Subsurface in thin-walled mode, small clarification (#258)

* Merge v1.1 development to main (#222)

This changelist merges v1.1 development from dev_1.1 to main, in preparation for marking the release of OpenPBR v1.1.

* Subsurface in thin-walled mode, small clarification

* Subsurface in thin-walled mode, small clarification

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Co-authored-by: Jonathan Stone <jstone@lucasfilm.com>

Author: portsmouth

index.html

@@ -1245,7 +1245,9 @@
          E_R[f^R_\mathrm{diffuse}] + E_T[f^T_\mathrm{diffuse}] = S \le 1 \ .
       \end{equation}
       At the default of zero anisotropy ($g=0$) the energy is balanced equally between diffuse reflection and transmission.
-      The diffuse transmission lobe shape (in both hemispheres) is assumed to be controlled by the **`base_diffuse_roughness`** parameter. Typically the diffuse lobes $f_+$, $f_-$ will be represented by an Oren-Nayar lobe flipped into the appropriate hemisphere (which technically should be modified due to the dielectric boundaries, though a renderer may choose to ignore this). This model is useful for rendering cases such as light scattering through a thin sheet of paper (Figure [thinwalled]).
+      The diffuse transmission lobe shape (in both hemispheres) is assumed to be controlled by the **`base_diffuse_roughness`** parameter. Typically the diffuse lobes $f_+$, $f_-$ will be represented by an Oren-Nayar lobe flipped into the appropriate hemisphere (which technically should be modified due to the dielectric boundaries, though a renderer may choose to ignore this). Note that in this mode, the **`subsurface_radius`** and **`subsurface_radius_scale`** are ignored and have no effect, since the scattering MFP is infinitesimal.
+
+      This model is useful for rendering cases such as light scattering through a thin sheet of paper (Figure [thinwalled]).
 
 
 ![](images/thin_walled1.jpg width=99% align=right) ![](images/thin_walled2.jpg width=99% align=left)