# Modelling an artificial sky : help needed

Hi,

We
are building an artificial sky for an architecture school where we
teach, and before
starting we prepare a Radiance model, to simulate the interior of
the artificial sky. The idea is to help us to choose the right bulbs,
the ceiling, etc.

To
simulate the overcast sky (Moon and Spencer) distribution in Radiance
mirror material and ceiling is filled with fluorescent tubes
(very close to one another) and covered with translucent material. Then
we tried to render few
images of interior, with several sets of rendering parameters, but
final images still aren’t good enough with a lot of spots.

Does anyone on the list have an experience with similar modelling,
where mirror objects are included?

Why do we get strange spots on the floor even when ambient parameters
are really high?

What combination of parameters may lead us to good image, for
acceptable rendering time?

Ljubica, JeanDo, Marija

Hi,

We are building an artificial sky for an architecture school where we teach, and before starting we prepare a Radiance model, to simulate the interior of the artificial sky. The idea is to help us to choose the right bulbs, the ceiling, etc.

Interesting approach. I would have used physical mock ups to decide on
materials (diffusers, layout and spacing) and measurements on a number
of lamps of the same type to check for manufacturing inconsistencies.

With Radiance you introduce the new complexity of modelling the
physical materials _correctly_ in a virtual world and in some cases
you have to deal with the optimisation of the algorithms.

To simulate the overcast sky (Moon and Spencer) distribution in Radiance we made walls of mirror material and ceiling is filled with fluorescent tubes (very close to one another) and covered with translucent material. Then we tried to render few images of interior, with several sets of rendering parameters, but final images still aren't good enough with a lot of spots.

At a first look I'd say your spotty images are the result of "-av 0 0 0" and a rather low
"-ab 4". Note that the trans material will swallow one bounce just to let light pass. That
leaves you 3 bounces to hit a light source which is not a lot.

As a first step I'd set "-av" to something realistic. You should be able to calculate a
reasonably good approximation of the ambient value from your physical set up.

Next: Your rendering times might benefit from an 'illum' calculation. Replace the
"trans" material ceiling with an "illum" to make it a direct light source. This will affect
the appearance of the ceiling but not the accuracy of the rendering. However, this
may go against your idea of simulating the physical set up.

In your last image you're disabling the ambient cache with "-aa 0". Now every
ray is calculated with your high settings. You can probably reduce "-ad" and "-as"
a lot in your uniform environment. I'd also set "-lr" back to 8 again. There is a
difference but it has not such a big visual impact.

Regards,
Thomas

···

On 22 Oct 2008, at 10:04, jeando wrote:

Hi Ljubica,

I just want to add to what Thomas wrote. Quoting your webpage:

Materials are basic :
walls are mirrors (80% reflexion - mirror material)
ceiling is translucent (60% transmission all diffuse, trans material)
floor is gray (60% reflexion, plastic material)
half-sphere is blue plastic

Since your trans surface blocks visibility of the light sources, I wouldn't recommend using the mirror type for this at all. I'd just use an 80% reflective metal material (pure specular) and let the interreflection calculation handle it. I don't think an illum is suitable, due to the high variance in luminance over the surface of each panel.

I'm actually quite surprised at how bad these results look. I think something else must be going on that I don't understand, like leaks at the sides of the structure. Anyway, try it without the mirror type and see if that helps.

Your calculation time could probably be improved without too much loss in accuracy by using a glow material for the cylindrical light sources -- set the distance limit to something like 4 times the size of one of your light panel widths.

Finally, I don't think a pure diffuse translucent material can have a transmission over 50%.

Best,
-Greg

···

From: jeando <[email protected]>
Date: October 22, 2008 2:04:22 AM PDT

Hi,

We are building an artificial sky for an architecture school where we teach, and before starting we prepare a Radiance model, to simulate the interior of the artificial sky. The idea is to help us to choose the right bulbs, the ceiling, etc.

To simulate the overcast sky (Moon and Spencer) distribution in Radiance we made walls of mirror material and ceiling is filled with fluorescent tubes (very close to one another) and covered with translucent material. Then we tried to render few images of interior, with several sets of rendering parameters, but final images still aren't good enough with a lot of spots.

Does anyone on the list have an experience with similar modelling, where mirror objects are included?
Why do we get strange spots on the floor even when ambient parameters are really high?
What combination of parameters may lead us to good image, for acceptable rendering time?

Ljubica, JeanDo, Marija

Thanks to you two

JeanDo

Greg Ward a écrit :

···

Hi Ljubica,

I just want to add to what Thomas wrote. Quoting your webpage:

Materials are basic :
walls are mirrors (80% reflexion - mirror material)
ceiling is translucent (60% transmission all diffuse, trans material)
floor is gray (60% reflexion, plastic material)
half-sphere is blue plastic

Since your trans surface blocks visibility of the light sources, I wouldn't recommend using the mirror type for this at all. I'd just use an 80% reflective metal material (pure specular) and let the interreflection calculation handle it. I don't think an illum is suitable, due to the high variance in luminance over the surface of each panel.

I'm actually quite surprised at how bad these results look. I think something else must be going on that I don't understand, like leaks at the sides of the structure. Anyway, try it without the mirror type and see if that helps.

Your calculation time could probably be improved without too much loss in accuracy by using a glow material for the cylindrical light sources -- set the distance limit to something like 4 times the size of one of your light panel widths.

Finally, I don't think a pure diffuse translucent material can have a transmission over 50%.

Best,
-Greg

From: jeando <[email protected]>
Date: October 22, 2008 2:04:22 AM PDT

Hi,

We are building an artificial sky for an architecture school where we teach, and before starting we prepare a Radiance model, to simulate the interior of the artificial sky. The idea is to help us to choose the right bulbs, the ceiling, etc.

To simulate the overcast sky (Moon and Spencer) distribution in Radiance we made walls of mirror material and ceiling is filled with fluorescent tubes (very close to one another) and covered with translucent material. Then we tried to render few images of interior, with several sets of rendering parameters, but final images still aren't good enough with a lot of spots.