How to divide an image into blocks and find the RGB values of each pixels in a block?

How to divide an image into blocks and find the RGB values of each pixels in a block? I used the following code to divide an image into blocks of size 32*32

J = imresize(M1, [256 256]);
[r c]=size(J);
bs=32; % Block Size (32x32)
nob=64 % Total number of 32x32 Blocks
% Dividing the image into 32x32 Blocks
kk=0;
l=0;
[F]=zeros(1,64);
%[B0]=zeros(32,32);
for i=1:(r/bs)
  for j=1:(c/bs)    
    B0=J((bs*(i-1)+1:bs*(i-1)+bs),(bs*(j-1)+1:bs*(j-1)+bs),3);      
  end
  kk=kk+(r/bs);
end
% And to get the RGB values I used the following code
for x=1:32
  for y=1:32
    R=B0(x,y,1);
    G=B0(x,y,2);
    B=B0(x,y,3);  
  end
end

But it shows the error

Attempted to access B0(1,1,2); index out of bounds because size(B0)=[32,32,1].

Answer 1

[Thorsten]

To get the i,j 32x32 block from an RGB image, use

BL = I((i-1)*32+[1:32], (j-1)*32+[1:32], 1:3);

to select the R,G,B planes from this block, use

R = BL(:,:,1); G = BL(:,:,2); B = BL(:,:,3);

[Anushka]

Sir Im getting the error

Index exceeds matrix dimensions.for the line BL = I((i-1)*32+[1:32], (j-1)*32+[1:32], 1:3);

[Thorsten]

You have to ensure that i and j are not larger than the image height and width divided by 32. For a 256x256 image you have 256/32 = 8 blocks in each direction, so your i and j must be <= 8.

[Anushka]

i used it this way J = imresize(M1, [256 256]);

[r c]=size(J); bs=32; % Block Size (32x32)

nob=64 % Total number of 32x32 Blocks

% Dividing the image into 32x32 Blocks kk=0; l=0;

for i=1:(r/bs) for j=1:(c/bs) BL=J((i-1)*32+[1:32],(j-1)*32+[1:32],1:3);

end kk=kk+(r/bs); end

[Thorsten]

I tested it with

J = imread('peppers.png');
J = J(1:256, 1:256, :);

and it worked smoothly.

Note that you do not need the kk = kk + ... line and the nob = 64 line.

Also, so far you are just assigning 64 times a different subimage of J to BL, without doing anything with BL. So at the end of the loop, BL ist just the bottom right 32x32 subimage of J.

[Anushka]

Sir how did you initialized the value of BL?

[Thorsten]

I do not have to initialize BL; There is a bug in your code in the innocent looking line

 [r c]=size(J);

For color images, c would be the number of columns *3, and this results in out of range errors in the for-loop. Instead, use

r = size(J, 1); c = size(J, 2);

Here is the full example:

    J = imread('peppers.png');
    J = J(1:256, 1:256, :);
    r = size(J, 1); c = size(J, 2);
    bs=32; % Block Size (32x32)

    for i=1:(r/bs)
      for j=1:(c/bs)
        BL=J((i-1)*32+[1:32],(j-1)*32+[1:32],1:3); 
        imshow(BL), pause % show for testing
        R = BL(:,:,1); G = BL(:,:,2); B = BL(:,:,3);
        imshow([R G B]), pause % show for testing
      end
    end