The mental ray renderer can generate shadows by ray tracing. Ray tracing traces the path of rays sampled from the light source. Shadows appear where rays have been blocked by objects. Ray-traced shadows have sharp edges.
Turning off caustics makes the outlines of shadows in this scene easier to see.
Light objects in 3ds Max Design let you choose a shadow generator: Ray Traced, Advanced Ray Traced, Shadow Map, and so on. Because the mental ray renderer supports only two kinds of shadow generation, ray tracing and shadow maps, some of the 3ds Max Design shadow generators aren't fully supported.
In 3ds Max Design, a special shadow generator type, mental ray Shadow Map, is provided to support the mental ray renderer. If shadows are enabled (on the of the Render Setup dialog) but shadow maps are not enabled, then shadows for all lights are generated using the mental ray ray-tracing algorithm. If shadow maps are enabled, then shadow generation is based on each light’s choice of shadow generator: