CODEWITHSUNDEEP
pythonmathangle
cmca
cmca

Reputation: 153

calculating angle between three points but only anticlockwise in python

So I'm trying to calculate the angle between three points. for example

a = [14, 140]
b = [13, 120]
c = [12, 130]
d = [11, 110]

|
|    c    a
|
|  d    b
|____________

say i want to calculate the angle between ABC i use the follow code

#create vectors
ba = a - b
bc = c- b
# calculate angle
cosine_angle = numpy.dot(ba,bc) / (numpy.linalg.norm(ba) * numpy.linalg.norm(bc))

angle = numpy.arccos(cosine_angle) 
pAngle = numpy.degrees(angle)

My script runs and the output angle works, my issue though is when i want to calculate BCD I want the angle on the outside not the inside so say instead of the angle being 120 degrees i want the angle 240. So i only want the anti-clockwise angles.

Not sure how to get this value, can anyone point me in the right direction?

*edit: in other terms i want to identify angles that are over 180 degrees anticlockwise

*edit2: the duplicate answer does not answer the question for me as i have not used alot of java so not sure how to code that in python

Upvotes: 4

Views: 3705

Answers (2)

Semih Yagcioglu
Semih Yagcioglu

Reputation: 4101

Now using angular cosine distance to calculate the angle between two vectors is quite good, but in your case it might be better to use arc tangent as mentioned in the comments.

Now assuming you want to calculate the counterclockwise angle between BCD, you can do this by using the numpy's atan2 function. atan2(x, y) will give the angle between the origin point y to x.

import numpy as np

def calculate_angle(point_a, point_b):
    """ Calculate angle between two points """
    ang_a = np.arctan2(*point_a[::-1])
    ang_b = np.arctan2(*point_b[::-1])
    return np.rad2deg((ang_a - ang_b) % (2 * np.pi))

a = np.array([14, 140])
b = np.array([13, 120])
c = np.array([12, 130])
d = np.array([11, 110])

# create vectors
ba = a - b
bc = c - b
cd = d - c

# calculate angle
cosine_angle = np.dot(ba, bc) / (np.linalg.norm(ba) * np.linalg.norm(bc))

angle = np.arccos(cosine_angle)
inner_angle = np.degrees(angle)

print inner_angle  # 8.57299836361


# see how changing the direction changes the angle
print calculate_angle(bc, cd) # 188.572998364
print calculate_angle(cd, bc) # 171.427001636

Upvotes: 6

Tusk
Tusk

Reputation: 1727

You are able to calculate the inner angle, but you want the reversed angle? why not just calculating 360 - angle

Upvotes: 0

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