Add fft equivalence test

changelog/58.breaking.rst

@@ -1 +1,2 @@
 Optional parameters are now keyword only for the :mod:`xrayvision.transform`, :mod:`xrayvision.imaging` and :mod:`xrayvision.visibility` modules.
+Remove ``natural`` keyword in favour of ``scheme`` keyword which can be either 'natural' or 'uniform'.

xrayvision/tests/test_transform.py

@@ -2,7 +2,8 @@
 import numpy as np
 import pytest
 from astropy.convolution import Gaussian2DKernel
-from numpy.testing import assert_allclose, assert_array_equal
+from numpy.fft import fft2, fftshift, ifft2, ifftshift
+from numpy.testing import assert_allclose, assert_array_almost_equal, assert_array_equal
 from scipy import signal
 
 from xrayvision.transform import dft_map, generate_uv, generate_xy, idft_map
@@ -361,3 +362,32 @@ def test_phase_centre_equivalence():
     img2 = idft_map(vis_shifted, u=u, v=v, weights=1/u.size, pixel_size=[1, 1] * apu.arcsec/apu.pix,
                     shape=data.shape*apu.pix, phase_centre=[5, 5]*apu.arcsec)
     assert np.allclose(data, img2)
+
+
+def test_fft_equivalence():
+    # chose so xy values run from 0 to 2 the same as in fft
+    shape = (3, 3)
+    pixel = (1, 1)
+    center = (1, 1)
+
+    data = np.arange(np.prod(shape)).reshape(shape)
+    uu = generate_uv(shape[0] * apu.pix, phase_centre=center[0] * apu.arcsec,
+                     pixel_size=pixel[0] * apu.arcsec / apu.pix)
+    vv = generate_uv(shape[1] * apu.pix, phase_centre=center[1] * apu.arcsec,
+                     pixel_size=pixel[1] * apu.arcsec / apu.pix)
+    u, v = np.meshgrid(uu, vv, indexing='ij')
+    u = u.flatten()
+    v = v.flatten()
+
+    vis = dft_map(data, u=u, v=v, pixel_size=pixel * apu.arcsec / apu.pix, phase_centre=center * apu.arcsec)
+
+    ft = fft2(data)
+    fts = fftshift(ft)
+    vis = vis.reshape(3, 3)
+    # Convention in xrayvison has the minus sign on the forward transform but numpy has it on reverse
+    vis_conj = np.conjugate(vis)
+    assert_array_almost_equal(fts, vis_conj)
+
+    vis_ft = ifftshift(vis_conj)
+    img = ifft2(vis_ft)
+    assert_array_almost_equal(np.real(img), data)