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What's the proper way to employ portals? Do you design a room with a window, put glass in it to have reflections and/or refractions etc and then put a portal over a second plane next to the glass? Or can portals only be used on empty openings and thus have no glass effects etc, i.e. glass or portal?

Hey!

you find a note about that in the manual:

"Currently, portals cannot be placed in openings which are not open, eg a window with a portal cannot contain glass at this time. In some complex scenes and situations, portals might slow down the render, so a bit of experimentation with/without should be done. Portals only apply to pathtracing type kernels, eg pathtracing and PMC. (not directlighting/ambient occlusion)"

so if it is not changed yet your portal will become useless if you put a windowglas behind or in front of it - the opening must be complete open - but you need portals only if you have rather small windows, if your window is big enoug the rays will find it anyway - also note that portals can slow down the rendering - its not always better for your scene!