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?
"...if you have faith as small as a mustard seed... Nothing will be impossible for you."
  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
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.
Quote: "Until you do what you believe in, how do you know whether you believe in it or not?" -Leo Tolstoy
Kai Pedersen
  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
What is the source of that reflection an HDRI, a luminant object, the reflection of a light source on another object?

My first suggestion is to reduce the HDR clamp and also perhaps clam the HDRI image itself a bit if that is the source of illumination. The next thing is if it is a luminant object, is it set to be an arealight?
Quote: "Until you do what you believe in, how do you know whether you believe in it or not?" -Leo Tolstoy
Kai Pedersen
  01 January 2013
The source is a spherical HDR I captured on set, it's got a huge range, so I'll try clamping it a bit. Thanks
  01 January 2013
You could have saved yourself all that typing by attaching a file

Is there a volumetric light in the scene ?
Try switching things off, an item at a time, to see what's what.

Even copy/paste the scene into a new file "might" work too.
Just shots in the dark without the file.

iMac 3.06 ghz Intel Core 2 Duo / 16 GB / OS 10.13.4 / C4D StudioR12 / CS6
  01 January 2013
Originally Posted by LucentDreams: My first suggestion is to reduce the HDR clamp and also perhaps clam the HDRI image itself a bit if that is the source of illumination

Ok so its definitely related to this. If I clamped the image in C4D with a Filter on the shader, all of the white grain goes away, of course those hot sexy contrasty reflections along the top, etc get muted to an ugly grey, but now we know its related to the HDR.

Vray samples HDRIs adaptively ( so the super hot areas of high contrast get sampled more than other areas of the image, which prevents this), and I can only assume that C4D's Physical renderer does not, and which is why this is happening.

Bummer! I'll have to try tonemapping the HDR to bring down some of the super hot areas but still maintain the contrast so I can keep those hot reflections... hopefully that'll help.

Sure can't wait until the next C4D Vray update that fixes so many of the problems that prevent me from using it, on the occasions that I have to use C4D.

Anyway, thanks for pointing me in the right direction!
  01 January 2013
C4D does too but not as effectively as some apps, and with R14 and radiosity maps you have even more control.

You may not have to actually clamp the HDR image, the HDR threshold setting may be the way to go.

I also personally always use two HDR's no, one that is the original and a second that has a small limiting of the brights and a blur. Clean map is for reflections, and blurred map is for GI. Guarantees cleaner faster renders. with little change to the overall look. too much blur may kill shadows so the amount of blur is the one thing to be careful of.
Quote: "Until you do what you believe in, how do you know whether you believe in it or not?" -Leo Tolstoy
Kai Pedersen
  01 January 2013
looking at your settings you are only using 2 samples for your GI!
and your illumination is coming from that...if you are using an HDR that is not blurred there will be too much contrast...but even if you blur it you will still need more than just 2 samples for your GI.

Also, CINEMA's renderer also samples adaptively, but you have to provide it with enough samples to start.

a scene would be the best help though.
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 replying - the issue is in the reflection, whether or not GI is on. The HDR I'm using ( for reflection only ) is too contrasty and the ramp from bright to way overbright is to steep from one pixel to the next for C4D to handle with reasonable settings. I'd love to provide a scene file, but the nature of the project prohibits that. Had to do some creative blurring in post to use the existing renders in time to get it out the door in time.
  01 January 2013
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