--- ray/doc/ray.1	2017/12/24 22:01:00	1.33
+++ ray/doc/ray.1	2018/10/08 20:04:09	1.36
@@ -1,4 +1,4 @@
-.\" RCSid "$Id: ray.1,v 1.33 2017/12/24 22:01:00 greg Exp $"
+.\" RCSid "$Id: ray.1,v 1.36 2018/10/08 20:04:09 greg Exp $"
 .\" Print using the -ms macro package
 .DA 07/10/2016
 .LP
@@ -947,6 +947,30 @@ 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 aBSDF
+.PP
+The aBSDF 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 aBSDF is assumed to have some specular through component
+(the 'a' stands for "aperture"), 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 aBSDF 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 aBSDF.
+.LP
 .UL Antimatter
 .PP
 Antimatter is a material that can "subtract" volumes from other volumes.
@@ -1569,6 +1593,12 @@ the Ecole Polytechnique Federale de Lausanne (EPFL Uni
 in Lausanne, Switzerland.
 .NH 1
 References
+.LP
+Wang, Taoning, Gregory Ward, Eleanor Lee,
+``Efficient modeling of optically-complex, non-coplanar
+exterior shading: Validation of matrix algebraic methods,''
+.I "Energy & Buildings",
+vol. 174, pp. 464-83, Sept. 2018.
 .LP
 Lee, Eleanor S., David Geisler-Moroder, Gregory Ward,
 ``Modeling the direct sun component in buildings using matrix