jeanphi wrote:It could also be stopped by russian roulette
I don't use RR in the eye pass for the same reason. SPPM has a kind of "natural" habit to terminate eye paths by stopping as soon as they hit a diffuse surface (i.e. the average eye path is very short). Terminating an eye path not on a diffuse surface will totality waste the successive photon pass for that particular hit point: this is the reason why RR is no used at all for the eye pass.
@guibou: it is a good a clever solution however it is not trivial to apply in the current Lux/PBRT BSDF interfaces.