Merge pull request #2 from DanRyanIrish/cadair/fix_resampled

Writing tests and updated fix

Author: Cadair

ndcube/wcs/wrappers/resampled_wcs.py

@@ -43,13 +43,13 @@ def _top_to_underlying_pixels(self, top_pixels):
         # Convert user-facing pixel indices to the pixel grid of underlying WCS.
         factor = self._pad_dims(self._factor, top_pixels.ndim)
         offset = self._pad_dims(self._offset, top_pixels.ndim)
-        return (top_pixels * factor) + 1 + offset
+        return (top_pixels + 0.5) * factor - 0.5 + offset
 
     def _underlying_to_top_pixels(self, underlying_pixels):
         # Convert pixel indices of underlying pixel grid to user-facing grid.
         factor = self._pad_dims(self._factor, underlying_pixels.ndim)
         offset = self._pad_dims(self._offset, underlying_pixels.ndim)
-        return (underlying_pixels - 1 - offset) / factor
+        return (underlying_pixels + 0.5 - offset) / factor - 0.5
 
     def _pad_dims(self, arr, ndim):
         # Pad array with trailing degenerate dimensions.

ndcube/wcs/wrappers/tests/test_resampled_wcs.py

@@ -185,7 +185,7 @@ def test_int_fraction_pixel_shape(celestial_wcs):
         assert isinstance(dim, numbers.Integral)
 
 @pytest.fixture
-def nine_nine_wcs():
+def four_five_wcs():
     from astropy.wcs import WCS
 
     wcs = WCS(naxis=2)
@@ -195,22 +195,21 @@ def nine_nine_wcs():
     wcs.wcs.crpix = [1, 1]
 
     wcs.wcs.set()
-    wcs.pixel_shape = [9, 9]
+    wcs.pixel_shape = [4, 5]
     return wcs
 
-def test_corner_pixels(nine_nine_wcs):
-    owcs = nine_nine_wcs
-    # original corner
-    ocorner = owcs.pixel_to_world_values(-0.5, -0.5)
-    # one super pixel along (3 original pixels)
-    osone = owcs.pixel_to_world_values(2.5, 2.5)
-
-    wcs = ResampledLowLevelWCS(owcs, 3)
-
-    # resampled corner == original corner
-    corner = wcs.pixel_to_world_values(-0.5, -0.5)
-    # One super pixel along
-    sone = wcs.pixel_to_world_values(0.5, 0.5)
-
-    assert np.allclose(ocorner, corner)
-    assert np.allclose(osone, sone)
+@pytest.mark.parametrize(('factor', 'offset', 'expected_over_pixels'),
+                         [([2, 3], [0, 0], np.meshgrid(np.arange(-0.5, 1.75, 0.25), np.arange(-0.5, 1.2, 1/6))),
+                          ([2, 3], [1, 2], np.meshgrid(np.arange(-1, 1.25, 0.25), np.arange(-7, 4) / 6)),
+                         ])
+def test_resampled_pixel_to_world_values(four_five_wcs, factor, offset, expected_over_pixels):
+    wcs = four_five_wcs
+    # Get world values of original pixel grid.
+    under_pixels = np.meshgrid(np.arange(-0.5, 4, 0.5), np.arange(-0.5, 5, 0.5))
+    expected_world = wcs.pixel_to_world_values(*under_pixels)
+
+    # Resample WCS
+    new_wcs = ResampledLowLevelWCS(wcs, factor, offset)
+    # Get expected pixel coords in resampled WCS of same pixel locations as above.
+    output_world = new_wcs.pixel_to_world_values(*expected_over_pixels)
+    assert_allclose(np.asarray(output_world), np.asarray(expected_world), atol=1e-15)