fix tth_to_d and d_to_tth

Author: yucongalicechen

src/diffpy/utils/scattering_objects/diffraction_objects.py

@@ -427,12 +427,12 @@ def tth_to_d(self):
             The array of :math:`d` values in the inverse of the units
             of ``wavelength``
         """
-        epsilon = 1e-10
         two_theta = np.asarray(np.deg2rad(self.on_tth[0]))
         wavelength = float(self.wavelength)
         sin_two_theta = np.sin(two_theta / 2)
-        sin_two_theta = np.where(np.abs(sin_two_theta) < epsilon, sin_two_theta + epsilon, sin_two_theta)
-        return wavelength / (2 * sin_two_theta)
+        d = np.where(sin_two_theta != 0, wavelength / (2 * sin_two_theta), np.inf)
+        d = np.minimum(d, DMAX)
+        return d[::-1]
 
     def d_to_tth(self):
         r"""
@@ -454,11 +454,10 @@ def d_to_tth(self):
         two_theta : array
             The array of :math:`2\theta` values in radians
         """
-        epsilon = 1e-10
         d = np.asarray(self.on_d[0])
-        d = np.where(np.abs(d) < epsilon, d + epsilon, d)
         wavelength = float(self.wavelength)
-        return np.rad2deg(2.0 * np.arcsin(wavelength / (2 * d)))
+        tth = np.where(d != 0, np.rad2deg(2.0 * np.arcsin(wavelength / (2 * d))), 180)
+        return tth[::-1]
 
     def set_all_arrays(self):
         master_array, xtype = self._get_original_array()

tests/test_diffraction_objects.py

@@ -268,18 +268,21 @@ def test_d_to_q():
 
 
 def test_tth_to_d():
-    actual = Diffraction_object(wavelength=0.71)
-    setattr(actual, "on_tth", [[0, 30], [1, 1]])
+    actual = Diffraction_object(wavelength=2)
+    setattr(actual, "on_tth", [[0, 30, 60, 90, 120, 180], [1, 2, 3, 4, 5, 6]])
     actual_d = actual.tth_to_d()
-    expected_d = [3550000000, 1.37161]
+    # expected d values are DMAX=100, 1/sin15, 1/sin30, 1/sin45, 1/sin60, 1/sin90, in reverse order
+    expected_d = [1, 1.1547, 1.41421, 2, 3.8637, 100]
     assert np.allclose(actual_d, expected_d)
 
 
 def test_d_to_tth():
-    actual = Diffraction_object(wavelength=0.71)
-    setattr(actual, "on_d", [[1e10, 1.37161], [1, 1]])
+    actual = Diffraction_object(wavelength=2)
+    setattr(actual, "on_d", [[0, 2, 4, 6, 8, 100], [1, 2, 3, 4, 5, 6]])
     actual_tth = actual.d_to_tth()
-    expected_tth = [0, 30]
+    # expected tth values are 2*arcsin(1/d), in reverse order
+    # when d is really small we have tth to be 180
+    expected_tth = [1.14593, 14.36151, 19.18814, 28.95502, 60, 180]
     assert np.allclose(actual_tth, expected_tth)