CODEWITHSUNDEEP
pythonmath
Tobias Stein
Tobias Stein

Reputation: 51

Find inner angles of a triangle

I have a triangle with three vertices (x,y,z) 

triangle = [[ 0.  0.  0.], [ 1.  0.  0.], [ 0.5    0.866  0.   ]]

I used the answer from link but somehow I don't get the right angles.

def angle(triangle):

def unit_vector(vector):
    print(vector)
    """ Returns the unit vector of the vector.  """
    return vector / np.linalg.norm(vector)

def angle_between(v1, v2):
    """ Returns the angle in radians between vectors 'v1' and 'v2'::

            >>> angle_between((1, 0, 0), (0, 1, 0))
            1.5707963267948966
            >>> angle_between((1, 0, 0), (1, 0, 0))
            0.0
            >>> angle_between((1, 0, 0), (-1, 0, 0))
            3.141592653589793
    """
    v1_u = unit_vector(v1)
    v2_u = unit_vector(v2)
    return np.arccos(np.clip(np.dot(v1_u, v2_u), -1.0, 1.0))

v1, v2, v3 = triangle[0], triangle[1], triangle[2]
vec1, vec2, vec3 = v2-v1, v3-v2, v3-v1
a1, a2, a3 = angle_between(vec1,vec2),angle_between(vec2,vec3),angle_between(vec3,vec1)


print(a1,a2,a3)
2.09440780623 1.04722295888 1.04718484736

The sum of the angles are above 3.1416rad what is not possible.

Where is the problem?

Upvotes: 0

Views: 248

Answers (1)

hiro protagonist
hiro protagonist

Reputation: 46921

you need to get the direction of your vectors right:

angle_between(vec1,-vec2)  # note the sign here

if the sign of the vector is wrong, you do not get the inner angle of your triangle at this corner (but 'pi - the angle'). drawing a sketch will surely clarify this.

Upvotes: 2

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