Understanding DMC Sampler

[trixian]

Well I was curious as I must admit I have never really dived too deep into sample rate debugging. Does the red in the sample rate element always equate to longer render times, and inversely, do the blue pixels always render faster?

[joconnell]

The sample rate just lets you know what level of anti aliasing the pixels are using. If you're using adaptive dmc anti aliasing, the blue pixels in your sample rate mean that they're using the minimum value of your anti aliasing. If you've got red pixels it means that they're using the very maximum level you've set in your adaptive dmc sampler. The sample rate element is mainly a guide as to how hard the anti aliaser is working to try and clean up your scene and it's up to you to decide if you need to either higher or lower your anti aliasing level to get the best balance of quality and speed. The way I'd tend to think is that anti aliasing is mainly there to work on the quality of your edges and your bump detail, so I like to play with the max aa level until I get smooth results in the main beauty render. When I now look at my sample rate element, if there's a line of red pixels just along the edges of all my objects and the rest is blue, then this is good for me - it means your anti aliasing is only high in the places that you need it. If you've got no red pixels at all, it means that the anti aliasing you've set has a max value that isn't being used at all so you can turn it down without harming your image quality.

The red versus blue doesn't really mean longer or shorter render times, all it shows you is how your anti aliasing is being used through your render. From that though, you can start to work out how all your other samplers are being affected, as the max anti aliasing value will turn down the settings of all other samplers. For example if you've got a render of a product sitting on a glossy white plane lit by a hdri, and the sample rate element shows green or red pixels on the flat areas of the plane, it means that whatever result the reflections and lighting is giving to the anti aliaser, it's not clean enough for the quality you've asked for with your noise thresholds, so the anti aliaser is trying to do extra work to clean things up. In the middle of the flat ground plane there's zero need for high anti-aliasing since you've got no edges and you've got no bump maps going on. If you've got red in your sample rate in these areas then it means that the reflection samples aren't doing a good enough job (or possibly you hdri light samples) so the anti aliaser is going up to it's max rate in an effort to clean it up. If you don't touch your anti aliasing settings at all, but gradually bring up your material reflection settings, the red pixels in the sample rate will gradually start to drop back down towards blue again. It's all about trying to get each of the samplers to do their job well.

Fred ruff wrote a good post about this back in earlier vray days when the sample rate was only dark blue up to light blue, so if you take the same ideas but use blue to red as your high / low then it's still valid - http://www.ruffstuffcg.com/journal/d...-aliasing.html

[trixian]

Yes. Well that is why I thought a render-time heat map for buckets would be ok for this purpose. Hopefully one could then use the heat map as a guide to look at the other elements when looking for noise contributors and / or slowdowns.
I think I'm going to modify my "maxstart.max" file to include all these elements for analysis in a projects early stages. I always forget to add them later on in the project when everything is complicated and messy and render times are already a lot higher. Hopefully this will force me to constantly optimize and clean up as I go along, resulting in less problems total in the end. At least in theory.

[joconnell]

Try this as an approach when optimizing:

1. Put on a grey override and hide all scene lights, turn off GI. Turn on default lights in the global switches so you get nothing but a boring grey render in your viewport. Use your sample rate element and max adaptive dmc control until you get clean enough edges on your geometry and get a nice looking sample rate with red only where it's needed.
2. Turn off default lights, turn on your scene lights and gi. Leave your grey override on and add in the raw lighting and raw global illumination passes. You know that your AA is probably in a good place now so lets not touch that. The sample rate element might go red again due to your lights or GI adding in noise but don't touch your AA. Try instead adding in more subdivs to your light sources so in the controls of the lights themselves and in the subdivs of brute force GI if you're using that. Change those settings until your raw light and raw GI are nice and clean (if you use irmap and LC instead of touching brute force then the raw gi is less important) and now when you go back to your sample rate then you should be back to having red only along the edges and blue everywhere else.
3. Turn off your material override and turn on the reflection, raw reflection, refraction and raw refraction elements. Again since these will likely be noisy, your sample rate element will go more red due to the aa sampler trying to clean up noisy materials. Again leave your AA as is, and gradually increase your material reflection / refraction settings until the render elements look clean and your sample rate has returned down to the usual red edge, blue everywhere else type of look.

[fewlo]

Hi joconnell,

after some testing based on your input I managed to get the render time of my original 1/16 0.03 render down to 2hrs 17 from 4.5hrs, just by upping my materials to 128 subdivs.

I then lowered clr threshold to 0.05 and now have a render time of 1hr 35 and clean render so I'm very pleased! Also I have a much better understanding of how to use the elements effectively from your workflow, its really helped me out so many thanks.

I'm starting another test on a live project now and have already hit a stumbling block...

1. Put on a grey override and hide all scene lights, turn off GI. Turn on default lights in the global switches so you get nothing but a boring grey render in your viewport. Use your sample rate element and max adaptive dmc control until you get clean enough edges on your geometry and get a nice looking sample rate with red only where it's needed.

When I do this step I simply get a fully black RGB pass and fully blue samplerate element, I must be missing something very simple?!?

[bardo]

joconnell's advices should go into the vray online help.... very well explained and very helpful.

If joconnell writes some mistakes, we wait for some Chaos clarifications.

I suppose that his suggestions will be good for animations... not only still images.

Thanks joconnell...

[joconnell]

Hi joconnell,

When I do this step I simply get a fully black RGB pass and fully blue samplerate element, I must be missing something very simple?!?

Brilliant to hear that this is working well for you too - I'm going to try and record a quick run through video of a scene that people can follow along with and hopefully get some benefit.

On the first step let me check that, I wrote it off the top of my head without having max in front of me, what I'm trying to achieve is a grey material render with the default scene lights so you look at the geometry edges only.

[fewlo]

just done a really simple scene and step one does work, must be a grouped light in my xrefs....

edit: no grouped lights, so still can't get step one working on live project.

[joconnell]

Hmm - did a test here and if I used the display panel to hide all lights it reverted to using the default lights - see if that behaves instead.

[fewlo]

dunno what the issue was, I rebuilt the scene from scratch and its all working fine now

[fewlo]

joconnell, is your workflow image size dependant? eg, I'm currently testing the workflow on an animation 1024 x 576, if I get settings I'm happy with in step one, are they applicable to a 3200 x 2400 high res?

[joconnell]

Yeah the same process will definitely work fine on any resolution, but a higher res image will mean finer detail, more and perhaps slightly higher aa. Vray seems to be quite clever in using screen related sizes for things like irmap and light cache so they should look pretty similar regardless what res you go to.

[zeronove]

dunno what the issue was, I rebuilt the scene from scratch and its all working fine now

could you verify if DR is checked or not?
Here I've issue with default lights and DR; without DR it works fine

[fewlo]

Alessandro,

Many thanks for the tip, I can confirm that DR was causing the problem.

[fewlo]

so I've done a test on a live job;

step1. with the default lights on I was happy with dmc settings of 1/4, clr 0.003
step2. added in my lights and upped subdivs of lights to 24, still happy with dmc settings
step3. globally changed my material subdivs to 64 and added them into the mix, all passes lovely and clean, but LOTS OF RED appears!
main areas of trouble are a bumpy leather material and reflection of daylight in glass.

Not sure where to go next in the workflow so messed with dmc settings to 1/12 clr 0.005 and was happy with the outcome and render times.

Does all that sound about right as a workflow?