MATLAB Answers

how images are divided into sub-images

Latest activity Commented on by Image Analyst
on 19 Aug 2015

the input image should be divided into many sub images. how to achieve this.

  0 Comments

2 Answers

Answer by Image Analyst
on 25 Aug 2012
 Accepted answer

Check out my 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 : blockSizeR : 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.');

  4 Comments

Image Analyst
on 27 Aug 2012

You're welcome.

Sheshjee
on 7 Feb 2013

Thanks for ur code. the first code is very helpfull to me.But the problem is after dividing an image into blocks, i want to remove some blocks which are not required to me and then combine the remaining blocks to form a image of the same size as the original was. Thanks for help.

Walter Roberson
on 7 Feb 2013

What do you want to put in place of the blocks that are "not required" ?


Answer by Michiel Van Duppen on 19 Aug 2015

i have the same problem. After dividing the picture, I want to work with some blocks, other can be deleted. Is this possible? If yes, how can I do it?

  1 Comment

Image Analyst
on 19 Aug 2015

Yes. How are you determining which blocks to operate on and which to ignore? Just put your "decision" code into the loop on the code I provided.

useThisBlock = DecisionFunction(theBlock);
% DecisionFunction is a function you devise.
if ~useThisBlock
    continue; % Skip to bottom of loop
end
% else operate on this block.

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