How to find out windowwise mean and standard deviation of some particular pixels.

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Mahua Nandy(Pal) 2013-2-17

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https://ww2.mathworks.cn/matlabcentral/answers/63760-how-to-find-out-windowwise-mean-and-standard-deviation-of-some-particular-pixels

I have a .txt file with 50 rows and 2 columns indicating 50 pixel positions in an image.I am reading that file with the following statement:

file_id=fopen('C:\Users\Mahua Nandy\PixelPositions_im1.txt','r'); Input=fscanf(file_id,'%d',[2,50]); fclose(file_id);

Input is coming in column order(2 rows and 50 columns).Now I want to find out 3x3 windowwise mean and standard deviation of those particular 50 pixels.

Please help me to code it

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Answer 1

Image Analyst 2013-2-17

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https://ww2.mathworks.cn/matlabcentral/answers/63760-how-to-find-out-windowwise-mean-and-standard-deviation-of-some-particular-pixels#answer_75342

在 MATLAB Online 中打开

First I guess you have to create an image - all black I guess.

maxRows = max(Input(1,:));
maxColumns = max(Input(2, :));
grayImage = zeros(maxrows, maxColumns, 'uint8');

Then you have to run down your list of coordinates, figuring out their gray level (from some other image I assume), then assigning/transferring them to the new image (the one that contains only the 50 pixels):

for c = 1 : maxColumns
  for r = 1 : maxRows
    row = Input(1, r);
    column = Input(2, c);
    % Transfer the gray level from the other image to
    % this location on the new image.
    grayImage(row, column) = initialImage(row, column);
  end
end

Then you have to use conv2() and stdfilt() to take the mean and std dev in a 3x3 window in the new image which contains the 50 pixels.

meanImage = conv2(grayImage, ones(3)/9, 'same');
stdImage = stdfilt(grayImage);

Be aware though that most of the image will be just black pixels so the mean will be very low. Upload your image first, if you have any follow up questions.

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Image Analyst 2013-2-18

在 MATLAB Online 中打开

% Gets mean and std dev of an image around 50 isolated coordinates.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures.
clear all;  % Erase all existing variables.
workspace;  % Make sure the workspace panel is showing.
fontSize = 20;
format compact;
Input = [...
  28 16
  28 17
  28 18
  28 27
  28 28
  30 57
  30 58
  30 59
  30 60
  30 61
  31 56
  31 57
  31 58
  31 59
  31 60
  31 115
  31 116
  31 117
  31 118
  31 119
  32 39
  32 40
  32 41
  32 42
  32 43
  33 28
  33 29
  33 30
  33 31
  33 32
  33 118
  33 119
  33 120
  33 121
  33 122
  34 39
  34 40
  34 41
  34 42
  34 43
  35 102
  35 103
  35 104
  35 105
  35 106
  37 74
  37 75
  37 76
  37 77
  37 78
  38 38
  38 39
  38 40
  38 41
  38 42
  111 131
  111 132
  111 133
  111 134
  111 135]
initialImage = imread('moon.tif');
maxRows = max(Input(:, 2)); % Y value
maxColumns = max(Input(:, 1)); % x value.
grayImage = zeros(maxRows, maxColumns, 'uint8');
numberOfCoordinates = size(Input, 1);
for c = 1 : numberOfCoordinates
  for r = 1 : numberOfCoordinates
    row = Input(r, 1);
    column = Input(c, 2);
    % Transfer the gray level from the other image to
    % this location on the new image.
    grayImage(row, column) = initialImage(row, column);
  end
end
meanImage = conv2(double(grayImage), ones(3)/9, 'same');
subplot(1, 2, 1);
imshow(meanImage, []);
title('Mean Image', 'FontSize', fontSize);
stdImage = stdfilt(grayImage);
subplot(1, 2, 2);
imshow(stdImage, []);
title('Std Dev Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);

Image Analyst 2013-2-19

That's what conv2 does. And what exactly is in the space between your 50 isolated pixels? In an image, there has to be SOMETHING.

Mahua Nandy(Pal) 2013-2-21

thank you

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Answer 2

Thorsten 2013-2-19

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https://ww2.mathworks.cn/matlabcentral/answers/63760-how-to-find-out-windowwise-mean-and-standard-deviation-of-some-particular-pixels#answer_75644

编辑：Thorsten 2013-2-19

在 MATLAB Online 中打开

Hi Mahua Nandy(Pal), that's all quite straight forward:

1. Let's say you have your image in I and your particular positions in variables x and y

 I = imread('cameraman.tif'); 
 x = 10; y = 20; % just for testing

To get the 3 x 3 neighborhood N around (x, y) from your image, use

N = I(y-1:y+1, x-1:x+1);

That gives you a 3 x 3 matrix. The mean and std are

 m = mean(N(:));
 s = std(N(:));

If you have 50 values of x and y stored in Input, you just do it in a loop:

 for i=1:size(Input, 1)
   x = Input(i, 1);
   y = Input(i, 2);
   N = I(y-1:y+1, x-1:x+1);
   m(i) =  mean(N(:));
   s(i) = std(N(:));
 end

Done!

Two caveats:

1. verify that each row of Input is [x y] and not [y x]; if the columns are given as y x use

x(i) = Input(i, 2); 
y(i) = Input(i, 1);

2. ensure that no x, y lies at the boundary of I, such that x-1, x+1m, y-1 and y+1 is always a valid index to the image. To be one the save side, you could use the statement

N = I(max(1, y-1):min(size(I, 1), y+1), ...
      max(1, x-1):min(size(I, 2), x+1);

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Mahua Nandy(Pal) 2013-2-21

thanks

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How to find out windowwise mean and standard deviation of some particular pixels.

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How to find out windowwise mean and standard deviation of some particular pixels.

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