Finding the edge of an image

Hello! I have an image, and I want to calculate the edge of it (as seen in red),. Then I want to take that edge and calculate the local min and max values of that edge. How would I go about doing this? Image is attached with and without annotation!

[DGM]

You'll have to attach an image without the annotation if you want someone to use it.

[Seejal Padhi]

sorry, I just attached it

[Image Analyst]

@Seejal Padhi did you overlook my answer below (scroll down)? Anything wrong with it?

[Seejal Padhi]

Thank you sm! It worked really well. I appreciate your help!

Answer 1

[Image Analyst]

Try this:

% 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 = 'wr_image.png';
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)

%--------------------------------------------------------------------------------------------------------
% Display the image.
subplot(1, 3, 1);
imshow(grayImage);
impixelinfo;
axis('on', 'image');
title('Original Gray Scale Image', 'FontSize', fontSize, 'Interpreter', 'None');
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 = grayImage(:, :, 3);
end
% 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;

%--------------------------------------------------------------------------------------------------------
% Threshold the image to get the bright blobs.
% https://www.mathworks.com/matlabcentral/fileexchange/29372-thresholding-an-image?s_tid=srchtitle
lowThreshold = 31;
highThreshold = 255;
% [lowThreshold, highThreshold, lastThresholdedBand] = threshold(lowThreshold, highThreshold, grayImage)
mask = grayImage >= lowThreshold & grayImage <= highThreshold;
% Take largest blob only.
mask = bwareafilt(mask, 1);
subplot(1, 3, 2);
imshow(mask)
title('Binary Mask Image', 'FontSize', fontSize, 'Interpreter', 'None');

%--------------------------------------------------------------------------------------------------------
% Get the top row for each column
topRows = nan(1, columns);
for col = 1 : columns
    thisColumn = mask(:, col);
    % Find top row
    t = find(thisColumn, 1, 'first');
    if ~isempty(t)
        topRows(col) = t;
    end
end
% Now overlay it onto the image
subplot(1, 3, 3);
imshow(grayImage)
hold on;
plot(topRows, 'r-', 'LineWidth', 4)
title('Top Row in Red', 'FontSize', fontSize, 'Interpreter', 'None');

[Seejal Padhi]

If I have an image like this, how would I figure the threshold to go below the lighter gray area (liquid) and to get the edge

[Image Analyst]

Try the attached triangle threshold.

Or else see my interactive threshold setting program in my FIle Exchange:

Image Analyst's File Exchange

or else just do trial and error after taking a look at the histogram.

[Carolina Vazquez]

I'm trying to use this code but trace the left edge of an image (first column), not the top row. I'm having some issues with this.

[Image Analyst]

@Carolina Vazquez please start your own question and include your image and your code (working or not).

[Carolina Vazquez]

Hi, https://www.mathworks.com/matlabcentral/answers/2024297-edge-tracing-left-side-only

Yes I posted the question here. Thank you!

[Image Analyst]

Great. I'll get you code this morning.