FVerbaas posted at 4:40 AM Mon, 29 May 2023 - #4466650

A reliable check whether an xyz point  is 'inside' or 'outside' a mesh is very costly. Mesh objects are not by definition 'watertight', so the simple 'cast any ray and count intersections' method does not work. On top of that, formally there is no 'inside' or 'outside' unless the mesh is 'watertight', while a labyrinth would effectively have the same effect as watertightness when it comes to casting shadows.

A volume check might be unnecessary. Most Poser meshes are single sided, however. If the light were only blocked by the front side as opposed to both sides, a lamp placed inside a globe could cast through to the outside of the mesh while the mesh still occluded lights placed outside of it as one would expect. (Also, I would have expected this as current behavior from the meshes being single sided)
Of course this would be pointless if forcing "render backsided polygons" made them block light still again.