That happens for example when a partly transparent object is reflective at the same time or has refraction aswell. It can be set how many rays XSI will handle at the same time and when they stop bouncing around, from reflection to reflection. Here we can set the maximums for reflections, refractions and the global maximum ray depth. But in this menu there is a lot more to find 2.2. Although scanline is much faster than raytracing, the missing features make the result look less realistic than a raytraced image.
Results look great, but rendering time can be very time consuming.
That is why mental ray supports two ways of acceleration methods to speed up rendering. When the scene is subdivided into those little boxes mentioned before, a BSP tree is build which contains information on how much stuff is in each cluster or box. Depending on how complex a model is, more boxes will be created where the object is positioned. That means that those clusters will be smaller than the big ones, where there is nothing complex happening. The clusters have different size, which greatly improves rendering speed.