This maya plugin....
RayDiffuse does this EXACT thing
https://www.lightengine3d.com/downloa...yDiffuse1.html
It's a GI tool, but a subsampling artifact makes it look like dirt
Check out the gallery, and you'll see what I mean.
The NEW version costs 50 bucks, but the OLD version is free.
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com