I've seen something about this a while ago. I think it worked like this: there was a simple underwater scene with a fish just under the water surface. when TIR was off the fish had no reflection in the watersurface because it was inside the water object. With TIR on the fish did have a a reflection even though it was inside the waterobject. A setting of 1 makes the renderer render just one reflection inside the object, with a setting of 2 you can render a reflection of a reflection and with a setting of 3 you can render a reflection of a reflection of a reflection and so on. I'm not completely sure but I think this is what TIR is.

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