Ashraf Aiad posts some information on what the FILTER setting in a Mental Ray Final Gather solution actually is doing by means of numeric values tied to sample images for each… great stuff!

Use this to control how Final Gather uses a speckle elimination filter to prevent samples with extreme brightness from skewing the overall energy stored in a Final Gather sampling region.

 

“FG will try to optimize/control the blotchiness in the scene by either merging or illuminating some of the rays that doesn’t return enough values. Thus getting rid of the extreme different in color (blotchiness). This will eventually lead to lower overall illumination.”