code: def _get_augment_params(self, img):
          center = img.shape[1::-1] * self._rand_range(
              self.center_range[0], self.center_range[1], (2,))
          deg = self._rand_range(-self.max_deg, self.max_deg)
          if self.step_deg:
              deg = deg // self.step_deg * self.step_deg
          
          mat = cv2.getRotationMatrix2D(tuple(center - 0.5), deg, 1)
          return WarpAffineTransform(
              mat, img.shape[1::-1], interp=self.interp,
              borderMode=self.border, borderValue=self.border_value)
docs: The correct center is shape*0.5-0.5. This can be verified by:
  
          SHAPE = 7
          arr = np.random.rand(SHAPE, SHAPE)
          orig = arr
          c = SHAPE * 0.5 - 0.5
          c = (c, c)
          for k in range(4):
              mat = cv2.getRotationMatrix2D(c, 90, 1)
              arr = cv2.warpAffine(arr, mat, arr.shape)
          assert np.all(arr == orig)