Sliding window minimum and maximum filter

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Lab Rat 2012-9-28

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回答： Dan 2017-6-19

I'm trying to apply a sliding window minimum and maximum filter to an image of a certain window size. Actually, I'm trying to find the optimum window size for it. But I really haven't gotten the hang of it. I presume that I should be using blockproc to implement the sliding window, but not really sure how to find the maximum and minimum filter. As to the implementation itself, should I use loops to slide the window across the entire area of the image ?

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Image Analyst 2012-12-1

From the description "A two-element vector, [V H], specifying the amount of border pixels to add to each block. The function adds V rows above and below each block and H columns left and right of each block." it looks like you'd have a windowSize of 1 so that it moves over in jumps of 1 pixel each time, but the 'BorderSize' would be half your window width so that it includes more than just the one pixel it's centered on. So if you wanted a 5 by 5 window, you'd set a windowSize of 1 and a BorderSize of 2. I think - I haven't tried it.

doudou 2012-12-1

@image Analyst thanks for your answer

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Answer 1

Image Analyst 2012-9-29

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If you have the Image Processing Toolbox, you're in luck!. The sliding max filter is called imdilate() and the sliding min filter is imerode(). These are called "morphological operations." No loops needed:

localMinImage = imerode(grayImage, true(3));
localMaxImage = imdilate(grayImage, true(3));

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Image Analyst 2012-9-29

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I did, above. At least I showed how to get them. How you USE them afterwards is up to you. If you need a more complete demo, see below. In that, I just "use" them by simply displaying them in the image.

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures.
clear;  % Erase all existing variables.
workspace;  % Make sure the workspace panel is showing.
format longg;
format compact;
fontSize = 20;
% Change the current folder to the folder of this m-file.
if(~isdeployed)
  cd(fileparts(which(mfilename)));
end
  
% Check that user has the Image Processing Toolbox installed.
hasIPT = license('test', 'image_toolbox');
if ~hasIPT
  % User does not have the toolbox installed.
  message = sprintf('Sorry, but you do not seem to have the Image Processing Toolbox.\nDo you want to try to continue anyway?');
  reply = questdlg(message, 'Toolbox missing', 'Yes', 'No', 'Yes');
  if strcmpi(reply, 'No')
    % User said No, so exit.
    return;
  end
end
% Read in a standard MATLAB gray scale demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'cameraman.tif';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
  % File doesn't exist -- 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 in the search path folders.', 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.
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]);
% Give a name to the title bar.
set(gcf,'name','Demo by ImageAnalyst','numbertitle','off') 
% Let's compute and display the histogram.
[pixelCount grayLevels] = imhist(grayImage);
subplot(2, 2, 2); 
bar(pixelCount);
grid on;
title('Histogram of original image', 'FontSize', fontSize);
xlim([0 grayLevels(end)]); % Scale x axis manually.
% Compute the local max and min images:
localMinImage = imerode(grayImage, true(3));
localMaxImage = imdilate(grayImage, true(3));
% Display them.
subplot(2, 2, 3);
imshow(localMinImage, []);
title('Local Min (eroded) Image', 'FontSize', fontSize);
subplot(2, 2, 4);
imshow(localMaxImage, []);
title('Local Max (dilated) Image', 'FontSize', fontSize);

Image Analyst 2012-9-29

Doing a max filter on the output of the min filter is called a morphological opening and is done by imopen(). It is not a way to get the illumination pattern that I've ever heard of. I do background correction all the time and this would not be a good way of getting the background unless your background was uniform and had small light-colored dust on it. But even then, there are flaws with that method such that you will increase the noise and have quantization/posterization errors. Do you have a uniform background that you can image? Or all you have is the image with your scene and illumination combined? If, unfortunately, you have the latter case, you can either try homomorphic filtering (basically assuming the light pattern is a very low pass filtered version of your scene) or try to find "holes" in your scene that is pure background and then try to fit a nice smooth model, like a 2D polynomial, to them. Finally, division is the right way to do it in most cases. For certain other cases (radiology and fluorescence) you should do subtraction. Do the math and you'll see why. Upload an image if you want further advice.

Image Analyst 2012-9-29

在 MATLAB Online 中打开

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures.
clear;  % Erase all existing variables.
workspace;  % Make sure the workspace panel is showing.
format longg;
format compact;
fontSize = 13;
% Read in a standard MATLAB gray scale demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'rice.png';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
  % File doesn't exist -- 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 in the search path folders.', 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.
subplot(2, 2, 1);
imshow(grayImage, []);
title('Original Grayscale Image', 'FontSize', fontSize);
% Do an opening to get the background.
% This is not a good method for this image.
openedImage = imopen(grayImage, true(13));
subplot(2, 2, 2);
imshow(openedImage, []);
title('Background Image', 'FontSize', fontSize);
% Divide them
backgroundCorrected = grayImage ./ openedImage;
subplot(2, 2, 3);
imshow(backgroundCorrected, []);
title('Background-corrected Image', 'FontSize', fontSize);

Matt J 2014-4-28

在 MATLAB Online 中打开

Padding doesn't require the IP toolbox, e.g.,

 p=2; %padding
 Apad=zeros(size(A)+2*p,class(A));
 Apad(p+1:end-p,p+1:end-p)=A;

Image Analyst 2014-4-28

What do you mean by shift? As long as the window size is odd, e.g. 3 or 5, there will be no shift. If the window size is even, e.g. 2 or 4, then there will be a half pixel shift.

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Answer 2

Matt J 2012-9-29

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编辑：per isakson 2014-10-25

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I would just use a for-loop to do 2 separable passes. BLOCKPROC can't take advantage of the separable nature of the max/min filter:

    A=rand(100);
    window=3;
    [m,n]=size(A);
    B=A;
    for ii=1:m+1-window
       B(ii,:)=max(A(ii:ii+window-1,:),[],1); 
    end
    for ii=1:n+1-window
       B(:,ii)=max(B(:,ii:ii+window-1),[],2); 
    end

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Royi Avital 2014-4-23

This is nice and fast. Yet it shift the matrix to the right.

How would replicate the results of `imerode` or `imdilate` with Image Processing Toolbox (no `padaaray`) most efficiently?

Thaks.

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Answer 3

Royi Avital 2014-4-23

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I would use:

localMaxImage = colfilt(inputImage, [winLength winLength], 'sliding', @max);
localMinImage = colfilt(inputImage, [winLength winLength], 'sliding', @min);

Though it is still requires patience. I wonder what would be the fastest way to do so without Image Processing Toolbox.

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Answer 4

tilak tenneti 2014-10-24

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编辑：per isakson 2014-10-25

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i want to apply a sliding window minimum filter on input image I and obtain Imin and also apply sliding window maximum filter on Imin to obtain Imax : the following is code

    N=1;
    Imin=ordfilt2(I, 1, true(N));
    N=10;
    Imax = ordfilt2(Imin, N*N, true(N));

here i assume N as window size... but i am confused as how should i take N for minimum filter and again N for maximum filter?

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Answer 5

Dan 2017-6-19

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function [minVals,maxVals] = minmaxfilt1(vector,nhoodSz)
vector = vector(:);
if nhoodSz < 3 || ~floor(mod(nhoodSz,2)) 
    error('nhoodSz must be odd scalar');
end
minVals = min(conv2(vector,eye(nhoodSz)),[],2);
maxVals = max(conv2(vector,eye(nhoodSz)),[],2);
minVals = minVals(((nhoodSz-1)/2)+1: end- ((nhoodSz-1)/2));
maxVals = maxVals(((nhoodSz-1)/2)+1: end - ((nhoodSz-1)/2));
end

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