error in splitting an image into blocks of pixels in matlab

Question

i am trying to split an image into blocks 16x16 here is my code

 A = imread('telec.jpg'); %reads file into a matrix

B = rgb2ycbcr(A); %reads file info

%convert to YCbCr
%if B.Format=='bmp'
%    A=rgb2ycbcr(A)

[width height]=size(B);
%detirmine number of 8x8 matrices, use ceil to round up
W=ceil(width/8);
H=ceil(height/8);

%create a matrix of zeros and add the image to it to fill out the 8x8
%matrices  (matrix will stay the same size if height and width are
%divisible by 8
I=zeros(H*8,W*8,'uint8');
I(1:height,1:width)=B(1:height,1:width);

%divide numbers into WxH 8x8 matrices
X=zeros(H,W,8,8);
for J=1:H
    for K=1:W
        for j=1:8
            for k=1:8
                X(J,K,j,k)=I((J-1)*8+j,(K-1)*8+k);
            end
        end
    end
end

i get this:

??? Index exceeds matrix dimensions.

Error in ==> projet at 18

I(1:height,1:width)=B(1:height,1:width); can anyone help?thanks in advance

change `B(1:height,1:width)` to `B(1:width,1:height)` or better than that change `[width height]=size(B);` to `[height width]=size(B);` — Rashid, Nov 20 '14 at 15:25

Divakar · Answer 1 · 2014-11-20T16:42:07.333

Assuming that the dimensions of 2D input matrix I are perfectly divisible by the blocksize 8, alternatively you can use a no-loop approach that employs couple of reshape's and permute's to achieve your 4D array output X -

%// About script: Blockwise split a 2D array into a 4D array
%// Input(s): I (Input 2D array), N (Blocksize)
%// Output(s): X (Output 4D array)
N = 8; %// blocksize
[m,n] = size(I); %// Size of 2D input array
X = permute(reshape(permute(reshape(I,N,m/N,[]),[1 3 2]),N,N,n/N,[]),[1 2 4 3])

Sample run (with blocksize = 2) -

I =
    0.6414    0.4333    0.1077    0.3413    0.0226    0.6582
    0.1388    0.5903    0.8644    0.7008    0.1167    0.8336
    0.0079    0.4648    0.4698    0.4513    0.6500    0.8422
    0.5720    0.1261    0.4916    0.7439    0.0378    0.8782
X(:,:,1,1) =
    0.6414    0.4333
    0.1388    0.5903
X(:,:,2,1) =
    0.0079    0.4648
    0.5720    0.1261
X(:,:,1,2) =
    0.1077    0.3413
    0.8644    0.7008
X(:,:,2,2) =
    0.4698    0.4513
    0.4916    0.7439
X(:,:,1,3) =
    0.0226    0.6582
    0.1167    0.8336
X(:,:,2,3) =
    0.6500    0.8422
    0.0378    0.8782

error in splitting an image into blocks of pixels in matlab

1 Answers1