matplotlib - extracting data from contour lines

Solution 1:

For a given path, you can get the points like this:

p = cs.collections[0].get_paths()[0]
v = p.vertices
x = v[:,0]
y = v[:,1]

Solution 2:

from: http://matplotlib.org/api/path_api.html#module-matplotlib.path

Users of Path objects should not access the vertices and codes arrays directly. Instead, they should use iter_segments() to get the vertex/code pairs. This is important, since many Path objects, as an optimization, do not store a codes at all, but have a default one provided for them by iter_segments().

Otherwise, I'm not really sure what your question is. [Zip] is a sometimes useful built in function when working with coordinates. 1

Solution 3:

I am facing a similar problem, and stumbled over this matplotlib list discussion.

Basically, it is possible to strip away the plotting and call the underlying functions directly, not super convenient, but possible. The solution is also not pixel precise, as there is probably some interpolation going on in the underlying code.

import matplotlib.pyplot as plt
import matplotlib._cntr as cntr
import scipy as sp

data = sp.zeros((6,6))
data[2:4,2:4] = 1

plt.imshow(data,interpolation='none')
level=0.5
X,Y = sp.meshgrid(sp.arange(data.shape[0]),sp.arange(data.shape[1]))
c = cntr.Cntr(X, Y, data.T)
nlist = c.trace(level, level, 0)
segs = nlist[:len(nlist)//2]
for seg in segs:
    plt.plot(seg[:,0],seg[:,1],color='white')

plt.show()