I recently ran into an issue where I’d like to use a piece of geometry as a sky dome, seen through a glass window. This accidentally disabled my sun directional light, as it turns out the sky geometry was blocking it.

My usual fix was to add the “Indirect Lighting” attribute, but in this case it also prevented the rays refracted through the glass from reaching the sky dome geometry. After a bit of resarch I found https://renderman.pixar.com/view/TGT_REYES_Lighting, which explains Trace Sets as being the workaround to fix this.

The article already explains it well, but this is how I applied it in my case:

  1. Create a set with the geometry you’d like to not cast shadow
  2. In the attribute editor, make sure the set is selected and add the trace set attribute via Renderman >> Mark as Trace Group
  3. Now select the light you’d like to ignore this object, go to it’s Shadows tab and choose the trace set you just set up