--- ray/doc/ray.1	2011/02/18 00:40:25	1.18
+++ ray/doc/ray.1	2011/06/21 23:58:00	1.20
@@ -869,11 +869,10 @@ mod transdata id
 .PP
 The BSDF material type loads an XML (eXtensible Markup Language)
 file describing a bidirectional scattering distribution function.
-
 Real arguments to this material may define additional
 diffuse components that augment the BSDF data.
-String arguments are used to define thickness for hidden
-objects and the "up" orientation for the material.
+String arguments are used to define thickness for proxied
+surfaces and the "up" orientation for the material.
 .DS
 mod BSDF id
 6+ thick BSDFfile ux uy uz funcfile transform
@@ -883,25 +882,27 @@ mod BSDF id
      rbdif gbdif bbdif
      rtdif gtdif btdif
 .DE
-The first string argument is a "thickness" parameter that is useful
-for hiding detail geometry for transmitting systems, e.g.,
-complex fenestration.
-If a view or shadow ray hits a BSDF surface with non-zero specular transmission
-and positive thickness, the ray will pass directly through with no
-reflection or transmission due to the BSDF.
+The first string argument is a "thickness" parameter that may be used
+to hide detail geometry being proxied by an aggregate BSDF material.
+If a view or shadow ray hits a BSDF proxy with non-zero thickness,
+it will pass directly through as if the surface were not there.
 Similar to the illum type, this permits direct viewing and
 shadow testing of complex geometry.
-In contrast, a scattered ray will use the BSDF transmission,
-offsetting transmitted sample rays by the thickness amount
-to avoid any intervening geometry.
-In this manner, BSDF surfaces may act as simplified stand-ins for detailed
-system geometry, which may still be present and visible in the simulation.
-If the BSDF has back-side reflection data, a parallel surface should be
-specified slightly less than the given thickness away from the front surface
-to enclose the system geometry on both sides.
-A zero thickness implies that the BSDF geomtery is all there is, and
-thickness is ignored if there is no transmitted component, or transmission is
-purely diffuse.
+The BSDF is used when a scattered (indirect) ray hits the surface,
+and any transmitted sample rays will be offset by the thickness amount
+to avoid the hidden geometry and gather samples from the other side.
+In this manner, BSDF surfaces can improve the results for indirect
+scattering from complex systems without sacrificing appearance or
+shadow accuracy.
+If the BSDF has transmission and back-side reflection data,
+a parallel BSDF surface may be
+placed slightly less than the given thickness away from the front surface
+to enclose the complex geometry on both sides.
+The sign of the thickness is important, as it indicates whether the
+proxied geometry is behind the.
+.I BSDF
+surface (when thickness is positive)
+or in front (when thickness is negative).
 .LP
 The second string argument is the name of the BSDF file, which is
 found in the usual auxiliary locations.