2D vector graphics Problem

[Scott David Daniels]

Karl Max wrote:
> def rotate(self, w):
>  # This method belongs to the class Vertex
>  # w = angle expressed in radiants
>  x, y = self.coords
>  xRel, yRel = self.relPoint # The rotation should be relative to this
>  sin, cos = math.sin(w), math.cos(w)
>  x = x * cos - y * sin - xRel * cos + yRel * sin + xRel
>  y = x * sin + y * cos - xRel * sin - yRel * cos + yRel
>  self.coords = (x,y)

Your equation for y uses the new x, not the old x.  Be more free with
names.  Here's one way to write it:

class ...
     def rotate(self, angle):
         '''Rotate point angle radians around relPoint'''
         x, y = self.coords
         xRel, yRel = self.relPoint
         sin, cos = math.sin(angle), math.cos(angle)
         newx = x * cos - y * sin - xRel * cos + yRel * sin + xRel
         newy = x * sin + y * cos - xRel * sin - yRel * cos + yRel
         self.coords = newx, newy

If you define a testcase or two, you can catch things like this early.


test = Point(1, 1)
test.rotate(math.pi / 2)
x, y = test.coords
assert (x - -1) ** 2 + (y - 1) ** 2 < .00001


--Scott David Daniels
Scott.Daniels at Acm.Org