Sky Portals

The Twilight Render Team shares tips, ideas, helpful hints, and more on using Twilight Render
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Chris
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Joined: Sun Mar 08, 2009 3:00 am
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Sky Portals

Post by Chris » Tue Feb 21, 2017 9:18 pm

There are often question about how to use the Sky Portal material, what it is, and when to use it.

A good answer to what it is and how to use it is found in this forum post: viewtopic.php?p=2670#p2670

The question about when to use it and what preset to use has been an evolving one. Where once we might have suggested using Sky Portals in any interior space, now that opinion has changed.

After a user asked the question recently about Sky Portals, I ran a series of tests on a very simple room with just one window. Below you will see the comparison of a set of images. Note that these are intentionally low light scenes, all the lighting coming through the window into an enclosed room.

Using Sun & Sky, Low+, from left to right: No Sky Portal (02s), Non-Visible Sky Portal (2min 47s), Visible Sky Portal (2min 48s)
Low+.jpg
Low+.jpg (114.53 KiB) Viewed 2438 times
While not immediately obvious, the improvement in quality is there. But look at the difference in render time! For that increase in render time we would expect a much more significant increase in quality.

Sun & Sky, Interior+ (10 passes), from left to right: No Sky Portal (5min 38s), Non-Visible Sky Portal(5min 45 s), Visible Sky Portal(5min 51s)
Interior+.jpg
Interior+.jpg (469.62 KiB) Viewed 2438 times
Note that we see virtually no increase in render time but we do see a noticeable difference in brightness in the scene, and somewhat less noise.

Now we look at using and HDR environment image. On Low+, from left to right: No Sky Portal (02s), Visible Sky Portal(1min 57s)
Low+_HDR.jpg
Low+_HDR.jpg (70.9 KiB) Viewed 2438 times
We see a definite increase in quality but still a significant increase in render time. Not as much of an increase as the previous test with Sun&Sky though.

Finally, HDR environment on Interior+ (10 passes), from left to right: No Sky Portal (5min 29s), Visible Sky Portal(5min 46s)
Interior+_HDR.jpg
Interior+_HDR.jpg (288.42 KiB) Viewed 2438 times
A negligible increase in render times but a very significant reduction in noise.

So we can see that a Sky Portal definitely makes a difference. It directs all the environment lighting into the scene through the portal, as if the portal were a light shining on the scene. But if we examine the tradeoff of quality vs. time, we can see that it is clearly not worth it to use with a biased preset (Low/+ through High/+). If we just took the same scene and rendered it on High+, we can get the same or better quality in only 08sec!
High+_08.jpg
High+_08.jpg (24.48 KiB) Viewed 2438 times
However, if we are using Interior/+, we see that we get a significant noise reduction for the same amount of time and passes!

So the verdict is, don't bother using Sky Portals with the biased presets, but if you are using the unbiased presets, it is definitely worth your time!

Now what about Visible vs. Non-Visible Sky Portals, what's the deal with that?
The easiest thing is to show you.
In Low+, from left to right, None, Non-Visible, Visible
Low+_Vis.jpg
Low+_Vis.jpg (177.47 KiB) Viewed 2436 times
In Interior+, from left to right, None, Non-Visible, Visible
Interior+_Vis.jpg
Interior+_Vis.jpg (289.14 KiB) Viewed 2436 times
So we can see that with no sky portal, we get a normal render; the sky emits light. With a Visible sky portal (let's skip the non-visible for a moment), you can see that the sky has been "turned off" and lighting is only occurring through the portal. That's good to know. But look carefully at the Non-Visible images. At first glance it looks a lot like the render with no sky portal. But if you study it you can see similar highlights with the Visible Sky Portal image. It appears that the Non-Visible sky portal produces lighting both from the environment and the portal, together. It's a combination of the two.
If you are planning on using the Sky Portal, it is good to know that by using the Visible flag you can control whether light only comes through the portal or both the portal and the environment. (Note that the Visible flag also makes the portal literally visible or not :D )

It's also worth noting that you don't have to use the Sky Portal in an enclosed space. If you are using it in an interior space, you definitely should do your best to fit the Sky Portal tightly into a "watertight" space (by improving my Sky Portal position and size I was able to cut the Low+ test above in half). But you don't have to. You can actually use the Sky Portal (set to Non-Visible) to direct the lighting at an exterior where you want extra lighting information (I would only recommend that for the unbiased presets). With Interior+ you will get improved lighting on those focus areas with little increase in render time.

Chris
Posts: 4842
Joined: Sun Mar 08, 2009 3:00 am
OS: Win10
SketchUp: 2016

Re: Sky Portals

Post by Chris » Tue Feb 21, 2017 11:04 pm

Just so you know, Sky Portals aren't just for windows. ;)
Attachments
Sink_Sphere.jpg
Sink_Sphere.jpg (96.04 KiB) Viewed 2432 times
SinkcubeA.jpg
SinkcubeA.jpg (93.48 KiB) Viewed 2432 times

posodrac
Posts: 208
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OS: i5 7th 16Gb W10
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Location: Portugal

Re: Sky Portals

Post by posodrac » Sun May 06, 2018 9:43 am

Just so you know, Sky Portals aren't just for windows. ;)
Chris, is this a teaser about sky portals ?
More then a year is time enough to let me in suspense. :D
Good efects you got there
Does it work with preset 11 ?
Is it that simple ? :hat:
Last edited by posodrac on Sun May 06, 2018 8:37 pm, edited 1 time in total.
This will look good in my "curriculum vitae": "Heterodox truck driver at TNT/FedEx"

Chris
Posts: 4842
Joined: Sun Mar 08, 2009 3:00 am
OS: Win10
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Re: Sky Portals

Post by Chris » Sun May 06, 2018 3:41 pm

Sky Portals have been part of Twilight Render from the beginning!

You can find the material templates under "Emitter".

posodrac
Posts: 208
Joined: Mon Feb 15, 2010 11:43 pm
OS: i5 7th 16Gb W10
SketchUp: SU 8
Location: Portugal

Re: Sky Portals

Post by posodrac » Sun May 06, 2018 8:39 pm

Can I have a sarcastic answer for the 2nd question?
This will look good in my "curriculum vitae": "Heterodox truck driver at TNT/FedEx"

Chris
Posts: 4842
Joined: Sun Mar 08, 2009 3:00 am
OS: Win10
SketchUp: 2016

Re: Sky Portals

Post by Chris » Mon May 07, 2018 1:01 am

Umm..... :halo:

Sky portals do work with preset 11. In fact they work best with the unbiased, Interior presets.
So yeah, I think it is that simple. :whistle:

Fletch
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Re: Sky Portals

Post by Fletch » Mon May 07, 2018 4:32 pm

Watch the video tutorial for Sky Portal with Twilight Render!

posodrac
Posts: 208
Joined: Mon Feb 15, 2010 11:43 pm
OS: i5 7th 16Gb W10
SketchUp: SU 8
Location: Portugal

Re: Sky Portals

Post by posodrac » Tue May 08, 2018 12:27 am

Com'on... :(
Do You and Chris believe that in 8 years using Twilight I don't know about Sky Portals or how to get the tutorials?
I was just peeking the forum and came across with Chris's one year old post with two images using solids has Sky
Portals
Just so you know, Sky Portals aren't just for windows. ;)
... ok... I made a joke.
Also a post by "admin" writing about lighting a room saying "Only for 9 & 10 Presets"
So... we'r good!? :totgm:
This will look good in my "curriculum vitae": "Heterodox truck driver at TNT/FedEx"

Fletch
Posts: 10323
Joined: Fri Mar 20, 2009 2:41 pm
OS: PC 64bit
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Re: Sky Portals

Post by Fletch » Tue May 08, 2018 7:17 am

:lol:

sarcasm is difficult to "hear" in a forum post. :hat: :hug:

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