Mental Ray Sky Portals

Although the use of the sky portals is a nice and recommended workflow for getting outside skylight into your interiors, it seems the Soft Shadow quality and Shadow Samples can be a killer for render times if you have a bunch of these in your scene. My latest scene required 16 of these, and that was with a lot of overlap on the exterior walls to minimize the number of Portals. Likewise, if I remember correctly, the Sky Portals render black from the backside if you are veiwing an exterior with the sky portals active (think of a transition from exterior to interior where the interior is lit primarily by these sky portals). I could be mistaken about this and need to double check the scenario in which I saw it. However, in scenes with large amounts of windows in a lot of different places, I used a workaround, creating Plane objects that mimic the sky portals but have a self illumiated Arch and Design Material assigned. Place these planes just inside the window and uncheck "Visible to Camera" in their properties. Tweak the Illumination settings to your liking. Although they don't cast shadows, their illumination seems to render much faster and gets that "outside" light look into your interiors.