i just wanted to know that blockproc(I,[m n],fun) function returns only one block of size mxn?

i.e block=blockproc(I,[m n],fun)

the above line returns only one block of size mxn oR it divides the whole image I into mxn blocks????

Answer 1

[Walter Roberson]

blockproc(I,[m n],fun) will, by default, call fun once for each non-overlapping m x n block (and partial block at the edges). Whatever fun returns will be arranged in the overall return matrix in the same order as the original blocks. fun can return scalars (e.g., the mean of the area) or arrays (e.g.., a filtered or processed area). If you use the default options and your fun returns an array the same size as the "data" field passed into it, then the output would be an array the same size as the original array "I".

If you want to divide up "I" into separate m x n blocks, such as a cell array of blocks, then you would not use blockproc() with the default options. If that is what you want to do, use mat2cell() instead.

[angel]

ok...but if i want to divide an input image into overlapping blocks instead of non-overlapping blocks thn which function sholud i use??

[Walter Roberson]

http://www.mathworks.co.uk/matlabcentral/answers/60198-how-to-partition-an-image

[angel]

i've gone through the above link whick u've given....

fun=@(block_struct) std2(block_struct.data)*ones(size(block_struct.data));

B=blockproc(J,[6 6],fun,'bordersize',[3 3]);

i need blocks like B1,B2 ,B3 and so on in my project.... what next should i do....

how can i make Blocks B1,B2,B3,..... from

B=blockproc(J,[6 6],fun,'bordersize',[3 3]);

[angel]

plz reply to my question

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2CA10_in_a_loop.3F

Note: Please take in to account that volunteers sleep or work or have other life activities to take care of.

[Image Analyst]

angel, blockproc does not return separate blocks. It processes an image block by block and returns a single image. If you want to divide an image into separate blocks, see my demo in my answer.

[angel]

u'have shown 2 methods for making blocks in the following demo...

1> by using mat2cell

2>by indexing

sir both methods are for non-overlapping blocks right????

[Walter Roberson]

Indexing can be used for overlapping blocks.

mat2cell() cannot directly be used for overlapping blocks. However, if you use the BorderSize and TrimBorder options of blockproc, the array you get back from blockproc will be suitable for dividing up by mat2cell. See Jassy's question about overlapping blocks.

Answer 2

[Image Analyst]

% Demo to divide an image up into blocks (non-overlapping tiles).
% The first way to divide an image up into blocks is by using mat2cell().
% In this demo, I demonstrate that with a color image.
% Another way to split the image up into blocks is to use indexing.
% In this demo, I demonstrate that method with a grayscale image.

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, 'FontSize', fontSize);
    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.');

[angel]

sir i've run ur code with the second method of indexing...

image extension is .png and its size is 142x158.

i've got an error at line

oneBlock = grayImage(row1:row2, col1:col2);

error=> Index exceeds matrix dimensions.

tell me plz what should i do?

[Walter Roberson]

You should read about how to use the debugger.

[Image Analyst]

You changed the demo. But you didn't change it enough to allow your changes to work. My demo had a 256x256 image and had 128 by 128 blocks so it fit 2 by 2 blocks in there perfectly. When you changed the size, now you can't fit two 128 wide blocks in your smaller image anymore. You should have also changed col2 and row2, but you didn't, so that's why it errored. I've done those fixes for you, so here is the corrected loop:

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.
    % Determine starting and ending rows.
    row1 = row;
    row2 = row1 + blockSizeR - 1;
    row2 = min(rows, row2); % Don't let it go outside the image.
    % Determine starting and ending columns.
    col1 = col;
    col2 = col1 + blockSizeC - 1;
    col2 = min(columns, col2); % Don't let it go outside the image.
    % 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, 'FontSize', fontSize);
    drawnow;
    % Assign this slice to the image we just extracted.
    if (row2-row1+1) == blockSizeR && (col2-col1+1) == blockSizeC
      % Then the block size is the tile size, 
      % so add a slice to our 3D image stack.
      image3D(:, :, sliceNumber) = oneBlock;
    else
      newTileSize = [(row2-row1+1), (col2-col1+1)];
      warningMessage = sprintf('Warning: this block size of %d rows and %d columns\ndoes not match the preset block size of %d rows and %d columns.\nIt will not be added to the 3D image stack.',...
        newTileSize(1), newTileSize(2), blockSizeR, blockSizeC);
      uiwait(warndlg(warningMessage));
    end
    sliceNumber = sliceNumber + 1;
  end
end

[angel]

ok sir...but again error arises in subplot...

m using png image of size 192x268

error=>

Index exceeds number of subplots.

subplot(2, 2, sliceNumber);

how to resolve it...