Troubleshoot volumetric
fog with depth map shadows
If the fog appears with
streaks, or the entire effect disappears from your scene when rendering
in mental ray for Maya, you can use one of the following solutions.
In the Attribute Editor for
the volume primitive shape node, turn on Volume Samples Override.
Set the Volume Samples value
to 5, or greater (between 5 - 20 is recommended). (You may need
to tune this setting repeatedly, until you achieve a good result.)