How to mapped largest contour from pathlines?

I need to find largest contour from the pathline.

Answer 1

[Image Analyst]

I don't know what pathline is. But if you can export your contour lines to a binary image, then you can extract the longest one with bwareafilt

longestContour = bwareafilt(binaryImageOfContours, 1); % Get only the blob with most number of pixels.

[Suman]

Dear Image Analyst,

Thank you for your comment, I have tried it but it does not work on mine one.

[Image Analyst]

You can use getframe to get a screenshot of that. But I don't see why the longest contour line has any relevance to anything. Why do you need it?

[Suman]

Dear Image Analyst,

The output pathline are driven from the vector field data, I need to cluster the field by using these pathlines which will be separated the different field and find out the largest closed contour from these path.

Thank you so much, I will try it from getframe, lests hops it will work on it.

Thank you.

[Image Analyst]

Now that you explain what you want to do in slightly more detail, I don't think the approach you're trying is the right one. If the input is an image, why not just threshold and use regionprops to measure the area(s) of the regions above the threshold. Can you attach the original image that you fed to the contour function?

[DGM]

It's not a contour plot. It's a plot of a vector field, so the goal is to find the largest closed path in the field. Since the plot lines are not necessarily drawn as contiguous lines, I'm going to say this isn't something that can be done by analyzing the screenshot.

I have to ask how valuable an answer would be, since it wouldn't be "the largest closed path", but rather it's only "the largest closed path that is contained entirely within the plot area". Is that adequate?

[Walter Roberson]

There are field lines present in the vector plot that would potentially be the longest if not for the fact that they encounter the edge, and so are not closed lines. Do you want the longest line, or the longest closed line? Which function call are you using to do the plotting (that will inform us as to what data structures are available.)

[Suman]

Dear Respected all,

Yes these are the pathlines obtained from the vector field, and if there is not any closed path lines it will catch the nearest pathlines and goes forward untill and unless it will catch edge or initial point( taken from the curve curvature). Finally if there is more than one closed contour I need to take largest contour from it.

I dont know any function over it that's why I am asking here. Now I am trying to build a fuction which maps the number of curve with its coordinate. If I got same repeated coordinate axes then it will be the closed curve or might be nearest coordinate value.

Note: Thank you so much for your answer and comments, I have tried all from above mentioned but none of the above works for mine. If you know any function which can be implemented for my issues, please let me know.

Thank you

[Image Analyst]

Did you try my suggestion of regionprops? With it you can get the number of pixels in each blue line, as well as the Euler Number (which says whether the line is closed or not)? The only thing required is that two lines to not touch or else they would be considered as a single line. So make sure you have enough spatial resolution, or adjust the line spacing level, so that touching does not occur.

% 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 = 'contours.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(2, 2, 1);
imshow(grayImage);
impixelinfo;
axis('on', 'image');
title('Original 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 red channel.
    grayImage = grayImage(:, :, 1);
    % 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)
end
% Maximize window.
g = gcf;
g.WindowState = 'maximized';
drawnow;
%--------------------------------------------------------------------------------------------------------
% Crop the image.
grayImage = grayImage(18:815, 60:1639);
% Show histogram
% histogram(grayImage);
subplot(2, 2, 2);
imshow(grayImage, [])
title('Cropped Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis('on', 'image');
impixelinfo
drawnow;
% Get a binary image of the dark lines:
threshold = 150; % Whatever.
mask = grayImage < threshold;
% Display the image.
subplot(2, 2, 3);
imshow(mask, [])
title('Mask Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis('on', 'image');
impixelinfo
drawnow;
% Count the pixels in each blob
[labeledImage, numBlobs] = bwlabel(mask);
props = regionprops(mask, 'Area');
% Get the max number of pixels
[maxArea, indexOfMask] = max([props.Area])
% Extract the largest blob
largestBlob = ismember(labeledImage, indexOfMask);
% Display the image.
subplot(2, 2, 4);
imshow(largestBlob, [])
title('Largest Blob', 'FontSize', fontSize, 'Interpreter', 'None');
axis('on', 'image');
impixelinfo
drawnow;

It's evident that a lot of your lines are connected because the single largest blob is composed of a lot of connected lines.

[Walter Roberson]

Which plotting function was used to create the plot you showed in https://www.mathworks.com/matlabcentral/answers/1883932-how-to-mapped-largest-contour-from-pathlines#comment_2533977 ?

[DGM]

If the field is represented with a limitation on maximum density of plotted lines, I don't see how you can discern whether a path is closed by looking at whether a plotted line is closed.

[Image Analyst]

I don't know. We're all just guessing here because he doesn't want to share his original data or code for how the vector field was created or how the contour lines were created. Obviously trying to figure it out from a screenshot is not the best way, and susceptible to the size of the plot on screen, but he's not sharing anything else so what can we do? We have no context at all other than it's a vector field. I'm going to quit soon unless more helpful information is forthcoming.

@Suman If you have any more questions, then attach your data and code to read it in with the paperclip icon after you read this:

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