FG/GI Flicker Tips

Quoted from Jim Rothrock's response on the mentalray mailing list...

First, disable photon mapping (caustics and GI) until you have adjusted your FG settings. You can try increasing the number of FG rays to 1000 or 1500, but the most important option for decreasing FG flicker is the FG maximum radius. Decreasing the radius reduces flicker. Enable the FG diagnostic mode so that you can see where FG points are being generated. Do not decrease the FG maximum radius to the point where the diagnostic display shows a solid mass of FG points. If you appear to have a sufficient FG point density and you are still getting flicker, increase the number of FG rays until the flicker is acceptable. Also, using FG points from a final gather file instead of using "finalgather rebuild" can reduce flicker. Remember to delete the file before rendering the first frame of the animation.

Once final gathering looks correct, disable it and enable photon mapping. For GI, I have gotten good results with an accuracy of 2000 to 4000 photons and a radius sufficient to encompass those photons. The accuracy radius is just a speed optimization; it should not be used to control quality. The accuracy radius tells the renderer "Don't bother looking outside this radius". In a well-illuminated area, you'll always find N (e.g. 2000 to 4000) photons within the radius. To get good photon "shadows" with an accuracy of 4000 photons, I made sure that 4 million photons were stored in the photon map. I was doing volume GI as well as surface GI, so you can probably store fewer photons.

Once the photon mapping looks correct, enable FG and render your scene.