Выдержка из доки по MR 2.1
1.19 OpenGL Acceleration
2.1By default, mental ray uses a scanline rendering algorithm for primary rays when no lens shaders are present that modify the ray direction. This algorithm can cope with both static and motion blurred scenes. In addition, mental ray supports OpenGL1.4 hardware to further accelerate scanline rendering of static scenes (without motion blurring). If the client host provides OpenGL acceleration with sufficient resolution, mental ray can use this to generate acceleration data that is subsequently used during regular rendering. This combines the speed of a hardware OpenGL accelerator with the full shading capabilities of mental ray, and is particularly effective for scenes with high polygon counts.
Shadow maps can also be rendering with OpenGL acceleration. This is particularly effective since shadow maps need no shading. Most of the computation involved is then performed by the OpenGL system, which greatly improves performance.
However, the accuracy of OpenGL acceleration is generally lower than that of the standard software scanline rendering algorithm.
В двух словах ясно сказано, что если ваш хост поддерживает Opengl то флаг в руки. Ускоряйтесь! И трассировку и тени, да, с потерей качества, но все же...
Это конечно все замечательно, но как? Все же как?
Уж очень 3dlabs поюзать охота.