Raising samples on the light with the new sampling system? (noise in alpha matte)

[PIXELBOX_SRO]

This has been an issue for us for some time, we end up just removing the noise as best we can in post and adding a blur to the shadow. It usually works fine for stills but for animation its a pain. We end up rendering the ground plane separately with higher settings as the rest of the scene is fine as is. It would be good to hear the correct workflow for a nice clean alpha that also doesn't take ages to render. We find disabling GI on the ground plane allows it to render much faster, so its just shadow from the HDRI. But the noise is still there :/

+1 totally agree

[PIXELBOX_SRO]

so i raised the MSR to 50 using 4/100 for AA and 0.002 for noise and its going quite quick with bearable amount of noise and no need to turn off the GI amount in matte properties.

[Guest]

so i raised the MSR to 50 using 4/100 for AA and 0.002 for noise and its going quite quick with bearable amount of noise and no need to turn off the GI amount in matte properties.

Interesting, I will have to see how an entire scene renders with these settings.

[SnipeyX]

Jumping on here to follow this because I just ran into this issue as well, thought I understood how the new sampling works with color threshold min/max, but my current scene wasn't working as I expected in the alpha. Admittedly the MSR is a black box to me, but from comments here it sounds like that's what we need to work with for clean alphas? It seems as if the color threshold value doesn't come into play with the alpha channel?

Screenshots of my zoomed in VFB below, alpha channel and SampleRate elements. MSR 6, min 1 max 36 color threshold 0.008 - I was baffled seeing the noise I was trying to remove still stuck at my minimum SampleRate. Ground plane is a vrayMtl inside VrayMtlWrapper with Matte Surface enabled, alpha contribution -1, and shadows and affect alpha enabled. Guess I'll play with MSR to see if results vary.

[PIXELBOX_SRO]

yes it indeed looks like the default MSR value cant be left at 6 when you want to be rendering clean alphas and who knows what other layers.
I bet you would need to play with this even for some other vray render elements - thats my guess though i will test it further.

The question is if Vlado can do something about this even at default MSR value.
Maybe have some multiplier for matte properties???

[super gnu]

ill say it again, do any of you guys get the same issue with the adaptive sampler? cos i had exactly this issue on a recent job, and could just not get it clean without stupid 100x longer renders. switched from progressive to adaptive and the problem totally went away.

[vlado]

ill say it again, do any of you guys get the same issue with the adaptive sampler? cos i had exactly this issue on a recent job, and could just not get it clean without stupid 100x longer renders. switched from progressive to adaptive and the problem totally went away.

Hm, I guess a part of the problem is that the adaptive sampler also adapts to the alpha channel, whereas the progressive one only looks at the RGB and totally ignores the alpha.

Best regards,
Vlado

[PIXELBOX_SRO]

i never used progressive for this scene...alway adaptive and unless i set the settings to the above mentioned, it noisy as hell.
the problém is only very evident if your bacplate is very clean...then the alpha noise is very evident and appears as atrifacts.

[joconnell]

Hmm - weird that you didn't get good results when you had the msr at 80 earlier on, it must have been the noise threshold dropping to 0.002 that made the big difference - maybe matte shadow objects over a dark background aren't being "seen" by the renderer as seriously as non matte objects or since they're being multiplied down by their alpha they're easily meeting the noise threshold you've set? Per object subdivs / per object AA would be handy for this, especially if it's a noise threshold issue. Must add this to the list of things to look at.

[PIXELBOX_SRO]

John...happy to share stripped down scene with a teapot instead of the character
i must say i have always had problems with noisy alpha and only this time i finally got it working although the rendertimes are not magical

[joconnell]

If it's only a teapot and a hdri light source I'll have a look over here using something similar. Since it's a common enough occurrence for all 3d artists it'd be worth me spending a bit of time at!

[SnipeyX]

Well I found my cause, not sure if it's the same as others. In the interest of posting a sample I started deconstructing my scene and in the process determined a cause: VrayExtraTex Render Element w/ VrayDirt & the "consider for antialiasing" + "affect matte objects" options. If both of the checkboxes are enabled, then my alpha gets noisey and the shadows lose samples according to the SampleRate RE. I don't understand the implications of these options enough to know if that's 'as designed' or a bug. Max 2016 Scene attached. Just watch the SampleRate render element. In it's current setup, it renders as you'd expect, sampling the shadowed area more. As soon as you toggle on the consider for antialiasing, then you see the samples redistribute. Aside from adaptive sampler isntead of progressive, everything else should be at default as I reset the renderer during troubleshooting.



vray AO affects image sampling.zip

[vlado]

Thanks, I will make a note to look into this. In general though, I usually recommend doing the AO in a separate pass - it requires way too different sampling from the main RGB image and in most cases just slows down and confuses the renderer.

Best regards,
Vlado