Physical Renderer, grain from reflections only

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  01 January 2013
Physical Renderer, grain from reflections only

Heya! I got brought onto a project in C4D that's using the physical renderer, with DOF and motion blur. I have a max/vray background lately and have little experience with the physical renderer.

I'm finding that I have to increase my Sampling Subdivisions have to be very very high in order to get less grain in my renders... This obviously increases the render time significantly. What I've noticed though, is that the reflections contribution to the final image is the source of the grain. At lower, reasonable sampling rates, if I look at the individual passes, the diffuse, the GI, etc are all silky smooth in the motion blurred and DOF'd areas, and its only the reflection that's insanely grainy. This is with R13, I've tried disabling Blurrines of the reflections to see if that was contributing to messing with the sampling of the reflections, but same effect.

Does anyone have any insight or thoughts on this, and how I might be able to smooth out my reflection grain without just cranking the Sampling subdivs in the render settings? I wish there was more control with this renderer, I've been spoiled by VRay's sampler.

  01 January 2013
If you have After Effects and are prepared to process your footage after render then using Neat Video which is a stella noise reduction plugin is very effective. In fact you can reduce the sampling overall and even render slightly larger in the knowledge that you will lose a bit of sharpness by reducing noise but you can bring it back a bit by scaling your picture down.

It totally depends on how little grain you deem to be acceptable and ultimately for the delivery. If it's for TV chances are the compression etc for broadcast may soften this anyway. Even Vimeo or Youtube will affect the quality of the final output.

Of course having to do this when you have experienced noise free loveliness in Vray is a pain. I have the same feeling and love the renderer in Modo, because it just works, although it too can get bogged down with reflective surfaces
  01 January 2013
Do you get banding as well, or only noise? Are you rendering at 8-bit or higher?
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  01 January 2013
you can adjust the blurry subdivisions...this is a multiplier for blurry reflections / transparency.
The views expressed in this post are by no means the opinion of those making the post or of any one person in particular.
  01 January 2013
or perhaps post some images of render settings / results...this should be a given...or we can just keep shooting in the dark.
The views expressed in this post are by no means the opinion of those making the post or of any one person in particular.
  01 January 2013
Thanks for the replies:

This is 32bit, no banding that I can notice whatsoever.

Chi, Adjusting the blurry subdivisions seems to have no effect on the object/camera motion blur where this "problem" is manifesting itself. The blurry subdvisions seem to have more of an effect on a shader level across a surface of glossy surfaces, and that is not the issue here really.

Settings are as follows:
Physical Renderer
DOF / Mblur = ON
Motion Subdivs 2 ( did a lot of tests, variations in this number yielded no effect )
Sampler: Adaptive
Sampling Quality: Custom
Sampling SubDs: 6
Shading SubD Min: 2
Shading SubD Max: 8
Shading Error Threshold: 0.1%

HDR Threshold 5
Blurriness SubD: 6
Shadow SubD: 2
AO SubD: 4

Indirect Illum: ON
DiffuseDepth 2
Sampling Subdivsions 2

Sample Image attached:
This scene is lit with and HDR on Sky object.

The top row is with the settings above, except the Sampling Subdivisions is lowered to 2. If you see the mat color / pass, its just as silky smooth as the one below that was rendered with 6 sampling subdivs, the difference is the amount of grain that the reflection pass is introducing. Granted, there is only a sliver of the HDR being reflected onto that grainy edge in a non motion blurred render,

I think what's happening is the ares of high contrast in the reflections that are small glints, or little hits are being significantly blurred as a result of their animation, but not sampled enough and there isn't any sort of global/post image sampler like in VRay ( maybe im wrong here ) to force it to smooth out, or go back and resample just those areas that are still grainy (like DMC threshold) to eliminate the noise that the reflections are contributing.

The quality of the grain is directly proportional to the amount of Sampling Subdivisions I employ, at the cost of exponentially increasing and production prohibitive render times.

I'm just wondering if I'm missing something as to why the other passes are so smooth and the reflection is not despite the varying contrast levels. I'm assuming I'm just SOL.
Attached Images
File Type: jpg cg_talk-help.jpg (79.1 KB, 95 views)
  01 January 2013
well if your cranking up the overall sampling subdivisions your other areas like GI can probably be lower first of all.

Second without seeing an image it's hard to know what kinds of reflections and such are the ones being grainy. It wasn't till half way through your description that you mention blurry reflections which was my initial assumption. But I still can't tell is it a reflection in the focused area that isn't moving, is is a reflection that is being both motion blurred and depth blurred? If so that's three factors that are all grainier that mean for that specific part of the image you will need a lot of samples.
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Kai Pedersen
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