Refraction is used to successfully exploit the "invisible in raytracing" state - where a refraction ray cannot intersect any part of the body, whether back-facing or not. The result is a tongue or teeth are only seen if the mouth is open, or the eyes or nose only seen if facing the camera (such as is not seen in seach's image above). The "invisible in raytracing" seems poorly named because even the camera ray is "ray traced" in SuperFly (but not FireFly) but whatever, that doesn't count.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)