sorting a complicated collection of 2d euclidian points in in clockwise/counterclockwise fashion to form a closed ring
This looks like a repeated question but I tried the solution that already exists and none seems to work so far for me. .
this solution gives a hint but it works only for a regular geometry. I have a rather complicated geometry from which I extract boundary points which are unsorted.
Below is a picture of the geometry and of the boundary vertices that I extract from the geometry. 
The (x,y) coordinates of points as in the image are :
import numpy as np
pts = np.array([[ 30. , -6.25 ],
[ 30. , -8.10127917],
[ 0. , -6.25 ],
[ 34.14082772, -6.75584268],
[ 36.49784598, -10. ],
[ 44.43561524, -10. ],
[ 100. , -10. ],
[ 100. , 10. ],
[ 84.1244615 , -10. ],
[ 84.1244615 , 10. ],
[ 36.49784598, 10. ],
[ 34.14082772, 6.75584268],
[ 44.43561524, 10. ],
[ 30. , 8.10127917],
[ 30. , 6.25 ],
[ 0. , 6.25 ],
[ -30. , 6.25 ],
[ -30. , 8.10127917],
[ -32.92183092, 9.05063958],
[ -35.84366185, 10. ],
[ -51.88274638, 10. ],
[-100. , 10. ],
[-100. , -10. ],
[ -83.96091546, 10. ],
[ -83.96091546, -10. ],
[ -35.84366185, -10. ],
[ -51.88274638, -10. ],
[ -32.92183092, -9.05063958],
[ -30. , -8.10127917],
[ -30. , -6.25 ],
[ -67.92183092, 10. ],
[ -67.92183092, -10. ],
[ 68.24892299, 10. ],
[ 52.37338449, 10. ],
[ 68.24892299, -10. ],
[ 52.37338449, -10. ]])
In the boundary vertex data, we can see that the points are unordered. Is there a way the points could be ordered clockwise/counter clockwise so that these points form a closed ring when connected successively?
My goal is to create a polygon or a linear ring as described here and later find if an arbitrary euclidean point lies inside the polyogn/ring
Update: the approach of computing angle between centroid of the pts and the individual euclidean point in pts also doesn't work. Here is a sample code of what I tried :
def sort_circular(pts):
cent = coords.mean(axis=0)
idx = list(np.arange(0, len(pts)+1, dtype=int))
angle = []
for i, cc in enumerate(coords):
dx,dy = cc[0] - center[0], cc[1]-center[1]
angle.append(math.degrees(math.atan2(float(dy), float(dx))))
#simultaneously sort angle and indices
_, idx_sorted = (list(t) for t in zip(*sorted(zip(angle, idx))))
pts_sorted = pts[idx_sorted]
return pts_sorted
The result from this is still not the way i expect it to be (image below):
Method 1:
Define a center point, compute the angle between every coordinate and the center point, then order them by angle:
import pandas as pd
# Define function to compute angle between vectors
import math
def clockwiseangle_and_distance(point, origin = [0,0], refvec = [1,0]):
# Vector between point and the origin: v = p - o
vector = [point[0]-origin[0], point[1]-origin[1]]
# Length of vector: ||v||
lenvector = math.hypot(vector[0], vector[1])
# If length is zero there is no angle
if lenvector == 0:
return -math.pi, 0
# Normalize vector: v/||v||
normalized = [vector[0]/lenvector, vector[1]/lenvector]
dotprod = normalized[0]*refvec[0] + normalized[1]*refvec[1] # x1*x2 + y1*y2
diffprod = refvec[1]*normalized[0] - refvec[0]*normalized[1] # x1*y2 - y1*x2
angle = math.atan2(diffprod, dotprod)
# Negative angles represent counter-clockwise angles so we need to subtract them
# from 2*pi (360 degrees)
if angle < 0:
return 2*math.pi+angle, lenvector
# I return first the angle because that's the primary sorting criterium
# but if two vectors have the same angle then the shorter distance should come first.
return angle, lenvector
import pandas as pd
# Compute the center point
center = pts.mean(axis=0)
angle = []
for i in range(len(pts)):
ang, dist = clockwiseangle_and_distance(pts[i,:] - center, origin=[0,0], refvec=[1,0])
angle.append(ang)
df = pd.DataFrame(pts)
df['angle'] = np.degrees(angle)
df = df.sort_values(by='angle')
df['clockwise_order'] = np.arange(len(df))
import matplotlib.pyplot as plt
# Create plot to show the ordering of the points
plt.figure()
df.plot(kind='scatter', x=0, y=1, s=100, alpha=0.5)
plt.title('Points by clockwise order')
for idx, row in df.iterrows():
plt.gca().annotate('{:.0f}'.format(row['clockwise_order']), (row[0], row[1]),
ha='center', va='center_baseline', fontsize=6, color='k', fontweight='bold')
plt.gca().annotate('Center', center,
ha='center', va='center')
If this clockwise ordering doesn't give you what you want, try Method 2.
Method 2:
To sort the points for the given geometry in clockwise order such that they form a closed ring, you can do the following:
- Divide the data set in quadrants
- Choose a center point such that the remaining points of the quadrant lie on about the arc of a circle centered at the center point
- Order each quadrant by clockwise angle
- Place each quadrant in clockwise order
# Compute the center point
center = pts.mean(axis=0)
df = pd.DataFrame(pts)
# Group points into quadrants
df['quadrant'] = 0
df.loc[(df[0] > center[0]) & (df[1] > center[1]), 'quadrant'] = 0
df.loc[(df[0] > center[0]) & (df[1] < center[1]), 'quadrant'] = 1
df.loc[(df[0] < center[0]) & (df[1] < center[1]), 'quadrant'] = 2
df.loc[(df[0] < center[0]) & (df[1] > center[1]), 'quadrant'] = 3
quadrant = {}
for i in range(4):
quadrant[i] = df[df.quadrant == i]
# Intelligently choose the quadrant centers
x = 35
y = 5
subcenter = [[ x, y],
[ x, -y],
[-x, -y],
[-x, y]]
# Compute the angle between each quadrant and respective center point
angle = {}
points = {}
df_sub = {}
for j in range(len(quadrant)):
angle[j] = []
points[j] = quadrant[j][[0,1]]
for i in range(len(points[j])):
ang, dist = clockwiseangle_and_distance(points[j].values[i,:] - subcenter[j], origin=[0,0], refvec=[1,0])
angle[j].append(ang)
df_sub[j] = quadrant[j]
df_sub[j]['angle'] = np.degrees(angle[j])
df_sub[j] = df_sub[j].sort_values(by='angle')
# Combine the data frames
df = pd.concat(df_sub)
df['clockwise_order'] = np.arange(len(df))
# Plot the points by clockwise order
import matplotlib.pyplot as plt
# Create plot to show the ordering of the points
fig, axis = plt.subplots()
df[[0,1]].plot(x=0, y=1, ax=axis, c='lightblue', legend=False, clip_on=False)
df.plot(kind='scatter', x=0, y=1, s=100, ax=axis, c='lightblue', clip_on=False)
plt.title('Points by quadrant in clockwise order')
plt.axis('off')
for idx, row in df.iterrows():
plt.gca().annotate('{:.0f}'.format(row['clockwise_order']), (row[0], row[1]),
ha='center', va='center_baseline', fontsize=6, color='k', fontweight='bold')
plt.gca().annotate('Center', center,
ha='center', va='center')
for i in range(len(subcenter)):
plt.scatter(subcenter[i][0], subcenter[i][1], alpha=0.5, s=80, marker='s')
plt.gca().annotate('Quadrant \n'+str(i)+'\n', subcenter[i],
ha='center', va='center_baseline', color='k', fontsize=8)
# Plot with axis equally-spaced
df2 = df[[0,1]].reset_index(drop=True)
df2.loc[len(df2),:] = df2.loc[0,:]
df2.plot(x=0, y=1, c='k', legend=False, clip_on=False)
plt.axis('equal')
plt.axis('off')
If this doesn't give you what you want, you may have to order the coordinates by hand.