Extract specific data from 3D matrix

I would like to kindly ask you for the help with the issue of extracting specific values from 3D matrix. I have the variable A(10,10,3) containing the elements 1, 2, 3, and the second variable B(10,10,3) containing pixel values. I need to extract and store the elements from each layer of B which are in the same position as elements 1 from A. Thus, these values should be stored in a cell array (it should contain 3 cells). I was able to perform this task for one 2D matrix, but in the case of 3D matrix I am not able to do that. I would like to kindly ask you for help.

[Iuliu Ardelean]

Could you give an example?

Answer 1

[Iuliu Ardelean]

size = 10;
A = randi(3, [size size 3]); % matrix containing elements 1, 2 and 3
B = randi(255, [size size 3]); % RGB matrix
[row, col] = find(A==1);

% I wasn't sure if you wanted to extract only one of R, G, or B, 
% OR you wanted to extract all three R, G, and B at a same time...

% so option 1: if you want to extract only one of R, G, and B at a time:
pixel_values = zeros(size, size, 3);
pixel_values(A==1) = B(A==1)

% and option 2: if you want to extract all three R, G, and B at the same time:
pixel_values = zeros(size, size, 3);
col = mod(col,size); % this is because find returns linear indices if matrix is multidimensional
col(col == 0) = size;
pixel_values(row, col, :) = B(row, col, :)

Check find behaviour here: https://uk.mathworks.com/help/matlab/ref/find.html#d122e398961

[Jan Kubicek]

Thank you very much for the reaction. It is a nice approach. Nevertheless, I need to get only arrays of the pixels from each layer of B in the same positions as A==1 . I am using the first option, I tried to make a cell array, containg only these pixels, but it probably does not work well, please can you help me with that:

for i=1:3

pix{i}=pixel_values(pixel_values(:,:,i)>0);

end

My task is I need to compute histogram of the pixel values from each layer of the matrix B.

Thank you very much.

Answer 2

[Walter Roberson]

A = randi(4, 5,5,3)

A = 
A(:,:,1) =

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


A(:,:,2) =

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


A(:,:,3) =

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

B = randi(9, 5,5,3)

B = 
B(:,:,1) =

     9     5     4     9     4
     1     9     8     9     1
     9     7     7     9     9
     6     4     9     3     6
     1     1     3     2     2


B(:,:,2) =

     8     1     4     7     9
     6     1     2     9     1
     2     7     2     1     7
     2     5     4     8     4
     9     6     6     5     4


B(:,:,3) =

     5     9     2     8     3
     3     7     1     8     5
     7     2     1     6     7
     2     5     9     1     4
     9     3     4     1     1

idx = find(A==1);
temp = B(idx);
[R,C,P] = ind2sub(size(A),idx);
pix = arrayfun(@(p) temp(P==p).', 1:max(P), 'uniform', 0);
celldisp(pix)

 
pix{1} =
 
     6     8     3     4     6     2

 
 
pix{2} =
 
     7     4     2     6

 
 
pix{3} =
 
     9     3     9     8     3     5     7

 

[Walter Roberson]

A = randi(4, 5,5,3)

A = 
A(:,:,1) =

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


A(:,:,2) =

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


A(:,:,3) =

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

B = randi(9, 5,5,3)

B = 
B(:,:,1) =

     6     9     1     1     1
     1     7     5     7     5
     3     3     2     2     6
     3     1     7     4     3
     8     2     8     4     7


B(:,:,2) =

     3     5     2     8     4
     7     6     6     1     8
     7     3     4     4     3
     3     2     9     1     9
     8     8     8     4     3


B(:,:,3) =

     9     8     8     4     2
     4     6     5     5     9
     9     7     1     7     7
     2     1     6     4     9
     6     2     1     2     5

SELECT = @(M,W) M(W);
pix = arrayfun(@(p) SELECT(B(:,:,p), A(:,:,p)==1).', 1:size(A,3), 'uniform', 0);
celldisp(pix)

 
pix{1} =
 
     9     1     8     1     4     3

 
 
pix{2} =
 
     8     2     8     4     4     9

 
 
pix{3} =
 
     6     8     7     5     1     4     4     2     7

 

[Jan Kubicek]

I would like to kindly thank you very much for your contributions, they work very well :-)