[Radiance-general] confused by turbidity

[Greg Ward]

Hi George,

I just want to add to what Lars said, which is correct.  Turbidity does not affect the light distribution in gensky.  

Sadly, we don't have a genocean to create lighting environments for underwater -- that would be a nice addition!  If you are trying to simulate absorption and scattering in a participating medium, you can try playing around with the -m* options to affect the global environment.  There is also the "mist" primitive for controlling local media and scattering from specific light sources.  The single-scatter approximation used in Radiance is very approximate, however, and specifying a non-zero global extinction coefficient then having light sources infinitely distant (a la the "source" type) results in a lamentable zero contribution to the scene.

However, there's no reason in the case of the ocean that you couldn't use one mist sphere for water surface, indicating the sun as the source, and you should get roughly the correct behavior down below.

Cheers,
-Greg

> From: "Lars O. Grobe" <grobe at gmx.net>
> Date: January 30, 2014 6:30:56 AM PST
> 
> Hi George,
> 
> gensky makes use of the standard sky distributions overcast (cloudy), clear (sunny) plus intermediate. The distributions are thus given, not affected by a turbidity. The turbidity value would be used to "scale" the distributions if you rely on the model-based assumptions instead of using measured / otherwise expected illuminance or luminance, but not when you pass that value using the -B parameter. Thus, in your case, you get a sunny sky and the turbidity is simply ignored. If you skip the -B parameter, then the sky luminance will be scaled based on the models included in gensky, and these would make use of the turbidity parameter. You can check this by varying -t and removing -B 55.87.
> 
> Cheers, Lars.
> _______________________________________________


> From: P George Lovell <p.g.lovell at st-andrews.ac.uk>
> Date: January 30, 2014 3:37:52 AM PST
> 
> Hi Everyone, 
> 
> I've been hacking around with some code trying to see how turbidity changes my rendering, so far I can't see to find any change in my images. Which leads me to suppose that I must be doing something wrong. I'm using the commands in Bold which result in the world.rad files below.
> 
> 
> Another question.
> 
> I'm actually interested in simulating turbidity underwater, don't know if anyone can comment upon whether the turbidity numbers for air relate to those for water? Is there a better renderer for such things.
> 
> Best wishes,
> 
> George
> 
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> Dr P. George Lovell,
> 
> Lecturer in Psychology
> University of Abertay Dundee