Puzzled why SampleRate Element turns red in dark, low-contrast areas

[Art48]

Hi,

We recently completed a large project involving a rendering of a big machine in front of some abstract/architectural/block wall background.
Since we rendered at over 10k by 10k pixels we sent it off to rebus to render. I am still getting used to the new sampling appraoch/settings so I cannot rule out a user error on the following issue:
As I understand it high-contrast areas generally need more sampling than low-contrast areas do. The render we made was rather dark and surprisingly the SampleRate turned up all the way to red (with 1/64 and 0.01 that is - rather crazy it comes up to red) in some low-contrast areas (especially the background wall which also has a very simple material) - which made the rendering quite expensive and I did not see that coming. What I'm asking myself now is why these areas were sampling so much while at the same time being very very low contrast and dark.

Due to NDA reasons I cannot post any images, but I am uploading a scene to the ChaosGroup FTP as we speak although it'll take a while (8-9 hrs)
I will write to support linking to this thread and I think it would be best if support guys answer here in case anyone else had a similar issue.

Windows 7 Ultimate x64 SP1
3ds Max Design 2014 SP5
V-Ray Adv 3.40.02

[vlado]

The gamma correction of the images boosts dark areas a lot - f.e. a color that is 0.0001 value in linear physical space and not visible normally, will get boosted up to 0.0152 with gamma 2.2 - or about 152 times (!) brighter than the linear color. Consequently, in these areas noise is amplified many times over and even small noise fluctuations of the colors lead to large differences in the final RGB output, needing many more samples to clean up properly.

Best regards,
Vlado

[Art48]

I see - good explanation. Thank you. Is there a possibility or plans to have some kind of adaptivity there, basically meaning a similarish behaviours as cameras have. I don't know how to explain. I am a very visual person and I don't really care if very dark areas of the image are a bit noisier than the rest. And while I typed this sentence I realized that you can do that with VRay by simply reducing max subdivs, right? Then "automatically" the darker areas get noisier since VRay can't clean it up the same as the bright areas due to too low max subdivs, right? So I am happy I guess - just need to find good values then which still give enough samples to the majority of the image.

[LudvikKoutny]

Hmm, I am confused now.

I thought that with some of the older V-Ray versions, there was a change where adaptive sampler was made to evaluate gamma corrected image rather than original linear one, so that it doesn't waste samples on areas that make no difference to human eye watching gamma corrected image. Of course only as long as I use gamma mode set to color mapping only, and sRGB correction in VFB.

I remember prior to that, I had excessive rendertimes, and once I switched to the correct workflow, everything ran a bit faster. But this explanation sounds like adaptive sampler still doesn't evaluate image in the same ranges human eye does.

[vlado]

But this explanation sounds like adaptive sampler still doesn't evaluate image in the same ranges human eye does.

Human eyes are way more sensitive to changes of intensity in the dark areas (this is what the whole sRGB/gamma correction thing is about, after all), so the sampler does the correct thing by default.

Best regards,
Vlado

[vlado]

And while I typed this sentence I realized that you can do that with VRay by simply reducing max subdivs, right? Then "automatically" the darker areas get noisier since VRay can't clean it up the same as the bright areas due to too low max subdivs, right? So I am happy I guess - just need to find good values then which still give enough samples to the majority of the image.

You could switch the color mapping to "Linear" and reduce the dark multiplier - this will make the sampler a little less sensitive to dark areas. However make sure you set the mode to "None (don't apply anything)" so it doesn't affect your render. In RenderMan they solved this issue by adding a parameter to offset the colors a little bit before doing the gamma correction for adaptation purposes- the logic is that there is no pure black color in the real world and there is usually some ambient light anyways. I've been meaning to try it out and see if it works better.

Best regards,
Vlado

[LudvikKoutny]

Human eyes are way more sensitive to changes of intensity in the dark areas (this is what the whole sRGB/gamma correction thing is about, after all), so the sampler does the correct thing by default.

Best regards,
Vlado

Ah, so it does what it should after all. Thanks!

[Art48]

You could switch the color mapping to "Linear" and reduce the dark multiplier - this will make the sampler a little less sensitive to dark areas. However make sure you set the mode to "None (don't apply anything)" so it doesn't affect your render. In RenderMan they solved this issue by adding a parameter to offset the colors a little bit before doing the gamma correction for adaptation purposes- the logic is that there is no pure black color in the real world and there is usually some ambient light anyways. I've been meaning to try it out and see if it works better.

Best regards,
Vlado

Interesting approach. When I get some time I'll try it out.

[^Lele^]

I've been meaning to try it out and see if it works better.

Yeah, me too.
Still a bit to go, i guess, for r21 NC.