--- ray/doc/ray.1	2017/12/24 22:01:00	1.33
+++ ray/doc/ray.1	2018/06/25 20:49:10	1.34
@@ -1,4 +1,4 @@
-.\" RCSid "$Id: ray.1,v 1.33 2017/12/24 22:01:00 greg Exp $"
+.\" RCSid "$Id: ray.1,v 1.34 2018/06/25 20:49:10 greg Exp $"
 .\" Print using the -ms macro package
 .DA 07/10/2016
 .LP
@@ -946,6 +946,29 @@ transmission or back-side reflection, only then the su
 invisible from behind.
 Unlike other data-driven material types, the BSDF type is fully
 supported and all parts of the distribution are properly sampled.
+.LP
+.UL sBSDF
+.PP
+The sBSDF material is identical to the BSDF type with two important
+differences.
+First, proxy geometry is not supported, so there is no thickness parameter.
+Second, an sBSDF is assumed to have some specular through component, which
+is treated specially during the direct calculation and when viewing the
+material.
+Based on the BSDF data, the coefficient of specular transmission is
+determined and used for modifying unscattered shadow and view rays.
+.DS
+mod sBSDF id
+5+ BSDFfile ux uy uz funcfile transform
+0
+0|3|6|9
+     rfdif gfdif bfdif
+     rbdif gbdif bbdif
+     rtdif gtdif btdif
+.DE
+.LP
+If a material has no specular transmitted component, it is much better
+to use the BSDF type with a zero thickness than to use sBSDF.
 .LP
 .UL Antimatter
 .PP