QMC for first bounce

[Lupaz]

I´m working on an exterior scene with many little details as complex windows frames. With irradiance as first bounce I had to take settings to very high in order to get rid of splotches on thin joints.
Someone here (an amateur btw ) asked me what if I use qmc for first bounce. I not only get a smoother result (a little noisy, but...) but I get it faster than with irradiance!

How come no one use it? Would it be slower when materials are applied? Is it better to stuck with irradiance for some reason?

Thanks

[Morbid Angel]

I use QMC for first bounce all the time. Infact this is known that irradience will sacrifice detail level for speed. In advance animations, irmap settings have to be so high that they are as slow as qmc if not slower when rendering. The reason people dont use qmc is because (my guess) they render interrior/exterrior stills, and for that you can get away with much. In my case I render animations with moving objects all the time, so QMC has served me very well. I would suggest going with qmc and 15 samples for primary and light cache for secondary. Turn op global subdivs in qmc sampler to 3-4 and your image should be pretty good. All tho render time will be much slower.

[RErender]

I also use QMC + LC for animated objects, specifically cars for exterior animations. I don't change the global subdivs but instead bump down the noise thresh. to .003 - .004 with subs of around 24.

[natty]

QMC + LC is a good option as the qmc adds an extra quality to your render that IRR Maps cannot do.

[Lupaz]

rerender: I agree with those settings. But I use .002, and QMC for second bounce too (never tryed LC for an exterior though). Turning up global subdivs to 3 or 4 makes render times way too long.

I got to say, this qmc for first bounce is all of a discovery for me .

[flipside]

Did you try it on 3000px or more renders too? I imagine it will get very slow then, while IR map can use lower min/max settings and at least the ir map pass will not increase in time with such resolutions.

[Morbid Angel]

flipside...i was rendering 7000x4550 image of a car using qmc and lc. It took 14 dual xeons combined time of 55 hours to render that image :P
so about 8 hours per strip.

[RErender]

I am working primarily at 480p so rendertimes are reasonable :P

[voltron7]

follow-up

Hi,
I'm new to the forum, so please excuse me if I'm posting a bad question.
I've been using mostly IR map/LC for architectural stills, and have recently been trying to use QMC for first bounce. Seems very good at small render sizes (640,480)but most of my stills must render at ~3000 pixels for large format prints. So, to the point...does anyone have any setting suggestions for using QMC/LC to render at this resolution? I've tried some of the tips in this post, but the render takes waaay too long and still has much grain. Am I just better off using IR/LC?

Thanks in advance.

J

[Morbid Angel]

voltron, all i can suggest is to try adjusting qmc sampler. This has great deal effect on qmc gi, for example havinf 20 samples is more then enough when qmc sampler noise is set to 0.003 global subdivs 4.
But, I must say that I heard you can get away with rendering irmap at larger scenes without increasing much quality to it....I cant say since I dont care for render times that much.

[voltron7]

Thanks for the suggstion Morbid,

I'll try those settings right now. Should the Vray quasi monte carlo GI subdivs be left at the default of 8?

Thanks again,
(now testing at 3200pix)

[Morbid Angel]

no, they can vary between 15-30 samples. But, by making qmc sampler noise less then 0.005 and by increasing global subdivisions you can allow yourself to drop the number of qmc gi samples, try 15 for a start.
Also, make sure your anti aliasing is qmc and not adaptive, since adaptive doesnt work with qmc sampler.

[Nickh]

Morbid,

what sort of settings do you tweak for your light cache settings? would it be calculated every frame or can you save a solution out to save time - or is this not possible whne objects are moving?

Cheers
Nick

[Morbid Angel]

Nick, unfortinantly lc has to be calculated perframe and cannot be stored, in same way as if you would do for fly through animations. So here are some settings you can find useful:
For LC you can set subdivs to 800-1500 (depending on your scene) for example if you have sss surfaces you will need more samples. Set samples to screen size 0.01 or 0.005, the less the samples the more presice lc is. Also screen works well because it calculated gi based on camera view. Check prefilter and set it to 30, filter fixed with 0.04. Also check use lc samples for glossy rays. This can improve speed of glossy surfaces.
My suggestion to any one who is trying to figure out lc is to take a simple scene and just test the heck out of it, with different settings until you find good result. This is how I came up with these settings, which work for me every time.

P.S. I also have to mention that sample size for screen is very dependant on the resolution you are trying to render. Since I developed this technique for tv res 720x486, Im most certain that one would need more samples for bigger frames as 1500 subdivs would spread out much thinner accross 2k renders as they would with 720x486.

[Nickh]

Thanks, morbid (or maybe I should call you dmitry...sounds a bit nicer)

I`m fairly new to vray and am still getting the feel for all the different GI settings, so this is all useful info.