Optimisations have been changed and updated - the BSP and Grid options were new to Bryce5.

Previously (from Bryce2, not sure about Bryce1) the three options were: Spatial Optimisation off; Spatial Optimisation low; and Spatial Optimisation high. Now we get the two new ones, each with the three levels (renamed and maybe reconfigured).

As to WHY. Well, as my mother used to say to me, "Because I bloody well said so, that's why. Now... be - quiet!" But that was last week. This week:

I think it's to do with Bryce having knowledge of where objects are in the scene and using that knowledge to not waste time sending rays to where there are no objects. So, depending on the layout of objects, Bryce uses different algorithms to trace the rays.