Update class2d docstring and add comment about log polar

Author: garrettwrong

src/aspire/classification/averager2d.py

@@ -621,10 +621,17 @@ def _reddychatterji(self, images, class_k, reflection_k):
             # Compute the Cross_Correlation to estimate rotation
             # Note that _phase_cross_correlation uses the mangnitudes (abs()),
             #  ie it is using both freq and phase information.
-            cross_correlation, shift = self._phase_cross_correlation(
+            cross_correlation, _ = self._phase_cross_correlation(
                 warped_fixed_img_fs, warped_regis_img_fs
             )
 
+            # Rotating Cartesian space translates the angular log polar component.
+            # Scaling Cartesian space translates the radial log polar component.
+            # In common image resgistration problems, both components are used
+            #   to simultaneously estimate scaling and rotation.
+            # Since we are not currently concerned with scaling transformation,
+            #   disregard the second axis of the `cross_correlation` returned by
+            #   `_phase_cross_correlation`.
             cross_correlation_score = cross_correlation[:, 0].ravel()
 
             self._rotation_cross_corr_diagnostic(

src/aspire/classification/class2d.py

@@ -54,5 +54,5 @@ def classify(self):
     @abstractmethod
     def averages(self, classes, refl, distances):
         """
-        Returns class averages using prescribed `aligner`.
+        Returns class averages.
         """