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explanation of ray threshold?

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I've been reading through the manual and am confused about the ray threshold settings.  I cannot determine whether setting the threshold higher or lower is meant to decrease rendering time.  Can anyone explain?

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I'll precede everything I am about to say with the caveat that my interpretation of Ray Threshold is largely experience based, and not a result of any formal expertise in render technology.

The first thing to say is that this only makes much difference if you have tons of complex transparent / refractive things in your scene - otherwise you can pretty much ignore it.

As I understand it, it works by killing rays once they reach the brightness threshold you specify. The 0.1 default is very low, and doesn't kill the rays until they are 0.1% of their maximum brightness, (therefore almost black) before they die. Turning RT up kills the rays sooner, which stops them being calculated thereafter, thus saving render time. You know when you've turned it up too much because your scene will be far too dark, because you've killed too many rays too early.

But I do agree that the last line in the manual seems to suggest that the more you turn it up the longer the render time gets, which is the opposite of what actually happens as far as I can see.

Perhaps someone from MAXON can comment on that, or correct our understanding of it ?


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  • That is what I had thought. I think that the manual is misleading in it's explanation. I always turn it up to 10% anyway.

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