MATLAB: Iterative threshold selection on an input gray-level image

Question

Perform iterative threshold selection on an input gray-level image to include a variable that counts the number of iterations and an array that stores the values of T for each iteration.Please, this is what i have tried but i know i am wrong, somebody help me out.

I = imread('coins.png');

Id = im2double(I);

Imax = max(Id(:));

Imin = min(Id(:));

T = 0.5*(min(Id(:)) + max(Id(:)));

deltaT = 0.01;

done = false;

while ~done

g = Id >= T;

Tnext = 0.5*(mean(Id(g)) + mean(Id(~g)));

done = abs(T – Tnext) < deltaT;

T = Tnext;

end

imshow(g);

Answer 1

You need to save T into an array that is indexed by the loop iteration counter. Like this:

clc;    % Clear the command window.
clearvars;
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;
grayImage = imread('coins.png');
subplot(2, 2, 1);
imshow(grayImage);
title('Original Image', 'FontSize', fontSize);
% Let's compute and display the histogram.
[pixelCount, grayLevels] = imhist(grayImage);
subplot(2, 2, 2); 
bar(grayLevels, pixelCount); % Plot it as a bar chart.
grid on;
title('Histogram of original image', 'FontSize', fontSize, 'Interpreter', 'None');
xlabel('Gray Level', 'FontSize', fontSize);
ylabel('Pixel Count', 'FontSize', fontSize);
xlim([0 grayLevels(end)]); % Scale x axis manually.
Id = im2double(grayImage);
Imax = max(Id(:));
Imin = min(Id(:));
T = 0.5*(min(Id(:)) + max(Id(:)));
deltaT = 0.01;
done = false;
counter = 1;
while ~done
  savedThresholds(counter) = T;
  
  g = Id >= T;
  Tnext = 0.5*(mean(Id(g)) + mean(Id(~g)));
  done = abs(T - Tnext) < deltaT;
  T = Tnext;
  
  % Display g
  subplot(2, 2, 3);
  imshow(g);
  title('Binary Image', 'FontSize', fontSize);
  
  % Plot savedThresholds
  subplot(2, 2, 4);
  plot(savedThresholds, 'b*-', 'MarkerSize', 15, 'LineWidth', 2);
  grid on;
  title('Thresholds', 'FontSize', fontSize);
  xlabel('Iteration', 'FontSize', fontSize);
  ylabel('Threshold', 'FontSize', fontSize);
  % Enlarge figure to full screen.
  set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
  
  promptMessage = sprintf('Threshold for iteration %d is %f.\nDo you want to Continue processing,\nor Quit processing?', ...
    counter, savedThresholds(counter));
  titleBarCaption = 'Continue?';
  buttonText = questdlg(promptMessage, titleBarCaption, 'Continue', 'Quit', 'Continue');
  if strcmpi(buttonText, 'Quit')
    return;
  end
  
  % Increment loop interation counter.
  counter = counter + 1;
end