What is the purpose of sky portals?

[thomthom]

I don't understand the use of sky portals. In what scenarios do you use them and what do they do?

[dalomar_cg]

Re: What is the purpose of sky portals?

What do they do?
Skylight portals take their color and intensity from behind the light.

Why use them?
To understand their use, you have to understand the difference between how direct light and indirect light (GI) is calculated. When V-Ray samples a given point in the scene it will go through each light source in the scene and cast a ray to see if that light is visible from the sampled point. This is a relatively quick thing to do (since it will require a set number of rays (subDs*2)) compared to GI sampling, which, if you're using brute force, can be much larger number of rays. The other thing that this does is "promote" any illumination on the other side of the light (which presumably is mostly from the environment) to a direct light calculation, thus allowing light that would have gone straight to GI calculations an extra "first bounce", effectively shifting the primary bounce to calculating the second bounce. This helps the accuracy in the sense that they "put off" being sampled by largely inaccurate secondary bounce methods. The downside is that the light itself does have to sample what's behind it, which takes a little time.

In what scenarios do you use them?
I'm not really the one to ask on this one as I personally never use them. In my mind the benefits are relatively negligible and they over complicate what is originally a uncomplicated situation. So for that reason, I stay away. Now I did hear that Vlado did a post where he put them in front of a light emitting object as to make the emissive material be handled by the direct light calculation. This seams like a viable use, but I just haven't put it into practice yet (and if I have my way we can have a) direct light calculations for emissive materials and/or b) the ability to define a mesh as a light source).

[thomthom]

Re: What is the purpose of sky portals?

Yea, I read that thread too - about emmissive materials.

But I've seen people mention them in regards to interior scenes where they've been placed by the windows. I wasn't sure why, so I tried it one a scene I was playing with:


Original Scene



Skyportals by each window - visible



Skyportals by each window - invisible


What I saw from that was I got some more defined interior shadow. More light coming in. But also very flat looking right wall. And increased render time. Sky portals didn't leave a result I desired in this case.

[dalomar_cg]

Re: What is the purpose of sky portals?

I personally haven't found many situations where they've really been useful, so I'm not entirely surprised at your tests. The time increase is a little bit concerning though.

[thomthom]

Re: What is the purpose of sky portals?

The time increase might be me that set the IR Max Rate to 1 instead of 0 by accident... Not 100% sure...

[Micha_cg]

Re: What is the purpose of sky portals?

The time increase dosn't wonder me, it's the emitter vs. light render time effect. Additional emitters dosn't slow down the GI calculation, but each light cause additional raytracing.

[dalomar_cg]

Re: What is the purpose of sky portals?

Emitters don't slow down the calculation because they are not calculated like lights. Lights require that their light be traced from the light source to the point in question in the scene. This must be done for all lights, for each point. Emitters are essentially sampled in the same manner as materials, which simply means that they have to be sampled by whatever GI algorithm your using. There is no way to "bump up" the quality of the emitter other than increasing the quality of your GI solution, so if you're experiencing artifacts when using emitters, then you're most likely looking at a fairly costly GI solution. I really don't really like recommending emitters because IMHO they have the potential to cause more harm than good. There are certain situations where they are needed, but relying on them too much can cause issues

BTW Micha, there are no emitters in this scene as far as I know.

[thomthom]

Re: What is the purpose of sky portals?

Nope. No emitters in this test.

[Micha_cg]

Re: What is the purpose of sky portals?

I see no big difference between a HDRI environment (or the physical sky [not the sun]) and an emitter, both are random sampled by the GI calculation. Additional sky portals are like adding lights were was emitters before.

A little bit OT: so far I understand is IBL something like a textured light and I'm curious to get my hand on it for product viz:
http://www.spot3d.com/vray/help/150S...lights.htm#ex5

So, for me is emitter vs. light the same like GI env vs. IBL - random GI sampling vs. determinated light raytracing. Maybe I oversimplify it to much, but for the daily use it help me.

[dalomar_cg]

Re: What is the purpose of sky portals?

No, I think your simplification is a good one and cuts straight to the chase. I will add one little aside to that though, that extremely bright emitters seam to cause more artifacts than a bright environment does. I'm not exactly sure why that is.

I do hope to get IBL stuff in the next full version. There were some improvements made to lighting in VfMax 1.5 SP3 that I would really like to get into Rhino/SU, although I don't know if they are in the SDK yet. Specifically I'm referring to mesh light objects, the ability to sample emitters as direct lights, and I believe the ability to texture any light source was in there as well.