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

[Guedi Capeluto]

Thank you very much for your answer.

My intention was not simulating light extinction through the atmosphere but
once light reaches a mist object which is on the ground, (I exactly know the
extinction coefficient within it), analyze the light (direct from sun and
diffuse from sky dome) extinction inside this small volume.

Since I need to know the light levels (illuminance) inside the volume at
different depths, it seems to me that the -av approach will not work.

Any suggestion?

Thanks in advance,
Guedi



-----Original Message-----
From: Greg Ward [mailto:gward at lmi.net]
Sent: Friday, April 16, 2004 7:12 PM
To: Radiance general discussion
Cc: pillo.pillo at tin.it; arrguedi at techunix.technion.ac.il
Subject: Re: Using MIST for overcast sky

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