How can I randomly select 1000 small matrix like 11X11 from 7081X7811 matrix size?

I have 7081X7811 size of matrix which have image pixel values, want to randomly select small matrix of size 11X11.

Right now, I am reading one small matrix like:

[A, R] = geotiffread('...tif'); %reading .tif image

%(200,1414) this center for below matrix

i0 = 200;

j0 = 1414; %center of small 11X11 matrix

[row, col] = size(A);

A_small = A((i0-5):(i0+5),(j0-5):(j0+5));

can you guys please help me in this?

Answer 1

[Voss]

[A, R]      = geotiffread('...tif'); %reading .tif image
[row, col]  = size(A); % row = 7081, col = 7811

N_small = 1000;
small_size = [11 11];
min_row = (small_size(1)+1)/2; % min_row = 6
max_row = row-min_row+1;       % max_row = 7076
min_col = (small_size(2)+1)/2; % min_col = 6
max_col = col-min_col+1;       % max_col = 7806

% A_small is a matrix of size 11-by-11-by-1000, each slice of 
% which will be a random 11-by-11 selection from A:
A_small = zeros(small_size(1),small_size(2),N_small);

for kk = 1:N_small
    
    % max_row-min_row+1 = 7071
    %   -> randi returns a random integer between 1 and 7071
    %   -> i0 is a random integer between 6 and 7076
    i0 = randi(max_row-min_row+1)+min_row-1;
    
    % max_col-min_col+1 = 7801
    %   -> randi returns a random integer between 1 and 7801
    %   -> j0 is a random integer between 6 and 7806
    j0 = randi(max_col-min_col+1)+min_col-1;
    
    % select from A the 11-by-11 matrix centered at i0,j0
    % and assign it to the kk-th slice of A_small
    A_small(:,:,kk) = A(i0-min_row+1:i0+min_row-1,j0-min_col+1:j0+min_col-1);
    
end

[Rahul Shah]

Hey, Thanks for the reply.

[Voss]

You're welcome!

[Rahul Shah]

Hey, Here in my A matrix, it also contains lots of 0 values and while randomly selecting the i0 and j0, I got matrix of which have only 0 values. So, can I get rid of this 0 values while randomly selecting i0 and j0.

[Voss]

The code below will do that, by picking another random 11x11 matrix in case the one selected was all 0:

[A, R]      = geotiffread('...tif'); %reading .tif image
[row, col]  = size(A); % row = 7081, col = 7811

N_small = 1000;
small_size = [11 11];
min_row = (small_size(1)+1)/2; % min_row = 6
max_row = row-min_row+1;       % max_row = 7076
min_col = (small_size(2)+1)/2; % min_col = 6
max_col = col-min_col+1;       % max_col = 7806

% A_small is a matrix of size 11-by-11-by-1000, each slice of 
% which will be a random 11-by-11 selection from A:
A_small = zeros(small_size(1),small_size(2),N_small);

% counter to keep track of which slice we're trying to get
kk = 1;

while kk <= N_small
    
    % max_row-min_row+1 = 7071
    %   -> randi returns a random integer between 1 and 7071
    %   -> i0 is a random integer between 6 and 7076
    i0 = randi(max_row-min_row+1)+min_row-1;
    
    % max_col-min_col+1 = 7801
    %   -> randi returns a random integer between 1 and 7801
    %   -> j0 is a random integer between 6 and 7806
    j0 = randi(max_col-min_col+1)+min_col-1;
    
    % select from A the 11-by-11 matrix centered at i0,j0
    % and assign it to the kk-th slice of A_small
    new_slice = A(i0-min_row+1:i0+min_row-1,j0-min_col+1:j0+min_col-1);
    
    % don't assign a new_slice that's all zeros to A_small ...
    if all(new_slice(:) == 0)
        continue % ... pick another random i0, j0 instead
    end
    
    % assign the valid new_slice to A_small:
    A_small(:,:,kk) = new_slice;
    % increment the counter:
    kk = kk+1;
    
end

[Rahul Shah]

Thanks for reply,

I will let you know by trying this.

[Voss]

You're welcome! Yes, please let me know if it works, because I didn't run it (I don't have the tif image).

Answer 2

[David Hill]

r=randi(7070,1000,1);
c=randi(7800,1000,1);
for k=1:1000
    newMatrix(:,:,k)=yourMatrix(r(k):r(k)+11,c(k):c(k)+11);
end

[Torsten]

I think

r=randi(7071,1000,1);
c=randi(7801,1000,1);
for k=1:1000
    newMatrix(:,:,k)=yourMatrix(r(k):r(k)+10,c(k):c(k)+10);
end

instead of

r=randi(7070,1000,1);
c=randi(7800,1000,1);
for k=1:1000
    newMatrix(:,:,k)=yourMatrix(r(k):r(k)+11,c(k):c(k)+11);
end