finding the third point in a triangle using python

Question

I am trying to create a python function to find the coordinates of a preestablished position given two reference points which can change.

At the start I have three points on the grid reference point 1 (X_r1,Y_r1), reference point 2 (X_r2,Y_r2), and the custom point (X_c,Y_c). Initialy all three points are known, but for subsequent movements reference point 1 and reference point 2 can be measured but the custom point is unknown. The shape and size of the triangle drawn between these 3 points dose not change.

Given this, is there a good way in python to take the coordinates of the initial 3 points and the coordinates of the new 2 reference points and calculate the coordinates of the new position of the custom point. (Green values are known, red values are not)

I have tried drawing circles around the new points using the distance between the reference points and the custom point as the radius and finding the intercept. however, this method was slow, and memory intensive as I am using very large coordinate distances, also it was rather unreliable in its implementation.

Any help on this would be most appreciated.

we need the length of all the three sides to successfully determine the third coordinates. — Talha Tayyab, Apr 25 '23 at 12:45
With the original 3 positions the length of the sides can easily be computed so that's a given. — Alain T., Apr 25 '23 at 12:57
This seems like a promising start. https://nghiaho.com/?page_id=671 — JonSG, Apr 25 '23 at 13:33
Checkout [Homogeneous transformations: an example in 2D with Python](https://alexsm.com/homogeneous-transforms/) — DarrylG, Apr 25 '23 at 14:04

Alain T. · Accepted Answer · 2023-04-25T16:16:52.233

Here's one possible solution using a function that rotates a point around the origin (0,0):

The strategy is to transpose the coordinates to a frame of reference around one of the known points. Then apply the relative rotation of the other known point to compute the unknown third point.

If we name the 3 points of the triangle A, B and C with C being the unknown after moving A and B, the process would go as follows:

Convert B and C into relative coordinates having A as their origin ==> B0 and C0
Compute the angle of B0 from the origin. That will be the original angle of the AB segment (which can easily be obtained using math.atan2)
Convert newB point to relative coordinates with newA as origin ==> newB0
Compute the angle of newB0 from the origin
The delta between angles of B0 and newB0 can be applied to the original C0 point to produce newC0 (using the point rotation function)
Transpose newC0 to newC by translation from newA

...

import math
def rotatePoint(x,y,angle):
    s,c = math.sin(angle), math.cos(angle)
    return (x*c-y*s, x*s+y*c)

def getNewC(A,B,C,newA,newB):
    Ax,Ay = A
    Bx,By = B
    Cx,Cy = C
    
    B0x,B0y    = Bx-Ax, By-Ay
    oldAngleAB = math.atan2(B0y,B0x)

    newAx,newAy   = newA
    newBx,newBy   = newB
    newB0x,newB0y = newBx-newAx, newBy-newAy
    newAngleAB    = math.atan2(newB0y,newB0x)

    C0x,C0y       = Cx-Ax, Cy-Ay
    newC0x,newC0y = rotatePoint(C0x,C0y,newAngleAB - oldAngleAB)
    newCx,newCy   = newAx+newC0x, newAy+newC0y
    
    return newCx,newCy

outut: (using turtle to illustrate)

import turtle
t = turtle.Turtle()

A = (10,10)
B = (60,10)
C = (30,120)        
t.penup()
t.goto(*A)
t.pendown()
t.goto(*B)
t.goto(*C)
t.goto(*A)

newA = (-10,-10)
newB = (-10,-60)
newC = getNewC(A,B,C,newA,newB)    
t.pen(pencolor="red")
t.penup()
t.goto(*newA)
t.pendown()
t.goto(*newB)
t.goto(*newC)
t.goto(*newA)

newA = (-40,40)
newB = (-70,80)
newC = getNewC(A,B,C,newA,newB)    
t.pen(pencolor="blue")
t.penup()
t.goto(*newA)
t.pendown()
t.goto(*newB)
t.goto(*newC)
t.goto(*newA)

Note that this does not perform any scaling. It assumes that the distance between the two base points remains the same after being moved. If, eventually, you do need scaling, it can be added at the end in the transposition from newC0 to newC:

scale         = ((newB0x**2+newB0y**2)/(B0x**2+B0y**2))**0.5
newCx,newCy   = newAx+newC0x*scale, newAy+newC0y*scale

output:

newA = (110,110)
newB = (150,220)
newC = getNewC(A,B,C,newA,newB)    
t.pen(pencolor="purple")
t.penup()
t.goto(*newA)
t.pendown()
t.goto(*newB)
t.goto(*newC)
t.goto(*newA)

finding the third point in a triangle using python

1 Answers1