Extracting image patches around each pixel

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VIJENDRA
VIJENDRA on 6 Oct 2016
Commented: Devkumar Das on 2 Aug 2017
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?
  2 Comments
Adam
Adam on 6 Oct 2016
Edited: Adam on 6 Oct 2016
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.

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Answers (1)

Guillaume
Guillaume on 7 Oct 2016
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
  1 Comment
Devkumar Das
Devkumar Das on 2 Aug 2017
Hi Guillaume, If I want to extract patch for a color image then nlfilter will not work. Can you suggest something for that?

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