trying to extend ndimage.interpolation.geometric_transform with Cython

geometric.pyx

import numpy as np
cimport numpy as np
from cpython.cobject cimport PyCObject_FromVoidPtrAndDesc
from scipy.ndimage import geometric_transform
from astropy.modeling import models, fitting

cdef int _shift_function(double* output_coordinates, double* input_coordinates,
            int output_rank, int input_rank, double* shift_data):
    cdef np.ndarray[double, ndim=1] shift = np.asarray(<double[:3]> shift_data)
    cdef Py_ssize_t ii
    for ii in range(input_rank):
        if ii == 0: input_coordinates[ii] = output_coordinates[ii]
        else: 
            input_coordinates[ii] = output_coordinates[ii] + (shift[0] + \
                                    shift[1]*output_coordinates[0] + \
                                    shift[2]*pow(output_coordinates[0],2))
    return 1


def undistort_function(np.ndarray[double, ndim=1] shift):
    """This is the function callable from python."""
    cdef double* shift_data = <double*> shift.data
    return PyCObject_FromVoidPtrAndDesc(&_shift_function,
                                        shift_data,
                                        NULL)

#my code calls geometric_transform from within a method to a cdef class that includes 
#a bunch of irrelevant stuff, so this is more of an adaptation than an accurate reproduction

def undistort_image(np.ndarray[double, ndim=2] image,
                    np.ndarray[double, ndim=1] trace_x,
                    np.ndarray[double, ndim=1] trace_y,
                    int fit_degree=2):
  cdef object poly = models.Polynomial1D(fit_degree)
  cdef object fitter = fitting.LinearLSQFitter()
  
  poly = fitter(poly, trace_x, trace_y)
  return geometric_transform(image, undistort_function(poly.parameters))