Mental Ray FG Artifacts in Relfections

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  04 April 2013
Mental Ray FG Artifacts in Relfections

I'm working on an interior shot using Mental ray in 3D Max Design 2013. Here is a reference image:

The issue I'm having is with the artifacting in the reflections. On the bottom of the overhead bins, and back of the seats, I'm getting this very grainy effect, also along the edge of the seats. Meanwhile, where lights are reflected I'm getting very spotty highlights. Additionally, it took 35 minutes to render this image, which I really need to cut down.

I've tried everything on every forum I've checked and nothing has solved the problem!!

Here are the specs for my scene/render:

-Scene to real world scale in cm
-A total of 33 photometric lights with 3 differrent settings
-2 mr sky portal lights with shadow samples set to 16

-Mental Ray
-1/16 sampling with Mitchell filter
-Max Trace Depth 8 / Reflections 6 / Refractions 8

-FG deafualt Draft settings
-Advanced tab: Noise Filtering set to High with Max Depth 6 / Reflections 4 / Refractions 6
-GI enabled with multiplier 1 / Max # photos per sample 500

All of my materials are Arch & Deisgn with glossy samples set to 32.

I know I need to up my FG settings, but even when using Medium and High presets I still get this artifacting.

Can anyone help me clean up my relfections and lower my render time?
  04 April 2013
You probably need to look at your glossy samples setting on your materials. Chances are the default isn't good enough to remove the grainy reflections. So increase that to 32 and see if it has an effect. This is a good place to start, and at least identify the source of the grain.
__________________ - 3D Visualization and Content Creation
  04 April 2013
The samples are all already 32 on all of my materials. I've even tried setting a few of them to 128 and although it helped, it didn't completely solve the problem.
  04 April 2013

if you have FG-Splotches in flat mirrors, use the option "Reflection Prerejection". I have this option included in Render Optimizer.

Look here:

hot chip
  04 April 2013
Originally Posted by hockey1237: Renderer
-1/16 sampling with Mitchell filter

No. Use unified sampling instead of adaptive. You'll have to tweak your scene (materials and lights) for it to work nice and fast. Explore it a bit, but generally glossy samples on everything need to go down. Way down. For materials 1 or 2, for lights 2 or 4. Quality is controled by unified sampling options.

Quote: -FG deafualt Draft settings

Try higher settings, not any of the presets. Like 0.2 for inital point density, 150 rays per FG point, play with interpolate over num. FG points. Set noise filtering back to Standard.

Quote: -GI enabled with multiplier 1 / Max # photos per sample 500

More Photons per sample. Set maximum sampling radius manually. If you leave it to MR to decide it will be 10% of the scene size, which in case of a long tubular interior may not be the best option.
  04 April 2013
Thanks for the tips. I'll give them a try and post results once I've had the chance.
  04 April 2013
Well, apart from what has already been said, it could be that your spatial contrast is set too high so it will use mostly 1-4 samples instead of the full range of 1-16 for everything. (Also, I'm not 100% sure but I don't think you're supposed to use Mitchell filtering with less than 4 samples).

Personally, I find I get good results with a contrast of 0.027 for all fields, 1/4-16 [or 64 if you REALLY need it. I use a max limit of 1 or 4 for test rendering, though], box filtering, jittering enabled.
You may be able to slightly raise the contrast to something like 0.031, though, with little perceived difference.

  05 May 2013
So I'm still working on cleaning up the artifacts, but in the meantime, I've noticed another issue. The area of the windows around the closed window shades are still illuminated as if there was no window shade. These should be darker, and the seats next to them shouldn't be receiveing as much light.

I've checked the normals of the shades and walls, but I have Force 2-sided on anyway. The light coming in is from an mr sky portal with shadows enabled. Any ideas why these areas are still so bright?
  05 May 2013
do you use an hdri image in the environment slot ?

if yes set the blur amount to 0.1 .

or/and you can use the environment blur as well
  05 May 2013
Originally Posted by hockey1237: I have Force 2-sided on anyway.

Check your geometry and if everything is OK, uncheck this if you want faster render.

Quote: The light coming in is from an mr sky portal with shadows enabled. Any ideas why these areas are still so bright?

- sky portal placement (placed inside or it collides with geometry)
- geometry set not to cast shadows

Less possible:
- low FG settings
- interpolate Over Num. FG Points to big
- low GI settings
- maximum sampling radius to big

Are you using Importons? If not, use them. That way you can crank up the GI photons to insane values and at the same time get very small photon map.

Last edited by blank000 : 05 May 2013 at 01:40 PM.
  05 May 2013
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