How to get data from contour

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Dmitry Borovoy
Dmitry Borovoy on 1 Sep 2011
Answered: Chad Greene on 15 Aug 2014
From help:
[X,Y] = meshgrid(-2:.2:2,-2:.2:3);
Z = X.*exp(-X.^2-Y.^2);
[C,h] = contour(X,Y,Z);
set(h,'ShowText','on','TextStep',get(h,'LevelStep')*2)
colormap cool
Now I want to take only levels equal to 0.4
[X,Y] = meshgrid(-2:.2:2,-2:.2:3);
Z = X.*exp(-X.^2-Y.^2);
[C,h] = contour(X,Y,Z);
set(h, 'LevelList', [ 0.4]);
set(h,'ShowText','on','TextStep',get(h,'LevelStep')*2)
colormap cool
And now I want to get data that was displayed at figure and display it with 'plot' function.
figure;plot(C(:,1),C(:,2))
How to get data which I needed?

Accepted Answer

Friedrich
Friedrich on 1 Sep 2011
EDIT: Code now handles different regions correctly.
Hi,
you can use the contour matrix together with inpolygon to get point inside the region:
For example:
hold on
[X,Y] = meshgrid(-2:.2:2,-2:.2:3);
Z = X.*exp(-X.^2-Y.^2);
[C,h] = contour(X,Y,Z);
set(h, 'LevelList', [ 0.1 0.2 0.3]);
set(h,'ShowText','on','TextStep',get(h,'LevelStep')*2)
colormap cool
x = reshape(X,1,numel(X));
y = reshape(Y,1,numel(Y));
cm = get(h,'ContourMatrix');
index_start = 2;
index_end = cm(2,1)+1;
IN = inpolygon(x,y,cm(1,index_start:index_end),cm(2,index_start:index_end));
for i=2:numel(get(h,'LevelList'))
index_start = index_end + 2;
index_end = index_start + cm(2,index_start-1) - 1;
tmp = inpolygon(x,y,cm(1,index_start:index_end),cm(2,index_start:index_end));
IN = IN | tmp;
end
plot(x(IN),y(IN),'r+');
hold off
figure
hold on
index_start = 2;
index_end = cm(2,1)+1;
plot(cm(1,index_start:index_end),cm(2,index_start:index_end))
for i=2:numel(get(h,'LevelList'))
index_start = index_end + 2;
index_end = index_start + cm(2,index_start-1) - 1;
plot(cm(1,index_start:index_end),cm(2,index_start:index_end))
end
plot(x(IN),y(IN),'r+')
hold off
  3 Comments
K E
K E on 15 Aug 2014
Really useful. Should be built in to the contour program or submitted to Fex.

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More Answers (1)

Chad Greene
Chad Greene on 15 Aug 2014
C2xyz converts the contour matrix to x,y,z values easily.

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