VRay 3.30 sampling tutorial ?

[joconnell]

Thank you again John for you reply !

I investigated my scene again and it appears that, when I enable the Detail Enhancement, no matter how high I set its subdivs or how high I set the Min shading rate, it is just impossible to get rid of the noise in the GI where the DE is involved. I just don't understand that point. And yes, I checked that "use local subdivs" is ticked off. Anyway, I had to find a solution to get a clean GI so I decided to disable the DE, I increased the IM subdivs to 300, and lowered the interp. samples down to 20 and the result is very satisfying: sharp GI without any splotchs. Of course I have to pay it with longer calculation times but I think I found a good trade-off between speed and quality, and it is still faster than BF calculations so I'm now happy with my GI

Good that you found something - irmap is still very useful! I'll be doing a canyon flythrough for some film shots so I'll probably be using irmap for it! DE is something I've never used personally, it seems like a useful option for stills to get localised sharp details but it seems like something that'd be problematic for animation, which is my main area. As you said you'd tried to up DE quality with both shade rate and subdivs (I presume use local subdivs was on for that test, otherwise it's ignored)

But I noticed something weird on my tests: when I increase the Min shading rate value, reflections become smoother for some materials (that's logical), but surprisingly, they become worse for other materials ... For instance: the reflections on the ceiling get regularly better and better as Min shading rate increase from 6 to 384 but the reflections on the floor only start to become better when Min shading rate reaches 192, and not before (please look at the screenshots attached). That does not seem logical to me.

This could be kind of silly but the only thing I could think of on that (and why I wanted to see the raw light) is that the ceiling is getting to reflect the floor which is cleanly hit by direct light and the floor's reflections are of the sky outside - it's quite a direct path to a clean source. The Floor on the other hand has to reflect the ceiling and none of the rays hitting that, either light or reflection are from quite as direct a path. If a reflective surface reflects a cleanly sampled surface, it'll more likely be clean - if it has to reflect a noisily sampled surface, it'll probably be noisy. The other thing is what bounce of the reflection it's calculating from also - again the ceiling might be getting it's sampling from the first reflection bounce, the floor might be tying to sample reflections of reflections

Another question I have about sampling quality is why don't you care about Adaptive amount and Noise threshold ?
In my tests, I found that lowering the Adaptive amount to something like 0.75 and Noise threshold to something like 0,003 gives less noise than the default values of 0.85 and 0.005 (obviously given a fixed Min shading rate value).

The noise threshold is still very relevant with the shade rate workflow, I'll have to do some tests with adaptive amount though. Adaptive 0.85 with the old dmc used to mean 15% of your rays were always taken and for the remaining 85%, keep testing after each ray to see if you'd met the quality asked for with your noise thresh. Lets do a quick example with wrong numbers but right ideas.

Lets say I do something stupid like use aa 1/4 and use light samples of 400 on something. We'll pretend adaptive amount is 0.5 So we get our usual division thing happening where the 400 samples get divided by our max 4 aa so 100 samples per round of aa (again not accurate numbers, lets just go with the idea). Say for example our lighting takes a total of 301 samples to clean up perfectly. So we go through our render - aa 1 gets us 100 samples on our light which isn't good enough, we go up to aa 2 which gets us another 100 samples but again 200 samples isn't good enough, then up to aa 3 and 300 samples - again just short so up to aa 4. If we need to get to 301 samples and we're getting another 100 when we hit our aa 4 cycle, here's where the adaptive amount becomes relevant. For our example we're at 0.5 adaptive for this. This means we're going to always use 50% of our rays regardless if they're needed or not and then 50% are going to be used adaptively. The problem with larger adaptive amounts (and I'd imagine this is where Vlado is hesitant to use big numbers for MSR) is that we only need 1 more sample to get our clean result but because of a high adaptive amount we're taking another 49 samples we don't need as well.

To me it's probably a tough one to get right. Again if you've a scene that needs 200 shading rays to clean up, having an 0.85 adaptive amount means that for 85% of the rays, you're taking a sample and then doing a test to see if the result has reached the noise threshold you want. This test has got to have some kind of overhead which will slow your render time so are you better off having less adaptivity so you can get up to your 200 samples quicker or are you better off having more adaptive testing and being able to stop sampling earlier? Similar to the example before, you using the minimum shading rate of 384 meant that for whatever your last aa ray was, you may have taken 384 rays when you only needed another 10 - that's the trouble with optimizing

One last thing I don't understand with VRay 3.30.05 is that I rendered images with various subdivs values for my VRaySun and Photometric Target light, but it does not have any effect. Only the min Shading rate affects shadows noise so why the subdivs are still accessible for these 2 particular lights ?

The guys might have forgotten to disable those when "use local subdivs" is turned off - it's that way for me in vray 3.304 anyway.

Thank you a lot for sharing your knowledge and experience with us, and for your patience to read me, it is very much appreciated

Always glad to - any time I explain something I have to think about it myself and figure out clear ways to explain it, hopefully I get better at putting ideas across each time

[AltoR]

Thank you so much for the relevant explanations John: I can now go on a clear basis with last VRay