mental ray Framebuffer Settings

Location:

The Render Manager > mental ray (Global Renderer)
The Render Manager > (expand a pass) > mental ray
Set a current pass and choose Render > Render > Renderer Options from the Render toolbar.
Tab: Sample Filtering [Properties Reference]

Motion Blur and Intensity Clipping

Options: Motion Data [Properties Reference]

By default, mental ray clips color samples after each ray, before filtering them into the final pixel value. When you render motion blurred objects, this color clipping causes objects with varying intensity values (above 1.0) to appear as though they all have the same intensity value when blurred.

Deactivating the Color Channel Clipping option forces mental ray to not perform color clipping, allowing the object's intensity to carry through to the blurred result.

To deactivate Color Channel clipping, set it to No Clip.

A	The image above shows two motion-blurred spheres. The one on the right has a much higher intensity, but mental ray's color clipping has caused it to render incorrectly.
B	Deactivating Color Channel Clipping causes the sphere on the right to render with its correct intensity.