How to divide images from folder into 4x4 blocks

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I want to divide all images in a folder into 4x4 blocks and then store the RGB information in a zeros matrix. A sample of what the images look like is attached. Does anyone know how to divide into these blocks and then store the information in the matrix.
Petrus van Aswegen
Petrus van Aswegen on 5 May 2021
Sorry for the poor wording mate. The end state is to use the RGB colour information in each 4x4 patch to isolate the properties of the solar panels, and then use support vector machine to detect solar panels in other images. I basically need to divide the images into patches, and then store the information within each patch into a matrix

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Accepted Answer

Turlough Hughes
Turlough Hughes on 5 May 2021
Hi Petrus, the mat2cell approach that Jonas pointed out can be done more generally as follows. Note, this handles the scenario where image dimensions are not a factor of the corresponding block dimensions (the remainder rows or columns are ommited)
1. Parameters
blockHeight = 4;
blockWidth = 4;
folderName = 'C://...'; % folder containing images
2. Directory. These three lines can be used by others to replicate the demo. Once you modify folderName you can replace the following three lines with the fourth one.
[fils(1:6).folder] = deal('');
fnames = cellfun(@(x) sprintf('office_%s.jpg',x),{'1','2','3','4','5','6'},'uni',false);
[] = fnames{:};
%fils = dir(fullfile(folderName,'*.PNG')); < Replace the above with this line
3. Load an initial image to get some metadata:
I0 = imread(fullfile(fils(1).folder,fils(1).name));
N1 = blockHeight*ones(floor(size(I0,1)/blockHeight),1);
N2 = blockWidth*ones(floor(size(I0,2)/blockWidth),1);
4. Loop through your directory:
L = cell(numel(fils),1);
for ii = 1:numel(fils)
I = imread(fullfile(fils(ii).folder,fils(ii).name));
b = mat2cell(I(1:blockHeight*numel(N1),1:blockWidth*numel(N2),:),N1,N2,3);
L{ii} = cat(4,b{:}); % alternatively L{ii} = b;
L = 6×1 cell array
{4×4×3×33750 uint8} {4×4×3×33750 uint8} {4×4×3×33750 uint8} {4×4×3×33750 uint8} {4×4×3×33750 uint8} {4×4×3×33750 uint8}
The result, L, is then a 6 by 1 cell array of matrices where each cell corresponds to an image. The matrix dimensions (4x4x3x3750) correspond to the block height, block width, RGB, and a linear index for the block.
  1 Comment
Petrus van Aswegen
Petrus van Aswegen on 5 May 2021
This worked perfectly, thank you so much mate. Appreciate your help

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More Answers (1)

Jonas on 5 May 2021
you could use mat2cell
A = rand(256,256,3); % your matrix here
N = 4*ones(1,64);
B = mat2cell(A,N,N,3);
adjusted from here
Petrus van Aswegen
Petrus van Aswegen on 5 May 2021
Legend. Thanks for the help mate, much appreciated.

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