Added some comments.

math/spvo.py

@@ -25,17 +25,26 @@ class SparseVoxelOctree:
         self.origin = (0, 0, 0)
 
     def _get_child_index(self, x, y, z, size):
+        """
+        Get the index of a child.
+        Index: The position of the node in the node parent's children list.
+        """
+
         index = 0
 
-        if x >= size // 2:
-            index |= 1
-            x -= size // 2
+        if x >= size // 2:  # x pos
+            # there are only 2 possible x coordinate values in a 2x2 cube, so we use the 1st bit in index for x
+            # xyz: the coordinates of the parent node
+            index |= 1  # set 1st bit to 1
+            x -= size // 2  # get parent x
 
-        if y >= size // 2:
+        # and the same for y and z
+
+        if y >= size // 2:  # y pos
             index |= 2
             y -= size // 2
 
-        if z >= size // 2:
+        if z >= size // 2:  # z pos
             index |= 4
             z -= size // 2