Well, that’s tricky and I render the scene twice to fix that. One renders only the stuff that casts the rays, and theres a commonfilter rendering for that buffer (invoke CommonFilters with that buffer instead of base.win).
The other renders the scene normally. The result from the filter is applied on top of the usual render.
I think I’m adding a trick to use the stencil buffer in 1.7.0 - that will be compatible with ancient GPU’s as well.