--- ray/doc/notes/BSDFdirections.txt	2015/08/26 03:19:27	1.1
+++ ray/doc/notes/BSDFdirections.txt	2015/08/27 15:31:52	1.2
@@ -26,11 +26,19 @@ The reason for this is two-fold.  First, the "Transmis
 matrix is the right way around for the 3-phase method already.
 Second, WINDOW 6 generally produces correct data for "Transmission
 Front," but in some cases just has dummy values as a placeholder
-for the "Transmission Back" matrix.  (Someone correct me on this
-if I am mistaken, but that's what I understood.)
+for the "Transmission Back" matrix.
 
+For the tensor tree representation, reciprocity can be used to
+derive all needed information for 4-D distributions, so only one
+tree is stored to save file space and memory.  However, isotropic
+(3-D) data does not use reciprocity for sampling, even though it
+can use it to answer BTDF queries, because it would require too
+much work to build up a 4-D structure then derive the 3-D information
+from it.  So, this is a caveat, and genBSDF will create reverse-flow
+isotropic BTDF data if run with +forward and +backward in this case.
+
 The situation is a bit simpler in the case of reflection, since one
-cannot construct the front reflection matrix from the rear or vice
+cannot construct the front reflection data from the rear or vice
 versa.  If one of the reflectance matrices is missing during
 rendering, then the surface gets no reflected contributions from
 that side.  The 3-phase method does not make use of reflectance,