[Radiance-general] Re: Using MIST for overcast sky

[Greg Ward]

Unfortunately, mist is never going to work with distant sources, 
because if your raise any extinction to an infinite power, no matter 
how close to one it starts out, you get a zero result.  (From RwR Chap. 
14, eq. 14.7, we see that light is absorbed as the power of distance.)  
I just had a similar discussion with another user (Pablo Escanilla 
<pabloe at ivo.cps.unizar.es>), who was simulating fog on a roadway.

Real atmospheres are quite a bit more complex than setting a global 
constant value for extinction and albedo.  Real atmospheres change with 
altitude, often dramatically, so it doesn't become pitch dark as soon 
as the fog rolls in.  Simulating large scale atmospherics in Radiance 
means creating enormous "slabs" of mist with different extinctions at 
different altitudes.  It can be done, but it's a bit of a pain and, as 
far as I know, no one has yet made the attempt.

As to sampling the sky dome, this is never going to work.  You are much 
better off setting a sensible value for -av, which gets figured into 
the mist calculation as an appropriate multiplier on albedo.  This was 
my advice to Pablo.

-Greg

> From: "Guedi Capeluto" <arrguedi at techunix.technion.ac.il>
> Date: April 16, 2004 2:06:43 AM PDT
>
Hi,

I am trying to simulate the extinction of light inside a MIST volume, 
for different sky conditions.

The simulations seems to work well using "sun" as the light source, but 
I have problems when using an overcast sky or a sunny sky with no sun 
(-s option).

The light sources for the MIST volume are "sun" and "sky" which is 
defined using glow.

Did MIST work for glow type light sources? If not, what do you 
recommend to use to simulate this case, with diffuse light coming from 
the sky?

Thank you in advance,

Guedi