In the 2-D case, the simplices
attribute of the ConvexHull
object holds the pairs of indices of the points that make up the line segments of the convex hull. One way to get just the indices is to get the unique elements of the flattened simplices
array. But note that the points will not be in an order that follows the convex hull around the set. (In scipy 0.13.0 and later, you can use the vertices
attribute to get the indices; see below.)
For example,
import numpy as np
from scipy.spatial import ConvexHull
import matplotlib.pyplot as plt
# Generate some random points for the demo.
np.random.seed(4321)
pts = 0.1 + 0.8*np.random.rand(15, 2)
ch = ConvexHull(pts)
# hull_indices = ch.vertices # This will work in the scipy 0.13
hull_indices = np.unique(ch.simplices.flat)
hull_pts = pts[hull_indices, :]
plt.plot(pts[:, 0], pts[:, 1], 'ko', markersize=10)
plt.plot(hull_pts[:, 0], hull_pts[:, 1], 'ro', alpha=.25, markersize=20)
plt.xlim(0, 1)
plt.ylim(0, 1)
plt.show()
This generates:
The vertices
attribute was added in scipy 0.13.0:
import numpy as np
from scipy.spatial import ConvexHull
import matplotlib.pyplot as plt
# Generate some random points for the demo.
np.random.seed(4321)
pts = 0.1 + 0.8*np.random.rand(15, 2)
ch = ConvexHull(pts)
# Get the indices of the hull points.
hull_indices = ch.vertices
# These are the actual points.
hull_pts = pts[hull_indices, :]
plt.plot(pts[:, 0], pts[:, 1], 'ko', markersize=10)
plt.fill(hull_pts[:,0], hull_pts[:,1], fill=False, edgecolor='b')
plt.xlim(0, 1)
plt.ylim(0, 1)
plt.show()