Noise in glass materials?

[marque_pierre]

Hello folks,

So... rendering glass in Mantra. (For me at least) It seems to be quite a noisy affair. 

I first used the glass shaders in the material library, but putting together a minimal shader with material builder (a PBR glass node: reflect 0, refract 1, roughness 0.2, IOR 1.05; a PBR reflect node: reflect 1; refract 0, roughness 0.1 and IOR 1.5) reduced the noise a lot. But... I still have a fair bit of noise in my indirect refraction and direct reflect layers. 

Increasing values in in my Mantra render node  of course improves things, but not nearly as much as I'd expect for the hit in render times.

Any suggestions as I tinker with this?

[toadstorm]

If you're seeing noise in direct reflect, try increasing your light samples before cranking reflect/refract quality on the ROP.

[davpe]

Not sure how much noise vs. render time are we talking about, but refraction rendering in Mantra is currently somewhat more noisy than competitive raytracers. I hope this issue to be addressed in upcoming Houdini release as that should be focused towards rendering (at least that's what I've heard). Right now, you have to bump up render samples to get rid of the noise. Note that you can manipulate direct and indirect samples separately (Quality multipliers on Mantra are only affecting indirect samples). Light samples should help too but beyond 3 samples it usually has no additional effect other than increasing render time. cheers.

[davpe]

also, thing with Mantra is that there is a great deal of render parameters that are not exposed (because there is a ton of it) and unfortunately some of them may impact the render time significantly. for new users this is not obvious and default values of some parameters should definitely be updated. for example, take a look at vm_samplecachesize (Sample Data Cache Size) - this is amount of ram dedicated for image sampling (I guess...), and something you definitely want to have at 2048 MB all the time and maybe even more for big scenes. it will not help you with noise but it can brutally slow down renders with lots of textures and displacements if you're leave it at default setting of 512 MB.

all rendering parameters can be accessed through the little gear icon at the top right corner of parameter window. also, see the docs page: http://www.sidefx.com/docs/houdini/props/mantra.html

Stochastic transparency can have an impact too. try to use the slider and bump it up to something ridiculous, like 128 (just to see if it has any effect). This parameter is exposed by default and is sitting at the mantra ROP in sampling tab all the way at the bottom. if this is on, separate sampling is used for transparency rays. most of the times, default value of 4 is fine but in some cases it can give quite noisy result and needs to be adjusted.

[davpe]

from the picture, I can also see you're using GGX specular distribution. that is the most noise prone specular you can have. Try using Phong. It may not give as plausible results but should reduce noise significantly and in most cases looks perfectly fine.