Transform Polar to Cartesian co-ordinates?

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I've written the following python code that 'unravels' an iris from a pupil (i.e. given a central point, it sweeps round 360 degrees and prints out the iris as a long strip (i.e. a rectangle).

The problem is, this code is very slow in Python; is there a good way to do this using OpenCV?

def unravel(img, pupil):
  height, width, depth = img.shape
  max_radius =  180
  min_radius = int(pupil[2])
  out = np.zeros((max_radius-min_radius,720,3), np.uint8)

  for phi in range(0,720):
    for r in range(min_radius, max_radius):
      target = cmath.rect(r,math.radians(phi/2))
      out[(r-min_radius,phi)] = img[(pupil[1] + target.real, pupil[0]+target.imag)]
  return out

Input:

(Cropped) output: