Find boundary points of xy coordinates-CodePudding

The

I want to retrieve the xy-coordinates of the points that form the boundary of the shape. Answers on similar questions suggest to use Concave Hull. With the help of this

Clearly the red points are not the boundary points. Some are enclosed by other points and they all are different from the original points. How do I define the boundary points in terms of the original points? Please advice

CodePudding user response：

When I try to reproduce your code the convexHull function works perfectly. I changed your code so that the positions of black and red circles are rounded in the same way. And I reduced the radius of the red circles so you can better see if everything fits.

import numpy as np import matplotlib.pyplot as plt from scipy import spatial xy = np.loadtxt('xy.txt') x, y = xy[:, 0].astype('float64'), xy[:, 1].astype('float64') hull = spatial.ConvexHull(xy, incremental=False, qhull_options='Qt') hull_indices = hull.vertices boundary_x = [] boundary_y = [] for i in range(len(hull_indices)): index = hull_indices[i] boundary_x.append(xy[index, 0].astype('float64')) boundary_y.append(xy[index, 1].astype('float64')) plt.plot(x, y, 'o', color='black', markersize=6) plt.plot(boundary_x, boundary_y, 'o', color='red', markersize=4) plt.show()

Result:

Update: If you need the concave parts of the boundary, too, you can use the python package