Dome light subdivs - how does this work?

[DPS]

How are samples distributed with a dome light?

I.e would a dome with 1 subdiv be the same as four spotlights placed in a circle each casting a single sample?

[Dariusz Makowski (Dadal)]

Dome is using MIS - Multiple Importance Sampling. Or maybe even more advance approach.

It basically spawn a grid of samples over your entire texture. Then the samples that find more light details get subdivided and spawn more samples in those areas. This way IBL finds ur light in HDR and samples them more than lets say ground or sky. Thanks to that u should get quicker and better renders.

[DPS]

Does that mean the behaviour with 1 subdiv would be 4 evenly distributed samples? and then from there it would subdivide as necessary?

And if the light is casting from the dome (?), does that mean it's sampling the scene?

I had two really solid mental ray books that explained this kind of stuff - I've yet to find one for Vray. A lot of the concepts are the same/similar at least.

[Dariusz Makowski (Dadal)]

not really... It depends on ur DMC amount
Its a bit complicated but I'll try to simplify it.

Not entirely sure if this is how it works in case of IBL tho but it might work this way... it would be better if some1 from chaos can confirm.
adaptive amount = 0.75
Dome subdivision = 32
So what happens is vray does this 32*32*0.25 = 256 samples initially get cast from IBL. So I believe that 256 samples is also sampling ur IBL for light importance and location of hot spots. Then as the threshold dictates noise and so on more samples are being cast. I'm just not sure if IBL texture is being samples this way as well or it does it differently...

with 1 subdiv vray cant produce any sort of usable data... u need minimum 8, that why minimum adaptive amount in dmc settings is set to 8. When u set subdiv to 1 in ur light, he is being overrided to 8 because of that value.

also 1*1 =1 not sure where u got 4 from...

From my experience MR and Vray are very different. Including concepts of working. Maybe on basic level they do the same thing but to get to the end from start road is very different. Also working with vray is a bit of art when as MR welll its like working with bricks

[AndresAhumada]

Thanks Dadal.

Very practical and clearly explained the logic of running a Dome Light.
Every day you learn something new.

Cheers

[joconnell]

How are samples distributed with a dome light?

I.e would a dome with 1 subdiv be the same as four spotlights placed in a circle each casting a single sample?

Not quite. For light samples, what vray does is take the normal direction of the polygon that it's trying to calculate lighting for and it fires rays in a half sphere around that direction - kind of like this blue polygon and all the rays coming from it:



The direction it chooses within that hemisphere is semi random, as if it followed in a pattern you'd start to see weird patterns appearing in your shading.

[Dariusz Makowski (Dadal)]

Not quite. For light samples, what vray does is take the normal direction of the polygon that it's trying to calculate lighting for and it fires rays in a half sphere around that direction - kind of like this blue polygon and all the rays coming from it:



The direction it chooses within that hemisphere is semi random, as if it followed in a pattern you'd start to see weird patterns appearing in your shading.

That means he fires a LOT of rays out of my 40 mil polygon objects... does he then takes the rays that hit light and subdivide it further to get better sampling or what?

[DPS]

not really... It depends on ur DMC amount
Its a bit complicated but I'll try to simplify it.

Not entirely sure if this is how it works in case of IBL tho but it might work this way... it would be better if some1 from chaos can confirm.
adaptive amount = 0.75
Dome subdivision = 32
So what happens is vray does this 32*32*0.25 = 256 samples initially get cast from IBL. So I believe that 256 samples is also sampling ur IBL for light importance and location of hot spots. Then as the threshold dictates noise and so on more samples are being cast. I'm just not sure if IBL texture is being samples this way as well or it does it differently...

with 1 subdiv vray cant produce any sort of usable data... u need minimum 8, that why minimum adaptive amount in dmc settings is set to 8. When u set subdiv to 1 in ur light, he is being overrided to 8 because of that value.

also 1*1 =1 not sure where u got 4 from...

From my experience MR and Vray are very different. Including concepts of working. Maybe on basic level they do the same thing but to get to the end from start road is very different. Also working with vray is a bit of art when as MR welll its like working with bricks

Lets forget the influence that the DMC sampler has for now and say it's not adaptive. I just want to understand the basic concepts. And doesn't 1 subdiv = 4 samples?

That means he fires a LOT of rays out of my 40 mil polygon objects... does he then takes the rays that hit light and subdivide it further to get better sampling or what?

Or would it only fire 1 subdiv (4 samples) hemispherically from the interstection of each eye ray from the camera perspective? or would it be from every scene polygon - so lighting calcs are still carried out for objects outside of the frame (not talking about GI)
EDIT - note, I was meant to quote both Joconnell and DADALs response.

[Deflaminis]

This is the best write up I've come across yet in terms of whats happening under the hood, though not specifically about dome lights. http://www.cggallery.com/tutorials/vray_optimization/

[DPS]

Hey

Cheers for the reply - and thats a good write up but taking that ray tracing diagram further - am I right in my understanding that the light rays are cast from the light source(s)? Evaluating for shadows as they trace back through the scene?

Say each light has 8 subdivs - would that mean 8 subdivs are used for every primary ray in the scene? Or is each primary ray intersection evaluated to establish whther it's in shadow and then 8 subdivs cast for all primary rays that are not in shadow?

And domelights? Anyone from chaos able to add?


Edit - the above is still ignoring the DMC adaptiveness. I should have probably written max 8 subdivs.