How to extract x and y position of contour line?

Question

K E 2015-7-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/230934-how-to-extract-x-and-y-position-of-contour-line

评论： KFoster 2021-10-1

In old matlab versions, I could extract x and y points along a contour line as shown below. This is incredibly useful in my work. Now in R2015 the approach doesn't work (hh.XData and hh.YData are the x and y values of the data you're contouring, rather than the x,y position of the contour line). In more recent versions of contour, how can I extract x,y points along a contour line? If I can't do it via contour, is there another way?

z = peaks; % Load in a dataset that all matlab users have
x = 1:size(peaks, 2); y = 1:size(peaks, 1);
figure;
pcolor(x, y, z); % Plot the data
shading flat;
hold on;
[cc, hh] = contour('v6', x, y, z, [0 10^5], 'k'); % Overlay contour line (will plot 3 continuous contours)
xPointsOfContour = get(hh(1), 'xdata'); % Get points along the first contour
yPointsOfContour = get(hh(1), 'ydata');
plot(xPointsOfContour, yPointsOfContour, 'w.'); % Demo that points on contour line were extracted

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Answer 1

K E 2015-7-23

3
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https://ww2.mathworks.cn/matlabcentral/answers/230934-how-to-extract-x-and-y-position-of-contour-line#answer_186957

在 MATLAB Online 中打开

Just realized that the contour matrix cc now seems to contain x and y location data,

pcolor(x, y, z); % Plot the data
hold on;
shading flat
[cc, hh] = contour(x, y, z, [0 10^5], 'k'); % Overlay contour line
hold on;
cc(cc<1) = NaN;
h = plot(cc(1,:), cc(2, :), 'w.');

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Ayush Tiwari 2021-7-29

what does 'w.' do here ?

Adam Danz 2021-7-29

w = white

'.' = point markers

w. = white point markers

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Answer 2

Yuhang Luo 2018-6-29

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/230934-how-to-extract-x-and-y-position-of-contour-line#answer_326887

https://ww2.mathworks.cn/matlabcentral/fileexchange/38863-extract-contour-data-from-contour-matrix-c this function might work.

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Adam Danz 2020-1-23

在 MATLAB Online 中打开

Also see

https://www.mathworks.com/matlabcentral/fileexchange/74010-getcontourlinecoordinates

[cm, h] = contour(. . .);
contourTable = getContourLineCoordinates(cm);
% --or--
contourTable = getContourLineCoordinates(h);
%     contourTable =
%      Level      Group        X             Y     
%     ________    _____    __________    __________
%      -5.8504      1       -0.040982        -1.625
%      -5.8504      1       -0.027916         -1.75
%      -5.8504      1               0       -1.7823
%      -5.8504      1        0.045032          -1.5
%     etc....

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Answer 3

John L 2017-6-9

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https://ww2.mathworks.cn/matlabcentral/answers/230934-how-to-extract-x-and-y-position-of-contour-line#answer_270085

Plot a single contour, then plot(hh.ContourMatrix(1,2:end),hh.ContourMatrix(2,2:end))

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Lars K. 2019-2-12

在 MATLAB Online 中打开

hi john,

with your answer you explained something very important using contour. I struggled and lost a lot of time, while i was trying to find intersections of two contour lines.

Plotting both contour matrix as lines and execute

[x,y,~,~]=intersection(contour1(1,:),contour1(2,:),contour2(1,:),contour2(2,:),true)

(btw thanks a lot to Douglas Schwarz) the intersection script found 6 intersections (4 wrong ones). Plotting both contour lines with markers i saw these other data points the first time.

The first column of the contour matrix M contains the value of the contour line and does not belong to the xy data points. With Johns little but important hint using

[x,y,~,~]=intersection(contour1(1,2:end),contour1(2,2:end),contour2(1,2:end),contour2(2,2:end),true)

the problem was solved and i got my two correct intersections.

KFoster 2021-10-1

The first column of the contour matrix M denotes what contour level and how many vertices are included in that particular line; similar columns are found throughout the M matrix between the coordinates for each line. It's in the matlab documentation here: https://www.mathworks.com/help/matlab/ref/contour.html#mw_6bc1813f-47cf-4c44-a454-f937ea210ab6

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