Christian Bauer; 8/2002


Lightscape, the quick and dirty way!

The Postsparkasse Scene is somehow different to normal Scenes. Light have to travel not only through one glas layer, but through two and the last layer is translucent. The other difficulty is that the Kassahalle is in a innercourt. Light have to travel down from the 6th floor to the second floor where is meets the first Glas.



My approach was to build the court and place on top of it a plane which i define as Opening.

In Lightscape you have two types of holes which let the sun into the model. You have a Window and a Opening. The difference is the diffuse distribution of the light. An opening will not diffuse the light, a window will diffuse the light depending the material.

Here you can see the face i have placed on top of the inner court.
You will need a face for the skylight opening



The second problem will be the very fine structures of the framework. Lightscape recognise this structure and will generate shadows. Normaly this wouldn't be a problem, because you can increase the meshrefinments, but we want to have fast results so we have to tweak a little bit.

See here the shadowleaks after two iterations(sun/sky) ~1:20min:
ther are some shadow leaks

the workaround is very easy and to be honest, it will result in some loss of quality but will speedup lightscape dramaticaly.

The framework have one material and over this material we can select the "sad" mesh very quickly
use the selection filter for selecting only faces with a specail material

Select the material and popup the menu with the RMB and "add to selection Filter". Now this Material is in our selection filter. The selection filter is a very handy tool since you can use it beside the normal layers to select what you need. You also can use multiple selections... i always select small structures too.

To enable the selection filter you have to switch it on and press the select all Icon
enable filtering

You have selected only the framework - now open the Processing parameters and deselect the reflecting and occluding checkbox:
the faces are selected, now disable occluding and reflecting in lightscape

While processing we can ignore the reflection of the frames because of the bright glass and if you want you can enable occluding after processing since this frames will NOT CAST SHADOWS now. ..that's the deal with occluding faces.

Anyway we now get this result after two iterations(sun/sky) in no time ~20sec.:
better result with less leaks



You can see i have assigned the materials before. So here my way of translucenty:our glass material

Transparency is ~25-30% and shininess is about the same value. I also changed the Refractionindex to 1, since i expect no refractions from a tranlucent surface. You can also see that i let the surface glow a little bit with 1cd/m2 it's low but not to low to not affect the material ... it would look completly different with 0 so it makes sense...even if it do not affect the solution.

You can see a small bump amount on the preview. This is because of the missing functionality of lightscape (~1999). I have decide to have a very small width of the bump:
Lightscape bump maping trick



The next point is the daylight. In our case it's very difficult to have a good setup. The problem is that most of the light will be filtered out by our translucent glass. To see the color you have to "oversaturate" the sun and sky. I normaly prefare less saturated sun, but in this case i wanted a warmer look so i used this settings:
the daylightsettings

You can see (or not) that the sun is more on the orange side and the sky is more on the cyan side. The translucent glass will filter a lot of color out.



Well, this is a difficult thing now. These settings are not optimized, but will give you on a 700mhz processor a result in 10minutes.

There is one problem in this scene... the translucent glass will act as indirect source, so you should decrease in the source category the Indirect Source min space to less than 0.5 of the Receiver min Spacing and increase the subdivison accuracy but this would blow up the calculation time ..i will solve this problem in a different way...

the process parameters

so how can i selective increase the resolution? I can select the surounding walls and floor and change the processing parameters : You can increase the Mesh Resolution to much higher values than 1, but in our case i wanted a fast result... this is another key for fast but accurate results.(you can see now the solutionfile after 15 minutes processing)
a much faster resolution output

If everything is ready you can run the calculation. On interior scenes you can't say ..wait until 80% is finished. I have waited until the glass looks good and bright enough (after 130 iterations ~80%). On different, more complex scenes, it may needs maybe to be processed until 95-99% . Sometimes you know "this one and only face" needs to be processed, but it won't until there are other faces with more stored energy. ;) i had luck here.. on Sibenik it needs full 98% for a satisfying result.



So here magic starts... since some month i use hdr shop which enables me to adjust brightness and gamma with an 48bit image. Lightscape let you render 48bit Tiff images. The trick is to render the scene with the lowest brightness ("0") and some morecontrast ("65") .... btw if you are in the GUI you will recognise that the result is very bright... you have to change the brightness to less than 40 to have a visualy realistic result.

Anyway, i always render from the comandline...


Here the rendering:
dark rendering

You can still see Shadowleaks, which is a results of our tweaking.
shadow leaks

anyway.. i opened that image in hdrshop, changed the gamma to 1,75 and opened the custom exposurecontrol and brighten the image:
change the exposure settings in hdrshop


Rollover to see the original image..... the shadowleak are not visible anymore, but please recognise some limitations:
the final result

You will miss the diffuse shadows from the pillars and the beams and framework. This is again a result of our tweaking. The sad point is that lightscape seems to have problems with shadows behind a second glass layer. You can turn occluding on (remember we had disabled it) but it will not show cast shadows.... maybe i have tweaked too much, but the time i have spend with this scen was not more than 30minutes. ...three times ;) since i doesn't get what i wanted.

So you see you can speedup lightscape with only view tricks and will not loose to much quality.

hope i helped you in any way.


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