James,

As far as I know, all clipping is done before the texture and lighting calcs. If the polygon (either the ones you've drawn, or the ones the object is converted to at render time, in the case of nurbs/subds) isn't visible then that's it for the polygon, no more time wasting calcs are performed.

Just checked, yep - it does work like this. I rendered a scene in 2 1/2 mins, then zoomed in on a tiny fraction of this scene and re-rendered. This time it took 1 min. In the second shot there were 2 - 3 times as many pixels being coloured (textured/lit) and it stil took 2/5ths of the time.

As for GI, not sure. It's not something I've delved into yet. If there was a setting for 2 sided lighting or something like that, it is probably there that you could control this.

Simon.