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

ghem
November 8th, 2008, 05:11 PM
Hello,

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.

WW
November 8th, 2008, 05:31 PM

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.

gschoep
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]