Update open_pbr_surface.mtlx

added a comment to avoid undos

Signed-off-by: krohmerNV <42233792+krohmerNV@users.noreply.github.com>

Author: krohmerNV

libraries/bxdf/open_pbr_surface.mtlx

@@ -409,6 +409,11 @@
       <input name="thinfilm_thickness" type="float" nodename="thin_film_thickness_nm" />
       <input name="thinfilm_ior" type="float" interfacename="thin_film_ior" />
     </dielectric_bsdf>
+
+    <!-- 
+      Explicitly using 'mix(layer(bsdf1, bsdf3), layer(bsdf2, bsdf3), weight)' instead of 'layer(mix(bsdf1, bsdf2, weight), bsdf3)' to overcome code gen limitations.
+      Both expressions are semantically equivalent.
+    -->
     <layer name="dielectric_base_tf" type="BSDF">
       <input name="top" type="BSDF" nodename="dielectric_reflection_tf" />
       <input name="base" type="BSDF" nodename="dielectric_substrate" />
@@ -422,6 +427,7 @@
       <input name="bg" type="BSDF" nodename="dielectric_base_notf" />
       <input name="mix" type="float" interfacename="thin_film_weight" />
     </mix>
+
     <!-- Metal Layer -->
     <multiply name="metal_reflectivity" type="color3">
       <input name="in1" type="color3" interfacename="base_color" />