finding angle between three points on a 2d graph

finding angle between three points on a 2d graph - Printable Version

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finding angle between three points on a 2d graph - qrani - Nov-19-2018

So, i've been trying to find this out for such a long time.
So say I have three points A, B, C. Each with X and Y values.
ax = 1
ay = 5
bx = 1
by = 2
cx = 5
cy = 1
which looks something like this on a graph:
5 | A . . . .
4 | . . . . .
3 | . . . . .
2 | B . . . .
1 | . . . . C
0 * - - - - -
* 0 1 2 3 4 5
and i want to find the angle between A & C as if it were around circle B
also, 0-360

RE: finding angle between three points on a 2d graph - buran - Nov-19-2018

what have you tried? We are glad to help, but we won't do your [home]work.
Post your code in python tags, any errors in error tags, ask specific questions.

RE: finding angle between three points on a 2d graph - Gribouillis - Nov-19-2018

The function math.atan2() may help you solve this.

RE: finding angle between three points on a 2d graph - qrani - Nov-20-2018

here is what i have so far:

import math
p1x = 0
p1y = 5
p2x = 0
p2y = 0
p3x = 5
p3y = 0
angle1 = (math.atan2(p3y - p1y, p3x - p1x) - math.atan2(p2y - p1y, p2x - p1x))
print(angle1)

based on something i found online, but not sure if it is correct, also it just gives
0.785(irrational) and i want a 0-360 number.
edit: i changed it to deg with math.degrees,
but still doesn't work to my full intentions
and gives me 45 deg when its 90 deg

RE: finding angle between three points on a 2d graph - Gribouillis - Nov-20-2018

It is almost correct. The order of the points matters

import math

def angle3pt(a, b, c):
    """Counterclockwise angle in degrees by turning from a to c around b
        Returns a float between 0.0 and 360.0"""
    ang = math.degrees(
        math.atan2(c[1]-b[1], c[0]-b[0]) - math.atan2(a[1]-b[1], a[0]-b[0]))
    return ang + 360 if ang < 0 else ang

print(angle3pt((5, 0), (0, 0), (0, 5)))

Output:
90.0

This function will not throw an error if a==b or c==b because math.atan2(0,0) returns 0.0. It would probably be a good idea to add this in the function, something like

    if b in (a, c):
        raise ValueError("Undefined angle, two identical points", (a, b, c))