Vlado:
I’m all for more statistics and predictability in the dmc sampler as you can guess by my stupid questions over the past while 
- The sample rate element is a really handy diagnostic tool to start with and a similar tool for reflection, refraction and lighting would be amazing too - I think you mentioned that it’d have a memory overhead and I’d imagine also a cpu overhead for the extra sums but during the tweaking stage I think I’d find it really useful
Michael - beware, long post that has to be split in two for forum letter counts
With your sample rate element, it only deals with your anti aliasing sampling and nothing else. In your first image of 1/50, you’re getting no red values so it means that no part of your image requires your upper 50 value of aa sampling, nothing anywhere near that in fact so it means your aa is more than the scene needs. It looks like it’s a geometrically and bump texture simple scene so it doesn’t need massive aa. With the second image, your aa has gone down a lot so now you’re getting way more red pixels as far more parts of the image are using your lowered ceiling of 16 aa samples. The lowered colour threshold is making vray use a fair finer level of quality before it can finish rendering too - I’d be interested to see your render times at 1/16 if you’d left the colour threshold at 1/16 and 0.007.
If I was to take a guess, I’d say your scene is being heavily lit from outside, so all of the complicated sampling in your image will be down to a really wide range of brightness, your glossy reflections on the floor covering such a large area, and then the very soft shadows coming in from your window light. If you have a look at the red pixels, they’re on the smooth curving lines of the chairs, and on the edges of the window frame. I’d say the wall lines at the window and around the skirting board aren’t getting hit with very bright light, so they get medium green sampling - geometrically they’re the exact same as the lines making up the window panes, the only difference is the pixels along the edges of the window panes are being blasted with whatever bright or complex light source is outside the window, so vray is putting additional AA samples there to deal with the difference in brightness values between the inside normal exposure and then the extremely bright rim pixels. So you probably don’t need to use much aa sampling, but you need to up your light samples for your main window light source, and likely the reflection samples on the floor material. Could you pop up a Raw GI, reflection and raw lighting pass along with the beauty to have another look? I’d say what’s happening with the bottom render is that your reflections are complicated but your floor material reflection samples aren’t that high, thus vray is having to use aa sampling closer to it’s max value to clean it up - hence the red pixels. If you open up your 1/16 scene, see what happens to the sample rate element and render times if you turn up the floor reflection samples by a factor of 4 and render, then set it back to what it was before, then turn up your external light source samples by 4 and render again. I think your scene is more complex in terms of material and light sampling rather than AA so it might make a difference.
In terms of the DMC tutorial there’s a lot in it! I’ve asked a lot of questions to the guys in chaos to try and clear up my understanding of DMC and Vlado and Svetlozar have been brilliant. I’ve to go back to Toni’s tutorial again and re read it knowing what I know now. If you want what I’ve found as relates to it, it’s as follows:
Vray always tries to balance your various samplers to get the cleanest result with the smallest amount of sampling.
Vray’s AA sampler makes the biggest final difference to how it decides sampling.
The sample settings of all of your materials get divided by your top AA setting, so the higher you go with your max AA, the more your material and light settings get divided.
If your material or light sampling is too low, it’ll force your AA to go to higher levels to try and clean up the noisy results that the light or material has returned to it (the cause of your red pixels in the 1/16 render).
Just because you use insane sample values in your materials or lights, it doesn’t mean that vray will actually use those numbers, it just gives it the option of using them.
At the minute with sampling, I find the AA is a good place to start as it provides the most measurable response via the sample rate element - you get a visual reference of exactly what it’s doing, and since the AA sampler is influencing everything else it’s kind of a good “pivot”. Now, the sample rate element WILL change as you tweak other parameters. For example in your 1/16 image above, if you were to either turn up your light samples, or turn up your material reflection samples on the floor, all of the red area on the sample rate element will start to drop off. Since your material is now doing more work, it’ll return a far cleaner result to the aa sampler, and it’ll have to use less of your max aa samples to reach the right level of cleanness for your final image, and this is the vray internal balancing act.
As one method you could try doing this to get things semi predictable:
- Turn on the default scene lights and hide all the others, turn your GI off. Stick on a grey override material so that you get a crap looking render, but the important thing is that you only want to see how vray is sampling your edges.
- Tweak your aa until you get really clean edges and your sample rate is mainly blue and then red at the edges where you need all of the quality.
- Next keep your middle grey material but turn on your proper lighting. With this you’ve got new levels of noise to deal with from the raytracing of your light sources and if you’re using brute force, from the light sampling here too. As a result your sample rate element will change again, as your previously clean, boring surfaces in your grey render now have shadow and highlight areas. The shadows if they’re particularly soft, will need more sampling and cause grain with the default light subdivs, so you’ll find that your sample rate element will now get more red, as your light is causing grain that forces your aa sampler to do more work. What you can do is leave your AA where it is, and turn on your Raw GI and Raw lighting elements. If you’ve got grain in your raw lighting pass, that means you don’t have enough samples in your light source, so gradually turn up the samples until you get something that’s clean enough. Don’t be afraid to use big changes in the value. If you start with 8, try going up to 25, 50 or 100. If I’m using a hdri in a domelight for example, I’ve ended up using values of 400 on the current film I’m working on. So ideally what you can do is maintain the AA settings you’ve had before, and alter your light samples until this is now clean also.
- Last turn on your proper materials. Turning these on means you’re now bringing in noise from glossy materials so again if they’re using low samples, they’ll likely cause some grain which again will cause the aa sampler to again do more work so your sample rate will again go red in the noisy areas. Again this means you’re going to have to up your material samples in the materials so turn on your reflection, raw reflection, specular, refraction and raw refraction elements. The non raw versions are the most important as they’re the level of reflections / refractions which finally end up added into your pixels. The reflection filter element can be useful here too as it’ll tell you exactly how much your reflection is conributing to the final materials - pure white pixels mean 100% reflective, dark grey mean nearly no reflections. If it’s noisy in the raw reflection for a material, but the reflection filter has almost black pixels then it’s quite likely that vray has decided it isn’t important enough to the final result to go up to any higher quality with. In terms of cleaning up reflections I’ve found that glossy reflections themselves are partially to do with their own material sample settings, and second of all the samples of the lighting solution that they’re reflecting. If you take a glossy surface and light it with a hdri, you can clean up noise by either turning up the material samples, or turning up the samples of the light that hits the surface. If you’re unsure which, take a look at your raw lighting channel. If the raw lighting of the surface is pretty clean but your reflections are noisy, up your material samples. If both your lighting and reflection channels are noisy, then up your light.
This is my current method of thinking and understanding vray, if anyone has enough time to read the above nonsense and I’ve got gaps or misunderstanding then I’d love to get any feedback.
