RC1 PMC sampling bug

[manalokos]

Hello,

I've noticed that PMC works differently, and now has a strange behaviour.

During the first samples the image looks OK and each sample reduces the noise of the image, but when sampling arrives to aproximatelly 16 s/px suddenly the image becomes far more noisy. Why is this happening?

Best regards
Filipe

[FooZe]

Hi manalokos,

Can i please have a couple of details...
What version are you comparing to?
Can you please post examples with PMC kernel settings or even your scene if it's not too complex/large and you don't mind.

Cheers
Chris.

[roeland]

We changed the behavior of PMC to give the first samples a lower weight. Due to how this kernel works, the first few samples are very similar to regular path tracing. In scenes with a lot of caustics this will result in noise that takes a long time to disappear. To avoid this noise we changed the sample weights for the first 15 samples per pixel. The downside is that the low frequency noise present in PMC renders becomes more apparent while rendering the next few samples.

--
Roeland

[manalokos]

Hello Fooze,

I am not comparing to any other version of Octane, but I noticed a difference.
I attached a strip of images with different sampling numbers in PMC using the default settings.
The noise suddenly increases around the 60th s/px, I've compared the image at 30s/px with the 80 s/px and the later has more noise although is slightly brighter.

I am guessing, this is because only later in the process the PMC algorithm starts doing "exploration"?
Maybe not as the same thing happens with a scene with diffuse only materials.

I also attached a sample scene, just press render and see it happen.

Best regards
Filipe

[roeland]

An increase in brightness is normal, PMC is often too dark with a low amount of samples per pixel. After the rendering reaches around 15 samples per pixel you may see some low frequency noise popping up, this is due to how the exploration works. In your images I don't really see an increase in noise from the low s/px to high s/px images.

PMC has an advantage over path tracing mainly in scenes with complicated lighting, like scenes with lots of light sources or scenes with lots of caustics. In diffuse only scenes path tracing may work better.

--
Roeland

[MattRM]

Hi all,

Maybe the changes in PMC cause also the line bug when do volumetric too, no ?

Matt

[Elvissuperstar007]

http://render.otoy.com/forum/viewtopic.php?f=33&t=25831