How to get data from contour

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?

Answer 1

[Friedrich]

EDIT: Code now handles different regions correctly.

Hi,

you can use the contour matrix together with inpolygon to get point inside the region:

http://www.mathworks.com/help/releases/R2011a/techdoc/ref/contourgroupproperties.html#ContourMatrix

http://www.mathworks.com/help/releases/R2011a/techdoc/ref/inpolygon.html

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

[Dmitry Borovoy]

thanks a lot. That is what I need. But seems like that it works only for one level. There appear some artifacts when I tried to get 2 regions simultaneously

[Friedrich]

Thanks for reporting. Fixed it. see modified code above.

[K E]

Really useful. Should be built in to the contour program or submitted to Fex.

Answer 2

[Chad Greene]

C2xyz converts the contour matrix to x,y,z values easily.