[Matplotlib-users] Extract the x- and y-locations from the contour matrix C

[Slavin, Jonathan]

Hi Nils,

Extracting the x and y locations from a contour is a bit of a pain in
matplotlib (or at least was last time I tried it). if you do something like
c = ax.contour(x,y,z,...)
(where ... stands in for the arguments you might want to supply)
then c is a QuadContourSet. To get at the contour data you need to do
something like
p = c.collections[i].get_paths()
(if you want the i'th collection, i.e. contour level number i, or you could
iterate of c.collections)
then p is a list of Path objects corresponding to each separate contour.
p.vertices is then an N x 2 array of the x and y values

An alternative to this is to use scikit-image package, in particular
skimage.measure.find_contours. However in that case you do something like:
c = find_contours(image,level)
(where image is a 2D array) and you get back a list of (n,2) arrays with
the indices of the image for the contour. To get the x,y values then you
need to scale those indices accordingly.

Jon

> Date: Wed, 6 Nov 2019 12:41:55 +0100
> From: Nils Wagner <nils106 at googlemail.com>
> To: matplotlib-users at python.org
> Subject: [Matplotlib-users] Extract the x- and y-locations from the
>         contour matrix C
> Message-ID:
>         <CAE7vhgN0cKXf54hvgT4Y=up_vYFL-o=w=
> ChSWUBNBMDnZ-nCPQ at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Hi All,
>
> in Octave you can do something like this
>
> % Define the input grid
> [x, y] = meshgrid(linspace(-2, 2));
> % Calculate the two surfaces
> %z1 = y.^2 + 2*x;
> z1 = exp(-(x.^2 + y.^2));
> %z2 = 2*y.^3 - x.^2;
> z2 =  0.4+0.0*x+0.0*y;
> % Visualize the two surfaces
> figure(1);
> surface(x, y, z1, 'FaceColor', [0.5 1.0 0.5], 'EdgeColor', 'none');
> surface(x, y, z2, 'FaceColor', [1.0 0.5 0.0], 'EdgeColor', 'none');
> view(3); camlight; axis vis3d
> % Take the difference between the two surface heights and find the contour
> % where that surface is zero.
> zdiff = z1 - z2;
> %C = contours(x, y, zdiff, [0 0]);
> figure(2);
> contour(x, y, zdiff, [0 0]);
> C = contour(x, y, zdiff, [0 0]);
> % Extract the x- and y-locations from the contour matrix C.
> xL = C(1, 2:end)
> yL = C(2, 2:end)
> % Interpolate on the first surface to find z-locations for the intersection
> % line.
> zL = interp2(x, y, z1, xL, yL);
> % Visualize the line.
> figure(3);
> line(xL, yL, zL, 'Color', 'k', 'LineWidth', 3);
>
> How can we extract the x- and y-locations from the contour matrix C in
> matplotlib?
>
-- 
Jonathan D. Slavin
Astrophysicist - High Energy Astrophysics Division
Center for Astrophysics | Harvard & Smithsonian
Office: (617) 496-7981 | Cell: (781) 363-0035
60 Garden Street | MS 83 | Cambridge, MA 02138
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