I am trying to use the Alpha Shapes 2D alpha shapes methods to create a polygon based on a group of points. I have found that when I try to plot the alpha shapes it appears that the points are not organised in such a way that it will make it possible to properly plot the polygon. See the image below for clarification,
The left plot shows the outcome when Alpha_shape_2 with the blue x showing the REGULAR finite vertices of the alpha shape.
The right shows the outcome when using the weighted alpha shape method.
In both examples you can see how the red polygon criss-crosses back and forth - which is the issue I am trying to resolve. Is this correct behaviour or is there something I can do to resolve this?
The (mostly)full code is shown below:
my_points = {load my points from file}
my_points_weighted = {load my weighted points from file}
fig, (ax, ax1) = plt.subplots(1, 2)
# fig, ax = plt.subplots(figsize = (4,8))
ax.scatter(my_points[:,0], my_points[:,1], marker='x',c = 'orange')
## alpha shape
lst = [Point_2(p[0], p[1]) for p in my_points]
t = Alpha_shape_2(lst, 10000, GENERAL)
t.make_alpha_shape(lst)
optimal_alphas = t.find_optimal_alpha(1)
print(optimal_alphas.next())
print("alpha", t.set_alpha(optimal_alphas.next()))
## weighted alpha shape
weighting = {load my point weightings from file}
lst_wp = [Weighted_point_2(Point_2(p[0], p[1]), weighting[int(p[1]),int(p[0])]) for p in my_points_weighted]
was = Weighted_alpha_shape_2()
was.make_alpha_shape(lst_wp)
# alpha_shape_vertices
# if t.classify(v) == REGULAR
alpha_vertices_weighted = np.array([(v.point().x(), v.point().y()) for v in was.finite_vertices() if was.classify(v) == REGULAR ], dtype=int)
alpha_vertices = np.array([(v.point().x(), v.point().y()) for v in t.finite_vertices() if t.classify(v) == REGULAR ], dtype=int)
poly = patches.Polygon(alpha_vertices, alpha = .3, color = 'r')
ax.scatter(alpha_vertices[:,0], alpha_vertices[:,1], marker='x')
ax.add_patch(poly)
ylim = ax.get_ylim()
ax.set_ylim(ylim[1], ylim[0])
poly = patches.Polygon(alpha_vertices_weighted, alpha = .3, color = 'r')
ax1.scatter(alpha_vertices_weighted[:,0], alpha_vertices_weighted[:,1], marker='x')
ax1.add_patch(poly)
ylim = ax1.get_ylim()
ax1.set_ylim(ylim[1], ylim[0])
plt.show()
I am trying to use the Alpha Shapes 2D alpha shapes methods to create a polygon based on a group of points. I have found that when I try to plot the alpha shapes it appears that the points are not organised in such a way that it will make it possible to properly plot the polygon. See the image below for clarification,
The left plot shows the outcome when Alpha_shape_2 with the blue x showing the REGULAR finite vertices of the alpha shape.
The right shows the outcome when using the weighted alpha shape method.
In both examples you can see how the red polygon criss-crosses back and forth - which is the issue I am trying to resolve. Is this correct behaviour or is there something I can do to resolve this?
The (mostly)full code is shown below: