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Divide the image into 4x4 sub images

Asked by M@lik Ali on 31 Jan 2013

Hi all, i want to divide the image into 4x4= 16 sub images. how i can do this and then further each sub image can be dividable into 2x2 square blocks. each sub images and each block should be accessible separately.

2 Comments

Lester Lim on 31 Jan 2013

for i=1:1:16

then try to use reshape(variable,2,2);

Hope this helps...

M@lik Ali on 31 Jan 2013

Thanks to reply, but first i need to divide the image into 4 rows and 4 columns, which should give me 16 sub images. variable is the image in above code?

M@lik Ali

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2 Answers

Answer by Image Analyst on 31 Jan 2013
Accepted answer

Run this demo:

% Demo to divide a color image up into blocks.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
workspace;  % Make sure the workspace panel is showing.
fontSize = 20;
% Read in a standard MATLAB color demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'peppers.png';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
if ~exist(fullFileName, 'file')
	% Didn't find it there.  Check the search path for it.
	fullFileName = baseFileName; % No path this time.
	if ~exist(fullFileName, 'file')
		% Still didn't find it.  Alert user.
		errorMessage = sprintf('Error: %s does not exist.', fullFileName);
		uiwait(warndlg(errorMessage));
		return;
	end
end
% Read the image from disk.
rgbImage = imread(fullFileName);
% Test code if you want to try it with a gray scale image.
% Uncomment line below if you want to see how it works with a gray scale image.
% rgbImage = rgb2gray(rgbImage);
% Display image full screen.
imshow(rgbImage);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
drawnow;
% Get the dimensions of the image.  numberOfColorBands should be = 3.
[rows columns numberOfColorBands] = size(rgbImage)
%==========================================================================
% The first way to divide an image up into blocks is by using mat2cell().
blockSizeR = 150; % Rows in block.
blockSizeC = 100; % Columns in block.
% Figure out the size of each block in rows. 
% Most will be blockSizeR but there may be a remainder amount of less than that.
wholeBlockRows = floor(rows / blockSizeR);
blockVectorR = [blockSizeR * ones(1, wholeBlockRows), rem(rows, blockSizeR)];
% Figure out the size of each block in columns. 
wholeBlockCols = floor(columns / blockSizeC);
blockVectorC = [blockSizeC * ones(1, wholeBlockCols), rem(columns, blockSizeC)];
% Create the cell array, ca.  
% Each cell (except for the remainder cells at the end of the image)
% in the array contains a blockSizeR by blockSizeC by 3 color array.
% This line is where the image is actually divided up into blocks.
if numberOfColorBands > 1
	% It's a color image.
	ca = mat2cell(rgbImage, blockVectorR, blockVectorC, numberOfColorBands);
else
	ca = mat2cell(rgbImage, blockVectorR, blockVectorC);
end
% Now display all the blocks.
plotIndex = 1;
numPlotsR = size(ca, 1);
numPlotsC = size(ca, 2);
for r = 1 : numPlotsR
	for c = 1 : numPlotsC
		fprintf('plotindex = %d,   c=%d, r=%d\n', plotIndex, c, r);
		% Specify the location for display of the image.
		subplot(numPlotsR, numPlotsC, plotIndex);
		% Extract the numerical array out of the cell
		% just for tutorial purposes.
		rgbBlock = ca{r,c};
		imshow(rgbBlock); % Could call imshow(ca{r,c}) if you wanted to.
		[rowsB columnsB numberOfColorBandsB] = size(rgbBlock);
		% Make the caption the block number.
		caption = sprintf('Block #%d of %d\n%d rows by %d columns', ...
			plotIndex, numPlotsR*numPlotsC, rowsB, columnsB);
		title(caption);
		drawnow;
		% Increment the subplot to the next location.
		plotIndex = plotIndex + 1;
	end
end
% Display the original image in the upper left.
subplot(4, 6, 1);
imshow(rgbImage);
title('Original Image');
%==============================================================================
% Another way to split the image up into blocks is to use indexing.
% Read in a standard MATLAB gray scale demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'cameraman.tif';
fullFileName = fullfile(folder, baseFileName);
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
if ~exist(fullFileName, 'file')
	% Didn't find it there.  Check the search path for it.
	fullFileName = baseFileName; % No path this time.
	if ~exist(fullFileName, 'file')
		% Still didn't find it.  Alert user.
		errorMessage = sprintf('Error: %s does not exist.', fullFileName);
		uiwait(warndlg(errorMessage));
		return;
	end
end
grayImage = imread(fullFileName);
% Get the dimensions of the image.  numberOfColorBands should be = 1.
[rows columns numberOfColorBands] = size(grayImage);
% Display the original gray scale image.
figure;
subplot(2, 2, 1);
imshow(grayImage, []);
title('Original Grayscale Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
% Divide the image up into 4 blocks.
% Let's assume we know the block size and that all blocks will be the same size.
blockSizeR = 128; % Rows in block.
blockSizeC = 128; % Columns in block.
% Figure out the size of each block. 
wholeBlockRows = floor(rows / blockSizeR);
wholeBlockCols = floor(columns / blockSizeC);
% Preallocate a 3D image
image3d = zeros(wholeBlockRows, wholeBlockCols, 3);
% Now scan though, getting each block and putting it as a slice of a 3D array.
sliceNumber = 1;
for row = 1 : blockSizeR : rows
	for col = 1 : blockSizeC : columns
		% Let's be a little explicit here in our variables
		% to make it easier to see what's going on.
		row1 = row;
		row2 = row1 + blockSizeR - 1;
		col1 = col;
		col2 = col1 + blockSizeC - 1;
		% Extract out the block into a single subimage.
		oneBlock = grayImage(row1:row2, col1:col2);
		% Specify the location for display of the image.
		subplot(2, 2, sliceNumber);
		imshow(oneBlock);
		% Make the caption the block number.
		caption = sprintf('Block #%d of 4', sliceNumber);
		title(caption);
		drawnow;
		% Assign this slice to the image we just extracted.
		image3D(:, :, sliceNumber) = oneBlock;
		sliceNumber = sliceNumber + 1;
	end
end
% Now image3D is a 3D image where each slice, 
% or plane, is one quadrant of the original 2D image.
msgbox('Done with demo!  Check out the two figures.');

2 Comments

M@lik Ali on 31 Jan 2013

This one is working fine, but how i can fix the number of blocks to 16. my image size is 256 x 384.

i want total blocks 16 only.

Image Analyst on 31 Jan 2013

Yes it runs but you were supposed to look over the code after you ran it to understand it. For example, see the line that mentions "blockSizeR"? Well you have to change that to your image height in rows divided by 4 or 16 or whatever.

Image Analyst
Answer by Matt J on 31 Jan 2013
Edited by Matt J on 31 Jan 2013

Using MAT2TILES available here

    Level2 = mat2tiles(Image,[2,2]);
    subimages = mat2tiles(Level2,[2,2]);

And now if you want the 4x4 block at tile coordinates (2,3) and then within this, the 2x2 block at tile coordinate (1,2), you would do

subimages{2,3}{1,2};

0 Comments

Matt J

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