View Full Version : [Python]Rotating a point about another point

November 8th, 2008, 05:11 PM

I am trying to write a small game in pyglet, in which several images will have to rotate around a point that is not the center of the image. I tried to write a function to do this, but it doesn't work:

from math import *

def prot(point, orig, ang):
point[0] -= orig[0]
point[1] -= orig[1]

point = [(point[0]*cos(ang)) - (point[1]*sin(ang)), (point[0]*sin(ang)) + (point[1]*cos(ang))]

point[0] -= orig[0]
point[1] -= orig[1]
return point

print prot([5, 5], [0, 0], 45)
The program runs fine, but it returns [-1.6279076785819435, 6.8811275667592415]. If it is rotating [5, 5] about the origin, then it should return [0, 5] or [5, 0] depending on which way it rotates.

Can someone please help me get this function to work?

November 8th, 2008, 05:31 PM
Just a few comments:

sin() and cos() use radians, not degrees. 45 degrees is pi/4 radians.
If you rotate (5,5) 45 degrees counter-clockwise, the new point is (0,7.071) (approximately), not (0,5).
Add the origin back to the point after rotating. You currently subtract it again.

January 6th, 2010, 08:36 AM
Don't forget that the second point can be y or z

This is complete code.

from math import *

def rotate2d(degrees,point,origin):
A rotation function that rotates a point around a point
to rotate around the origin use [0,0]
x = point[0] - origin[0]
yorz = point[1] - origin[1]
newx = (x*cos(radians(degrees))) - (yorz*sin(radians(degrees)))
newyorz = (x*sin(radians(degrees))) + (yorz*cos(radians(degrees)))
newx += origin[0]
newyorz += origin[1]

return newx,newyorz