Documentation

The bih constructor creates a Bounding Interval Heirarchy from a list of primitives. BIH is a type of data structure that groups primitives into a heirarchy of bounding objects, so that a ray need not be tested against every single primitive. This can make the difference betweeen a rendering job that takes days or seconds. BIH usually performs a little worse than a SAH-based KD-tree, but construction time is much better.

See http:en.wikipedia.orgwikiBounding_interval_hierarchy