Extracting image patches around each pixel

Suppose I have an image of size 256*256

Image = imread('cameraman.tif');

I have to extract patches of 7*7 around each pixel of image

patchSize = 7;

Around edges I padded image with zeros pixel

padcam = padarray(Image,[7 7],'both');

Total number of pixels is 256*256 therefore total patches would be 65536

[m,n] = size(padcam);
ImagePatches = zeros(patchSize,patchSize,256*256); 
tic
for k = 1:65536
  for i = 8:263
      for j = 8:263
          ImagePatches(:,:,k) = padcam(i-3:i+3, j-3:j+3);
      end
  end
end
toc

Due to 3 for-loops the operation is taking very long time. How can i optimize this code?

[Adam]

You should only need to loop twice. One loop is redundant. There are probably quicker methods that I don't have time to think about so someone else will likely suggest them, but instead of your outer k loop just keep a counter that you initialise and increment within your inner loop to give you your k index into the ImagePatches.

[VIJENDRA]

Thanks Adam

Answer 1

[Guillaume]

Since you have the imaging toolbox, a much simpler way to generate your output is by subverting nlfilter. You don't even need to pad your image as nlfilter does that for you:

ImagePatches = nlfilter(Image, [7 7], @(block) {block}); %store each block generated by nlfilter into scalar cell of cell array
ImagePatches = cat(3, ImagePatches{:}); %and concatenate in 3rd dimension

[Devkumar Das]

Hi Guillaume, If I want to extract patch for a color image then nlfilter will not work. Can you suggest something for that?