Block structures and image processing

2013 4 14
1 个回答
回答已采纳
更新时间：2017 9 16
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1 个投票

Hi I'm trying to write some code that achieves the following using the image processing toolbox:
1. Split a grayscale image (I) into blocks
2. Calculate the mean value of each block
3. For each element within each block, if it's > the mean of that block colour it black, if not colour it white.
Here's what I want to say:
fun=@(block_struct)...
for n=1:32
for m=1:32
if block_struct.data(n,m)>(mean2(block_struct.data));
block_struct.data(n,m)=0;
else
block_struct.data(n,m)=255;
end
end
end
I2=blockproc(I,[32 32],fun);
imshow(I2)
However you cannot have anonymous functions with more than one statement and I'm not sure how to use a block structure with a normal function. I have looked a little into nested functions but I haven't found an answer yet. Any help would be appreciated.

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 采纳的回答

Image Analyst
Image Analyst 2013-4-14

1 个投票

Here is how to use blockproc() with a custom (non-anonymous) function:
% Demo of blockproc in two different ways.
% One uses an anonymous function to return a block of pixels
% the same size as the sliding window block.
% The other uses a custom written function to return a
% single value for each sliding window position.
function blockproc_demo()
try
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
workspace; % Make sure the workspace panel is showing.
fontSize = 20;
% Change the current folder to the folder of this m-file.
if(~isdeployed)
cd(fileparts(which(mfilename)));
end
% Read in standard MATLAB demo image.
grayImage = imread('cameraman.tif');
[rows columns numberOfColorChannels] = size(grayImage);
% Display the original image.
subplot(2, 2, 1);
imshow(grayImage, []);
caption = sprintf('Original Image\n%d by %d pixels', ...
rows, columns);
title(caption, 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Position', get(0,'Screensize'));
set(gcf, 'name','Image Analysis Demo', 'numbertitle','off')
% Block process the image.
windowSize = 3;
% Each 3x3 block will get replaced by one value.
% Output image will be smaller by a factor of windowSize.
myFilterHandle = @myFilter;
blockyImage = blockproc(grayImage,[windowSize windowSize], myFilterHandle);
[rowsP columnsP numberOfColorChannelsP] = size(blockyImage);
% Display the processed image.
% It is smaller, but the display routine imshow() replicates
% the image so that it looks bigger than it really is.
subplot(2, 2, 2);
imshow(blockyImage, []);
caption = sprintf('Image Processed in %d by %d Blocks\n%d by %d pixels\nCustom Box Filter', ...
windowSize, windowSize, rowsP, columnsP);
title(caption, 'FontSize', fontSize);
% Now let's do it an alternate way where we use an anonymous function.
% We'll take the standard deviation in the blocks.
windowSize = 8;
myFilterHandle2 = @(block_struct) ...
std2(block_struct.data) * ones(size(block_struct.data));
blockyImageSD = blockproc(grayImage, [windowSize windowSize], myFilterHandle2);
[rowsSD columnsSD numberOfColorChannelsSD] = size(blockyImageSD);
subplot(2, 2, 4);
imshow(blockyImageSD, []);
caption = sprintf('Image Processed in %d by %d Blocks\n%d by %d pixels\nAnonymous Standard Deviation Filter', ...
windowSize, windowSize, rowsSD, columnsSD);
title(caption, 'FontSize', fontSize);
% Note: the image size of blockyImageSD is 256x256, NOT smaller.
% That's because we're returning an 8x8 array instead of a scalar.
uiwait(msgbox('Done with demo'));
catch ME
errorMessage = sprintf('Error in blockproc_demo():\n\nError Message:\n%s', ME.message);
uiwait(warndlg(errorMessage));
end
return;
% Takes one 3x3 block of image data and multiplies it
% element by element by the kernel and
% returns a single value.
function singleValue = myFilter(blockStruct)
try
% Assign default value.
% Will be used near sides of image (due to boundary effects),
% or in the case of errors, etc.
singleValue = 0;
% Create a 2D filter.
kernel = [0 0.2 0; 0.2 0.2 0.2; 0 0.2 0];
% kernel = ones(blockStruct.blockSize); % Box filter.
% Make sure filter size matches image block size.
if any(blockStruct.blockSize ~= size(kernel))
% If any of the dimensions don't match.
% You'll get here near the edges,
% if the image is not a multiple of the block size.
% warndlg('block size does not match kernel size');
return;
end
% Size matches if we get here, so we're okay.
% Extract our block out of the structure.
array3x3 = blockStruct.data;
% Do the filtering. Multiply by kernel and sum.
singleValue = sum(sum(double(array3x3) .* kernel));
catch ME
% Some kind of problem...
errorMessage = sprintf('Error in myFilter():\n\nError Message:\n%s', ME.message);
% uiwait(warndlg(errorMessage));
fprintf(1, '%s\n', errorMessage);
end
return;

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Robert
Robert 2013-4-14
Many thanks! I played around with your code a bit and, for the benefit of others who may read this later, here's the solution I arrived at:
function bpdemo2()
clc;
grayImage=imread('image1.jpg');
grayImage=rgb2gray(grayImage);
myfilter=@filter;
processedimage=blockproc(grayImage,[100 100],myfilter,'PadPartialBlocks',true);
imshow(processedimage);
end
function singlevalue = filter(block_struct)
singlevalue=ones([100 100]);
for n=1:100
for m=1:100
if block_struct.data(n,m)>(0.5*(mean2(block_struct.data)))
singlevalue(n,m)=0;
else
singlevalue(n,m)=255;
end
end
end
end
TUSHAR MURATKAR
TUSHAR MURATKAR 2017-9-16
@ image analyst, can you help me in converting your non overlapping block codes into overlapping blocks with overlapping factor = 4 and block size = 16. Eagerly waiting for your answer.
Image Analyst
Image Analyst 2017-9-16
You have to specify a bordersize parameter and you need to have TrimBorder false.

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