How to mark threshold value in image histogram?

My input is gray scale image. After drawing histogram of that image i want to mark the threshold value in the histogram using any red mark or line. I am using simple Otsu's thresholding method for segmentation.

[Rik]

A simple hold('on') followed by a call to plot should do it, or am I missing something? What did you try?

Also, this is your 68th question. By now you should be aware of the advice here. Try to make a MWE so we can run your code without any other dependencies and can reproduce your issue. Use the tools explained here to format your post properly.

Answer 1

[Image Analyst]

Try xline. Full Demo:

% Demo by Image Analyst
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 22;
markerSize = 40;

%--------------------------------------------------------------------------------------------------------
%    READ IN IMAGE
folder = [];
baseFileName = 'cameraman.tif';
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
    % The file doesn't exist -- didn't find it there in that folder.
    % Check the entire search path (other folders) for the file by stripping off the folder.
    fullFileNameOnSearchPath = baseFileName; % No path this time.
    if ~exist(fullFileNameOnSearchPath, '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.
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(grayImage)
if numberOfColorChannels > 1
    % It's not really gray scale like we expected - it's color.
    fprintf('It is not really gray scale like we expected - it is color\n');
    % Extract the blue channel.
    grayImage = rgb2gray(grayImage);
end

%--------------------------------------------------------------------------------------------------------
% Display the image.
subplot(2, 1, 1);
imshow(grayImage);
impixelinfo;
axis('on', 'image');
title('Original Gray Scale Image', 'FontSize', fontSize, 'Interpreter', 'None');
% Update the dimensions of the image.
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(grayImage)
% Maximize window.
g = gcf;
g.WindowState = 'maximized';
drawnow;

%--------------------------------------------------------------------------------------------------------
% Get threshold
threshold = 255 * graythresh(grayImage);

%--------------------------------------------------------------------------------------------------------
% Display histogram
subplot(2, 1, 2);
imhist(grayImage);
grid on;
xline(threshold, 'Color', 'r', 'LineWidth', 2)

%--------------------------------------------------------------------------------------------------------
% Display threshold on histogram.
caption = sprintf('Histogram with threshold of %.1f gray levels', threshold);
title(caption, 'FontSize', fontSize)

[Zara Khan]

@Image Analyst Why my red line is not upto the height? I am using R2022a and histogram() function.

[Image Analyst]

The red line is optional. You can delete it, or replace xline() by line() if you want. Or even with plot(). If it works other than that, can you Accept the Answer? 🙂

[Zara Khan]

@Image Analyst want this red line. But the fact is its not going upto the top like your. Please see the attached file.

[Image Analyst]

OK, so did you try any of the alternatives I listed? If not, why did you not try them? I expected you would.

yl = ylim;
line([threshold, threshold], [yl(1), yl(2)], 'LineWidth', 2, 'Color', 'r');

Full demo (tested - it works):

% Demo by Image Analyst
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 22;
markerSize = 40;
%--------------------------------------------------------------------------------------------------------
%    READ IN IMAGE
folder = [];
baseFileName = 'cameraman.tif';
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
    % The file doesn't exist -- didn't find it there in that folder.
    % Check the entire search path (other folders) for the file by stripping off the folder.
    fullFileNameOnSearchPath = baseFileName; % No path this time.
    if ~exist(fullFileNameOnSearchPath, '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.
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(grayImage)
if numberOfColorChannels > 1
    % It's not really gray scale like we expected - it's color.
    fprintf('It is not really gray scale like we expected - it is color\n');
    % Extract the blue channel.
    grayImage = rgb2gray(grayImage);
end
%--------------------------------------------------------------------------------------------------------
% Display the image.
subplot(2, 1, 1);
imshow(grayImage);
impixelinfo;
axis('on', 'image');
title('Original Gray Scale Image', 'FontSize', fontSize, 'Interpreter', 'None');
% Update the dimensions of the image.
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(grayImage)
% Maximize window.
g = gcf;
g.WindowState = 'maximized';
drawnow;
%--------------------------------------------------------------------------------------------------------
% Get threshold
threshold = 255 * graythresh(grayImage);
%--------------------------------------------------------------------------------------------------------
% Display histogram
subplot(2, 1, 2);
imhist(grayImage);
grid on;
yl = ylim;
line([threshold, threshold], [yl(1), yl(2)], 'LineWidth', 2, 'Color', 'r');
%--------------------------------------------------------------------------------------------------------
% Display threshold on histogram.
caption = sprintf('Histogram with threshold of %.1f gray levels', threshold);
title(caption, 'FontSize', fontSize)

[Zara Khan]

@Image Analyst yes I have tried. Thank you once again.

[Image Analyst]

Then I'd call tech support since line() and xline() are not working the same way for you as for everyone else.