Light fall off problem ?

[Thomas An._cg]

Hi,

I have been reading this info from Vlado, but I still think there is something not right about light fall-off in Vray.
http://www.chaosgroup.com/forum/php...ht+decay#110398
Edit: http://www.chaosgroup.com/forum/phpB...t+decay#110398

For example in this test the back room (up the stairs) is not illuminated enough. http://www.treddi.com/forum/index.php?showtopic=16020

There are two windows in that little room (one on the wall just above the stairs and one on the far end of that back room... not to mention the stair opening itself). There is no way that room should be so dark ... I think the Maxwell result is more correct ... how is it possible to avoid this pitfall in Vray ?

I don't think black or white points or a curves adjustment would do it. Also, when I put portals in all windows the problem is still there. It can only be remedied if portals are assigned different multiplier values (but that would be totally unacceptable in my book)

Any thoughts ?

[flipside]

Light fall off problem ?

It is related to the gamma setting... Maxwell uses a 2.2 gamma correction by default (not in a post effect, but in the renderer itsself). You can do this in vray by using gamma color mapping, with bright set to 0.455 (=1/2.2). But your materials will be washed out, because you created them with a gamma 1.0. So we need a gamma correction for colors and mat previews etc... This is being considered already.

In max this is perfectly possible because there you have a display gamma control, and you can control textures by using the color correct plugin.

Your vray forum link doesn't work. But here are some links on the gamma topic, or what it is called: working linear space workflow

http://www.gijsdezwart.nl/tutorials.php
http://www.chaosgroup.com/forum/phpB...=13724&start=0
http://www.chaosgroup.com/forum/phpB...=10854&start=0

[Thomas An._cg]

Light fall off problem ?

It is related to the gamma setting... Maxwell uses a 2.2 gamma correction by default (not in a post effect, but in the renderer itsself). You can do this in vray by using gamma color mapping, with bright set to 0.455 (=1/2.2). But your materials will be washed out, because you created them with a gamma 1.0. So we need a gamma correction for colors and mat previews etc... This is being considered already.

I see, so it is a problem of some sort, but it is being looked into ... Good to know

I will take a look in the links you provided. Thank You !

Sorry about the broken link ... this is what I was looking at.
http://www.chaosgroup.com/forum/phpB...t+decay#110398

[vlado]

Light fall off problem ?

I don't know why the back room is darker in their images... in my rendering below it appears a bit brighter... don't mind the colors, they don't match the maxwell rendering.



Best regards,
Vlado

[flipside]

Light fall off problem ?

Hey Vlado, I was testing this too, but I can't open the max file to see their light values. What are the direct light and skylight multipliers and colors? And the diffuse for the walls? In my test, the back room is also lighter, and with gamma correction it seriously lacks contrast, I'm not so sure anymore if it is really the way to go.

[flipside]

Light fall off problem ?

Here's my test with gamma color mapping, I think I had a wrong diffuse color in the beginning. Now contrast looks ok:



Rendertime a couple of minutes, don't remember and forgot to save it in the stamp...

[Micha_cg]

Light fall off problem ?

Wouter, your last image looks good could you tell me your GI settings? A small problem are the splotches at th right wall. Any hint how to avoid it?

[flipside]

Light fall off problem ?

Here is the scene:
http://www.aversis.be/vrayrhino/gamma_test01.zip

Note that I use gamma color mapping! All materials are adjusted for that purpose. If you look at the wall material, you may think it has a very dark diffuse but in fact it is not, because you should see this with a gamma curve 2.2 applied (which is not possible right now in rhino). Follow the links I posted before for more info about gamma correction and why we are working in a wrong way for years now. I'm really bad at explaining all this. Especially gijs' tutorial explains the starting problem very well.

I see no blotches, but If you want ultra smooth GI increase the hsph value to 75 (it is 60 now).

[Thomas An._cg]

Light fall off problem ?

hmm, I do not think it is just a gamma issue. I think there is a falloff issue as well. Somehow light does not appear to have enough bounces. There should be some glow entering the top window and hitting the back wall. In Vray, that back wall shows no signs of ambient light from the top window.

[vlado]

Light fall off problem ?

In the scene that I have, the light is positioned in such a way that no direct light comes through the top window (it's in shadow). So I'm not sure where maxwell gets it from...

Best regards,
Vlado

[Thomas An._cg]

Light fall off problem ?

In the scene that I have, the light is positioned in such a way that no direct light comes through the top window (it's in shadow). So I'm not sure where maxwell gets it from...

Best regards,
Vlado

There should be ambient light from the sky.
(The background illumination)

Even if it was a rainy day (no sun) there should be light coming in.

[vlado]

Light fall off problem ?

Even if it was a rainy day (no sun) there should be light coming in.

Of course - it's there, otherwise you would get pure black, and not a shade of gray.

Best regards,
Vlado

[vlado]

Light fall off problem ?

Sorry, I meant that no direct light from the "sun" enters the window; of course skylight does.

Best regards,
Vlado

[Micha_cg]

Light fall off problem ?

Thank you Wouter. Interesting to study your settings.

[flipside]

Light fall off problem ?

Hey thomas, I think my example pretty much is the same as maxwell. I had to guess how the sun was positioned, and I alsi had to guess the light values and colors (balance between sun/sky). I think with exactly same balance, the result will be the same. As you can see, in maxwell example the sun is stronger, that's where the difference comes from imo. I will try an example with darker sky and brighter sun.