rectangular light versus emmissive material for lots of lights

[andybot_cg]

So I wanted to figure out a good way of doing a ceiling in an office building with banks of fluorescent lights in the ceiling. I thought it would be quick to just do the lighting as emmissive areas. Works great. Then I was wondering if the quality would be better with vray rectangular lights. I tried that, but it ends up looking pretty much equal, however, it takes more than 3x as long to render.
I remembered a thread that talked about a way to reduce the calculation impact of many vray lights in the scene. Well, I tried to reduce the rectangular light settings (reduce the subdivisions from 8 to 2, also changing the photon subdivisions from 500 to 10), but could find no setting to help reduce the render time. Is there something I am missing in the settings? Thanks for any assistance!
here's the file: http://www.mediafire.com/?k3pky79582922by

[nomeradona_cg]

well some food to learn.. thanks for doing this test. honestly i dont know andy how to shorten the render time when it comes to rec lighting

[fpedrogo]

Hi
The sampling inside the rectangular light is more precise than the emissive material. The rectangular light light is actually direct light while the emissive need the GI and the quality and speed of the emissive depends of the GI solution quality. To prove this make a quick test. Disable the Indirect illumination and click render with the emissive light. You are not going to have any illumination from them. The emissive material is just a material with very hi intensity. In 3DS max we have a option to convert a emissive material to direct light, but unfortunately we don't have that option in VfSU or VfR. Then make another render with the rectangular lights and without GI. You will see the light effect. In term of how deal with a bunch of light, the all the V-Ray light have an option called cutoff threshold or cutoff, if you reduce that value you will limit the range of the rectangular light illumination, basically you can reduce the distance around the rectangular light in which they are going to illuminate. This is very useful to reduce the render time when you have a lot of v-ray light. When V-Ray meet the cutoff threshold value, it stop the light calculation.
I hope this make sense for you.

Best

[andybot_cg]

Thanks Fernando, I'll try the cutoff value see if that helps. Does the value reference distance? I understand about the emmissive not being used for lighting. That's why I have all the GI lighting turned on in this test because I am only depending on the emmissive for visual appearance, not as a light source.
What I'm confused about is the other thread I linked. What I understood from that thread was that reducing the sampling of the rectangular lights would help reduce the calculation time, but that does not seem to be the case. Do you have any explanation about that?
-Andy

[andybot_cg]

well, that almost worked! I changed the cutoff to .02 instead of .002
On the positive, my time was reduced slightly, but... now the lights create these patches of light on the walls. Oh well.

[nomeradona_cg]

thanks for the info fernando and test from andy.. im learning a lot from this one.

[ValeroStudio]

I never use emmissive for lighting a scene, it creates to many splotches on complex scenes. you can use emmitters to make the lights look like they are giving off light, but light the floors with a hidden rectangular light is my advice.

[andybot_cg]

@valerostudio: That's the point in this test. The scene is lit by GI, and the emmissive is only for appearance (instead of a dull ceiling.)

[bocomofo]

First Post...Hope its of assistance.

When I render outside views of offices I add in a ceiling material that has emissive lights set-up as part of the material. It makes it relatively easy to apply the material to multiple floor paltes and gives a good enough effect for external views.

zip file of the material can be found here if its of any use?

https://www.wetransfer.com/dl/464Ccn...9d4f8482abb6a7

I'd be happy to receive suggestions on how it can be improved though!

[ValeroStudio]

Emmissive materials are really messy to work with in the current release. They create so many splotches, it makes them useless as a light source. Hope this bug is fixed in the next beta.

[bocomofo]

this is true, however where you want to create the effect of lights in the ceiling it is a much easier way to do it as part of an emissive material, especially where you have an external scene and the lighting isnt contributing greatly to the scene