How to mapped largest contour from pathlines?

Question

Suman 2022-12-24

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https://ww2.mathworks.cn/matlabcentral/answers/1883932-how-to-mapped-largest-contour-from-pathlines

评论： Image Analyst 2022-12-31

采纳的回答： Image Analyst

I need to find largest contour from the pathline.

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Answer 1

Image Analyst 2022-12-24

1
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https://ww2.mathworks.cn/matlabcentral/answers/1883932-how-to-mapped-largest-contour-from-pathlines#answer_1135637

在 MATLAB Online 中打开

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.

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Image Analyst 2022-12-30

编辑：Image Analyst 2022-12-30

在 MATLAB Online 中打开

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.

DGM 2022-12-31

编辑：DGM 2022-12-31

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 2022-12-31

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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