Problem with refl. occ. with mr amb./refl. occ. shader

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  09 September 2013
Problem with refl. occ. with mr amb./refl. occ. shader

Hope the title wasn't confusing, anyways I'm having problems using the reflection mode with the "Ambient/Reflective Occlusion" shader in mental ray, what happens is that whenever the spread is lower than .65 it becomes really grainy when the viewing angle is around 90 - the lower the spread the worse it gets. Even with samples in the thousands it doesn't improve too much.

Is this normal?

By the way, I'm using it in the reflection slot in a blinn shader, if that makes any difference.
  09 September 2013
My first thought is to stay away from Blinns altogether. The mia_x_material can do everything a blinn, phong or lambert can and more. You're always better off sticking to MR shaders in general. There's a few reasons you want to do this, for one they're energy conserving and physically accurate.

In testing your refl occ make a new render layer (if you're not already doing this) and assign just the occ shader to everything. Your samples can be as low as Tweak until you get the result you like rather than doing tests on your beauty layer and you'll save alot of time.

In case you want to read up on the mia material you can find the docs here:

Hope this helps.

  09 September 2013
Thanks a ton for your reply, Mr. Justin. I will try out the A&D (max user here) and see if that helps.

I would like to have it all in the least number of render passes possible, since the stuff we produce at work is always in a rush and needs to render/be composited asap. That's why I opted with using it in the reflection slot, which can yield some interesting results if you ask me!

The setup I often use now is a composite map consisting of a dirty map ("refl map") multiplied on top of a fall off in refl slot - then occlusion shader in 'side' and then tweak the curve to mimic fresnel - then a color corrected "dirty map" used to control the spread ("gloss map"), with some times a fall off to map the spread together with the dirty map with a curve going from ~0.8 down to ~0.5 - and this little trick does make it possible to have spreads lower than .65 but not sure if that's physically accurate.

Here's an example of this setup:

It's better than nothing and renders super quickly, but definitely not realistic.
  09 September 2013
Another cool thing about the mia material is that it has built in fresnel shading. By ticking that box in BRDF settings, the reflection falloff is now controlled by the IOR in the refraction settings. All real world materials have an IOR, not just transparent ones so using that also gives you a more physically accurate render. Even if you're not going for photo-realism, using these techniques are worthwhile. Ever look at an image and say to yourself, something's not quite right even though you can't exactly put your finger on it? Between using physically accurate shading and lighting, as well as proper linear workflow, your renders will look noticeably improved. Alot of people think that working linearly isn't always necessary but I beg to differ. If you're not working linearly, you'll be fighting an uphill battle trying to compensate for that gamma curve by eye. Nothing looks right, the way light bounces and falls off, ramps and colors are all wrong. I highly suggest checking out MasterZap's fxphd on mental ray, he really is a master-

Anyways, best of luck and hopefully I didn't veer too far off topic.

- MrJ
  10 October 2013
It seems like using it in the surface slot in A&D works. I'll need to do some more testing to get it working with materials and what not, but at least there's no grain! Thanks!
  10 October 2013
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