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
