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

Question

Preet Lal 2021-11-15

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1587174-i-have-to-select-mode-from-matrix-with-sliding-window-of-3x3-how-to-do-that

评论： Preet Lal 2021-11-16

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

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Adam Danz 2021-11-15

"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?

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Answer 1

Adam Danz 2021-11-15

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1587174-i-have-to-select-mode-from-matrix-with-sliding-window-of-3x3-how-to-do-that#answer_832139

编辑：Adam Danz 2021-11-15

在 MATLAB Online 中打开

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.

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Preet Lal 2021-11-15

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 2021-11-16

@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.

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