fix: reconstruction volume rotated by 180° due to floating point inac…

…curacies (tilted bpg)

CHANGELOG

@@ -1,3 +1,8 @@
+0.2.5
+ - fix: reconstruction volume rotated by 180° due to floating point
+   inaccuracies (affects `backpropagate_3d_tilted`).
+0.2.4
+ - maintenance release
 0.2.3
  - enh: employ slice-wise padding to reduce the memory usage (and
    possibly the computation time) of

odtbrain/_alg3d_bppt.py

@@ -291,7 +291,12 @@ def sphere_points_from_angles_and_tilt(angles, tilted_axis):
 
     # (b) This is the polar angle measured in the x-z plane starting
     #     at the x-axis and measured towards the positive z-axis.
-    phi = np.arctan2(u, w)
+    if np.allclose(u, 0) and np.allclose(w, 0):
+        # Avoid flipping the axis of rotation due to numerical
+        # errors during its computation.
+        phi = 0
+    else:
+        phi = np.arctan2(u, w)
 
     # Determine the projection points on the unit sphere.
     # The resulting circle meets the x-z-plane at phi, and

tests/test_alg3d_bppt.py

@@ -90,6 +90,42 @@ def test_3d_backprop_plane_rotation():
         assert np.allclose(results[ii], results[ii-1], atol=.2, rtol=.2)
 
 
+def test_3d_backprop_plane_alignment_along_axes():
+    """
+    Tests whether the reconstruction is always aligned with
+    the rotational axis (and not antiparallel).
+    """
+    parameters = get_test_parameter_set(1)
+    p = parameters[0]
+    results = []
+
+    # These are specially selected angles that don't give high results.
+    # Probably due to phase-wrapping, errors >2 may appear. Hence, we
+    # call it a soft test.
+    tilts = [0, np.pi/2, np.pi, 3*np.pi/2, 2*np.pi]
+
+    for angz in tilts:
+        sino, angles = create_test_sino_3d_tilted(tilt_plane=angz, A=21)
+        rotmat = np.array([
+            [np.cos(angz), -np.sin(angz), 0],
+            [np.sin(angz),  np.cos(angz), 0],
+            [0,             0, 1],
+        ])
+        # rotate `tilted_axis` onto the y-z plane.
+        tilted_axis = np.dot(rotmat, [0, 1, 0])
+        fref = odtbrain.backpropagate_3d_tilted(sino, angles,
+                                                padval=None,
+                                                tilted_axis=tilted_axis,
+                                                padding=(False, False),
+                                                dtype=np.float64,
+                                                onlyreal=True,
+                                                **p)
+        results.append(fref)
+
+    for ii in np.arange(len(results)):
+        assert np.allclose(results[ii], results[ii-1], atol=.2, rtol=.2)
+
+
 if __name__ == "__main__":
     # Run all tests
     loc = locals()

tests/test_spherecoords_from_angles_and_axis.py

@@ -27,8 +27,7 @@ def test_simple_sphere():
                          [0.87965281125489458, s2/3*2, 0.063156230327168605],
                          [s2/3, s2/3*4, s2/3]],
                         ])
-
-    assert np.allclose(correct.flatten(), np.array(results).flatten())
+    assert np.allclose(correct, np.array(results))
 
 
 if __name__ == "__main__":