By default, mental ray for Maya ignores any previously generated final gather results each time final Gather render happens; final gather results are freshly calculated.

You can store final gather results so that later frames can use the results from a frame rendered earlier to speed up the Final Gather rendering process.

Store and reuse final gather results when you aren’t going to be changing any final gather settings between renders (for example, when rendering a still image). You may be able to reuse a final gather map for animation, as long as the irradiance values for moving objects don't change significantly.

By default the file is saved into the current project's mentalRay\finalgMap directory.

To store and reuse previously generated final gather results

1. Make sure Final Gather is turned on and the attributes are set. See Render with final gather.
2. Set the Final Gather file attributes:
   • Primary Final Gather File
   • Rebuild
3. Render.