Ivguerrero has created a Volume Particle Kill Field for Maya that allows you to make a Volume Particle Kill Field similar to that found in RealFlow, for both Maya particles and nParticles. This Volume Particle Kill Field for Maya is mostly an addition to MaxtorAG’s magKillField, which only handles classic maya particles not nParticles. Check out the freeware Volume Particle Kill Field for Maya Particles & nParticles script here.
