 1 3 2 3 3 3 3 3 5 1 1 1 5 3 3 2 2 3 5 1 1 1 2 5 5 2 2 3 3 1 5 1 4 5 5 2 2 2

LULC_3_km(i,j) is the mode of values within the 3x3 window with the upper left corner at LULC_01km(i,j).

n*m sliding mode; slides by intervals of m (horizontally) and n (vertically)

To use a boxcar moving mode, simply redefine the winX0 and winY0 variables as

winX0 = 1:winSz(1):n-winSz(1)+1;   % starting index of x-values for each window
winY0 = 1:winSz(2):m-winSz(2)+1;   % starting index of y-values for each window

For a 9x12 input matrix, this will create a 3x4 ouput matrix. To see an animation of a 3x3 boxcar filter, see this answer.

[Preet Lal]

Thank you, Now I found, i have to go for another method, please help me with code as:

Here you have created matrix of 9,12 and output as 7,10 but with moving window I need as output matrix 3,4. So above shown code by me is wrong. So one window will move one left and right, then second window will move column and row 4.

[Adam Danz]

You're describing a boxcar filter which is demonstrated in this answer.

I'll update my answer with a second option for boxcar modes.

[Preet Lal]

Hello Adam Danz,

Could you please look into this question: https://www.mathworks.com/matlabcentral/answers/1587494-solving-equation-for-different-matrix-size

Thank You in advance

[Preet Lal]

@Adam Danz

Could you please tell me how to move forward by keeping window from center, like i-1:i+1 for row 1 to 3 and next using similar condition how i can jump to 4 to 6. My code should work like this.