python - Get coordinates from the contour in matplotlib?

Question

Background

From the documentation example here, one can easily produce the following contour plot with the code snippet.

import matplotlib
import numpy as np
import matplotlib.cm as cm
import matplotlib.mlab as mlab
import matplotlib.pyplot as plt

matplotlib.rcParams['xtick.direction'] = 'out'
matplotlib.rcParams['ytick.direction'] = 'out'

delta = 0.025
x = np.arange(-3.0, 3.0, delta)
y = np.arange(-2.0, 2.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = mlab.bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
Z2 = mlab.bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
# difference of Gaussians
Z = 10.0 * (Z2 - Z1)

# Create a simple contour plot with labels using default colors.  The
# inline argument to clabel will control whether the labels are draw
# over the line segments of the contour, removing the lines beneath
# the label
plt.figure()
CS = plt.contour(X, Y, Z)
plt.clabel(CS, inline=1, fontsize=10)
plt.title('Simplest default with labels')

My Goal

I have obtained my contour plot and meanwhile got the matplotlib.contour.QuadContourSet instance CS. In the example snippet, CS is only used for clabel(). However for my case, I need to obtain either the equation of the contour line or the coordinate set for further computation.

How can I extract the coordinates of the contour line from the instance CS? OR How can I achieve it in other ways?

I bet there must be a way of doing so. Otherwise, the contour thing is only a "vase for visualization" then.

Answer 1

You can get the coordinates of the contours from the CS.allsegs list.

Try:

dat0= CS.allsegs[0][0]
plt.plot(dat0[:,0],dat0[:,1])

to plot the first (-1) contour level.