Default Sky Setting for Twilight... and speed up SU

The Twilight Render Team shares tips, ideas, helpful hints, and more on using Twilight Render
Chris
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Re: Default Sky Setting for Twilight... and speed up SU

Post by Chris » Mon Jul 09, 2012 2:17 pm

He means post the .skp scene, the file.

You are modeling 3500 meters above the origin (where the red, green, and blue lines all cross)? :shock: That's not really necessary to get the correct lighting, is it? I don't think SketchUp's lighting calculations take altitude into account.
If that is what you are doing, it might be possible for that to cause the sun to appear below the model in the render. I've never seen or tried a render like that, so I can't say for sure. The Twilight Render engine (Kerkythea) uses a different lighting type for the sun than what is used in SketchUp, so in extreme cases like very large scenes or scenes that are far from the origin you will get different shadows.

I recommend putting your model at the origin. I seriously doubt that will change any of the SketchUp shadows, but you should see significant difference with the rendered shadows.

NOTE: this is probably the reason why your early renders weren't showing any shadows. The ground was completely shadowing the scene.

SectionAA
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Re: Default Sky Setting for Twilight... and speed up SU

Post by SectionAA » Mon Jul 09, 2012 3:43 pm

Ah! the sketchp model file! as that is over 15.5mb maybe not a good idea.

But yes Chris, you are right. It hadnt occurred to me till earlier. The terrain for the building model is derived from an original 3D topo survey in ACAD with contours all drawn at correct altitude, about 3500m and above, and I guessed I just carried all that over into Sketchup.
I have corrected it for the section model by lowering all the geometry by 4000 metres, and Twilight now renders the shadows same as Sketchup.

I am not sure about the overall model of this building - where I have set up a view to align with a photo I took of some surrounding mountains, and which has taken me a lot of time.
Lowering the geometry could affect that view - but with some extra work could be re-aligned. Maybe I have to do that to get proper lighting.

So - there is an issue with Twilight sun positioning which should be addressed.
Twilight sun could also be a factor on this site, where the building is some 2-300 metres lower than the terrain to the west. If Twilight's sun is low, it would produce false shading by that terrain.
Fletch said: "Twilight's shadows are slightly different from SketchUp....... Twilight's sun is a different distance from the "earth" in Twilight than in SketchUp"
"Slightly different", in this instance is an understatement.
At the very least this problem should be highlighted in Twilight manual. There are a lot of high and hilly places in the world, with buildings, and Sketchup modellers.

Thanks again.

Chris
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Re: Default Sky Setting for Twilight... and speed up SU

Post by Chris » Mon Jul 09, 2012 4:10 pm

Under normal circumstances, the difference is usually small. When the model is considerably offset from the origin, it can obviously be much different. We'll look at adding mention of this in the manual. As it is now, there isn't an effective fix for this issue. The Kerkythea render engine doesn't support directional light (all the light coming from one direction in parallel rays). We will look at situations where the light is considerably off however; maybe there is a fix that can be made in situations like yours.

Fletch
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Re: Default Sky Setting for Twilight... and speed up SU

Post by Fletch » Mon Jul 09, 2012 8:32 pm

Twilight has been used for several years by thousands of people, and this is the first time I've heard of this situation where someone has placed their model 3.5-4 kilometers in the air.

In SketchUp you specify your location of your model, and this will carry over into Twilight, no problem. When you specify the location of the model (you are specifying the location of the SketchUp origin, where the 3 axis intersect) in SketchUp, and I believe SU then takes into account the elevation. In SU go to File>Geo-location>add location.

SketchUp doesn't handle large models very well, so I'm guessing you noticed while working in your file that the SketchUp cameras were not being very cooperative for you. Keeping things close to the origin in SU usually fixes that issue.

SectionAA
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Re: Default Sky Setting for Twilight... and speed up SU

Post by SectionAA » Tue Jul 10, 2012 8:42 am

First time for everything Fletch.
I used Sketchup for the location, choosing La Paz, Bolivia, and as you can see it handled that fine for shadows.
Whether it also adjusted for altitude - no way of knowing.

As I said, originally I just used a 3D topo survey. I must have adjusted the X, Y origin in Sketchup, but didnt give any thought to Z. It is where it is. Hadnt noticed any problems till now.
I expect Twilight didnt imagine that there was much consequence in their choice of location for the sun, which for almost all instances, there isnt.

As you know, surveyors like to use real world co-ordinates where possible, and in the UK the origin of their surveys is a matter of choice, but it always used to cost more to tie the drawing origin back to the National Ordnance Survey Origin, (True Origin or False Origin), and Ordnance Datum.
Nowadays, with GPS, these sorts of relationships are much easier to determine, and are indeed possible globally.

The implications for Twilight are that more and more data used in Sketchup, mapping, surveys etc, will derive from 3D data with real world X, Y, Z origins. I dont know anything about it, but the ability to put Sketchup models on Google earth, and vice versa, indicates how important it is already. Indeed, for this project, I brought an image from Google Earth and laid it on to the topo surface created from the survey contours and Sandbox.

Food for thought at least.
I think we learnt something.

Many thanks.

Fletch
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Re: Default Sky Setting for Twilight... and speed up SU

Post by Fletch » Tue Jul 10, 2012 2:57 pm

We agree, as chris mentioned we will see about a way to compensate for this situation in the future. ;)

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