What does the “eff” statistic mean, and why is it above 100%?
The “eff” statistic stands for Efficiency, and essentially measures how easy it is for LuxRender to “find the light”. Currently it measures how many light contributions each sample has, on average. Due to how the bidirectional path tracer works, it is very common to see an efficiency rating of over 1000%. While the bidirectional path tracer spends more time per sample (the Samples/sec count is lower), it has usually a much higher efficiency rating than the regular path tracer. In other words, you get more bang for the buck with the bidirectional path tracer.
