lightcache vs. qmc - exposure difference?

[oluv_cg]

i am not sure if this is a bug or if i am doing something wrong. i am wondering why others don't seem to have this problem, but lately when i tried to use lightcache as secondary engine instead of quasi monte carlo, the result was completely different. the image using lightcache was washed out and overexposed.
to see if i was doing something wrong, i setup a very simple scene without any materials and let it render with qmc first and then switched over to lighcache. again the lightcache version was much brighter. all other settings remained unchanged.

i remember that in vray for max the lightcache was also a bit less contrasty than qmc, but the difference was never that extreme.

so what is going on here?

[Thomas An._cg]

lightcache vs. qmc - exposure difference?

The thing is that the QMC does not use enough bounces.

The light cache is most likely the correct soluton. (it is not bounce dependent)

[oluv_cg]

lightcache vs. qmc - exposure difference?

but i have never had such an extreme difference in max. i remember there were contrast differences but not to such an amount. is this really how it should be?

[vlado]

lightcache vs. qmc - exposure difference?

It also depends on your materials, by default materials in rhino are perfectly white, I think, while in 3dsmax the VRayMtl is middle-grey by default.

Best regards,
Vlado

[Micha_cg]

lightcache vs. qmc - exposure difference?

Oluv, I have seen it here too, that images per IMAP are darker than other. Could be nice, if you would try to find the reason for the difference. I'm curious on a test with grey materials.

[oluv_cg]

lightcache vs. qmc - exposure difference?

i wanted to make some further tests yesterday, but during trying my objects disappeared suddenly and rhino crashed with a rcm-dump. i am still getting unpredictable results with vray for rhino. my lights didn't work yesterday. i wanted to add some sunlight, but didn't get any shadows. no idea what was going on.

[oluv_cg]

lightcache vs. qmc - exposure difference?

so here some further tests qmc vs lc with some interesting findings.
i took the same scene as yesterday put in a sunlight and hdri enviroment and rendered one outside and inside view with irradiance map as primary engine and then qmc or lightcache as secondary. all other settings remained the same.
interesting fact is that the inside-views differ quite a lot, whereas the outside view is more or less the same, except for the inner parts inside the building which are lit brighter when using lightcache. you can also see less artefacts inside when using lightcache.
rendering time was very similar with both engines in this case, although i made the experience that normally using lightcache results in a quicker rendering.

inside:



outside:



the only problem i have is glass. i can't get really convincing reflections. i already changed fresnel ior to 2.5 to get some brighter reflections in glass, but the windows remain still much too dark.
maybe one could share cool settings for convincing and realistic glass!

here some fresnel ior tests:





and here the real view:


i remember that i always got perfect glass when rendering with lightscape. i haven't it installed anymore because of open-gl problems with my actual graphics-card, so i cannot try it out now. but i would love to see the difference when rendering glass.

[Micha_cg]

lightcache vs. qmc - exposure difference?

Interesting, how much bounce did you use for the QMC? Maybe, here is the problem. Or the material was to bright.
I have done a test with a grey material (150,150,150) and 30 QMC bounce. In both GI mode cases the same look. Also I have tested different GI modes, allways the same brightness. Like Vlado says, we should avoid white materials.

Reflections: I know you problem well. I have it here at my daily work too. But, anything is right in Vray.
In the real world, the heaven is very bright. If you would use the same intensity for the GI env and the reflection env, than you would simulate a natural environment. But in this case, in linear color mapping mode, the background would be totaly burned out. Best would be, if we could simulate a human eye or a analog camera. In this case, anything would be visible like at your photo. A exponential color mapping should do it. Best you test it.
And you could change the fresnel ior. But in this case, the glass will look like a metal, the fresnel effect will be less visible.
I would wish, I could use a higher reflection intensity as 1. I don't know why it is limited now. I could bring in some cases bad effects for GI reflection caustics, but most I have disabled it for arch viz. So, I see no problem to give us an unlimited reflection intensity. In you examples it would be the easy render solution.

[oluv_cg]

lightcache vs. qmc - exposure difference?

i guess you are right, it depends on the bounces. i tried it with 30 bounces now in QMC, and the brightness was similar to lightcache, but the passes take ages to calculate. i think i will stick with lightcache

i must admit that you also seem to be right regarding the correlation of reflections and enviromental brightness. i tried to increase multiplier for enviroment background and had some nice results. increasing ior is not the best solution, because this way i am also losing lots of brightness inside, when light passes through the windows.

i think we would need 2 different brightness values for enviromental maps. one brightness setting should be for reflections only, the other brightness setting for true background display. that means that one can set the brightness of the backgroundmap (the one that is visible in the rendering) and of the same map to be used for reflections separately.

now when i increase enviroment map multiplier it looks right in the reflections, but is too bright in the background. now the only solution would be to make 2 different renderings and compose them in postprocessing afterwards.





(by the way, how can one turn off background or gi map and use color instead, once a map is set? i don't see any way how to turn it off again)

[Micha_cg]

lightcache vs. qmc - exposure difference?

An other intersting combination for quick test renders is to set both engines to QMC. Per higher noise setting and mouse track you get parts of the image rendered in very less time.
And you can fast render big scale image if you like the fine noise, for example at QMC/QMC + noise 0.003. Low LC settings would cause splotches.

I use 9 GI bounce for the QMC. You could test it, how much GI bounce are needed to get a nice lighting result. Maybe 5 is enough. In my previous test the rendertime was not so much bounce dependent.

(by the way, how can one turn off background or gi map and use color instead, once a map is set? i don't see any way how to turn it off again)

I can not open the texture dialog during a rendering at the moment and I don't remember the right name, but there is a "reset" button. This buttons is deleting the texture conection.

[oluv_cg]

lightcache vs. qmc - exposure difference?

i have to test your qmc and qmc setup.

but back to reflections and glass: i just realised that increasing enviroment multiplier is not the perfect solution. although glass starts to look better, all other reflective surfaces are much too bright as well. if i put a mirror-ball in the scene i see only white reflection instead of the sky which is logical somehow.



but still i have no idea how to make a good looking glass in vray... anyone?

[Micha_cg]

lightcache vs. qmc - exposure difference?

... it's time to leave the bug section.

http://www.asgvis.com/phpBB2/viewtopic.php?t=452

[oluv_cg]

lightcache vs. qmc - exposure difference?

but what about the glass? if it is not a bug that i can't get real-looking reflections in glass-windows, am i too stupid then?

i will install lightscape again today evening and try the same setup to see how my windows will look there.

i have the impression as if vray was handling fresnel reflections and ior not in the right way.

[Micha_cg]

lightcache vs. qmc - exposure difference?

I'm curious. My big wish is - a unlimited reflection intensity multiplier.

What do you think about my last mini tutorial? Looks the reflections wrong? Don't forget, the human eye works exponential and there is big contrast between heaven and erath brightness in the real world. If you take this in acount to your rendering, it should look better. I think, the problem is the linear mapping. It's like a standard CCD digital camera, the resolution of the brightness is not good. You can not get heaven and erath in the same image.

[oluv_cg]

lightcache vs. qmc - exposure difference?

your mini-tutorial is certainly cool! especially for all the newbies who are new to vray. yesterday i tried your qmc/qmc setup but i was getting either only noisy pictures, that were quick to render or still noisy pictures which took too long.
i will stick to imap and lightcache in most situations. for outside renders irradiance map and qmc works maybe even faster then lightcache, but inside you cannot go without lightcache. lightcache smooths out all the blotches and spots that you get with qmc. although they can still remain in critical situations even with lightcache, especially when tweaking levels/contrast a lot afterwards.

i know the problem with exposure. but to tell the truth i always try to simulate a photo, when doing renderings. because most of the time i compose the renders with a real photograph, so it would look weird if the rendering would show bright shadows whereas in the photo these parts were dark.
the problem with right exposure is not always easy to deal with. for example i still haven't understood what gamma is :P
but i try to rely on my eyes to get the right pictures. the only problem is that i am a bit color-blind too hahaha

later in the evening i will try the lightscape rendering for comparison so stay tuned!