CODEWITHSUNDEEP
pythonrotationpoint
cleacrij
cleacrij

Reputation: 45

python rotating a point counterclockwise by an angle

I have this method that rotates a point counter clockwise.

def rotate(self, rad):
    self.x = math.cos(rad)*self.x - math.sin(rad)*self.y
    self.y = math.sin(rad)*self.x + math.cos(rad)*self.y

but when i pass it a point to rotate, only the x coordinate is rotated correctly. For example I tried to rotate the point (0,25) by π/3 . I should be getting (-22,13) since i am rounding the answers. Instead I am getting (-22,-6).

Upvotes: 1

Views: 4975

Answers (2)

sabbahillel
sabbahillel

Reputation: 4425

Yes you have changed x in the first equation. Thus

self.x = math.cos(rad)*self.x - math.sin(rad)*self.y

self.x = 0 - 6

so the second equation is

 self.y = math.sin(rad)*self.x + math.cos(rad)*self.y
 self.y = math.sin(math.pi/3)*(-6) + math.cos(math.pi/3)*25

>>> def rotate(x, y, rad):
        print x, y, rad
        xx = math.cos(rad)*x - math.sin(rad)*y
        yy = math.sin(rad)*x + math.cos(rad)*y
        print xx, yy
        return(xx, yy)

>>> rotate(0, 25, math.pi/3)
0 25 1.0471975512
-21.6506350946 12.5
(-21.650635094610966, 12.500000000000004)

Upvotes: 0

Felix
Felix

Reputation: 6369

The Problem here is that you save the new value for self.x and use that same value as input for the calculation of self.y

Try this:

def rotate(self, rad):
    x = math.cos(rad)*self.x - math.sin(rad)*self.y
    self.y = math.sin(rad)*self.x + math.cos(rad)*self.y
    self.x = x

Upvotes: 2

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