Interpolation allows Carrara to guess at photon placement - the lower the precision setting, the more Carrara guesses. The payoff if faster render times. Miss Nancy's advice is good if you are doing a single image. However, if you're working with an animation, you may want to try settings between 85 and 100 percent on the interpolation slider because of time per frame on render. 100 percent precision IS different than the No Interpolation option you get when selecting the check box. The 100 percent accuracy will still provide faster results than those you get when simply unchecking the interpolation option. Additionally, one possible assist that I forgot to mention was to turn on the Improved Edges option. Sometimes this will help when using transparency and sometimes it does nothing at all. I've worked on several assignments very similar to what you show in your preview image. There is not a single "bullet-proof" method of fixing the issue because of other variables. The biggest variable is what is doing the GI lighting. If you are only using lights, it's much easier. If you're using an HDRI or Background map, things can get a little more complicated. Mark