[Radiance-general] Radiance Vs Viz

[Giulio Antonutto]

from the "Lord":

"That is correct -- Radiance only finds virtual source objects using 

mirror or prism material types directly. It doesn't even look if you 

have a modifier in front of the material in the chain (e.g., you 

reference a pattern before the material)."

 

cheers!

 

 

-----Original Message-----
From: Giulio Antonutto [mailto:Giulio.Antonutto at arup.com]
Sent: 19 May 2004 16:04
To: 'Radiance general discussion'
Subject: RE: [Radiance-general] Radiance Vs Viz


"3) You can combine Radiance plastic, metal and glass with the mirror
material and turn any Radiance surface that has a slight or large specular
component into a virtual light source. Again, the error is very significant
in radiosity since those specuar reflections are ignored. "
 
unfortunately you cannot use mixfunc with virtual sources...
....I think that virtual sources nested into complex material descriptions
are simply ignored... but I could be wrong (any confirm about this?).
 
nevertheless radiance rules... always!
 
giulio
 

-----Original Message-----
From: Martin Moeck [mailto:MMoeck at engr.psu.edu]
Sent: 19 May 2004 15:56
To: Radiance general discussion
Subject: RE: [Radiance-general] Radiance Vs Viz


Hi Mark,
 
see below.
 
Martin Moeck

-----Original Message----- 
From: Mark de la Fuente [mailto:MdelaFuente at wmtao.com] 
Sent: Tue 5/18/2004 7:07 PM 
To: radiance-general at radiance-online.org 
Cc: 
Subject: [Radiance-general] Radiance Vs Viz

 
 
Even though we are specifically talking about Radiance Vs Lightscape & Viz,
I think this is in part more a comparison about radiosity Vs raytracing.
>From what I know, the big problem with radiosity is that it simplifies
things and assumes lambertian reflections.  This is obviously a deviation
from reality, but I assume it's still possible to achieve a pretty accurate
calculation of some sort depending on the situation.  (I know the person I'm
having this discussion with is certainly convinced you can).  Therefore what
I am more interested in, is knowing what type of calculation you really
CAN'T do with radiosity (Viz or Lightscape) that you could do with Radiance.
 
 
1) Venetian blinds: If you cannot subdivide surfaces like Venetian blinds
into very small patches and override the automatic triangular subdivision
(i.e., Lumen Designer), the calculation is meaningless. Exception: The new
AGI32
 
2) Specular highlights are not considered in the hybrid raytracing method in
AGI32, Lumen Designer and VIZ4. You might see a small reflection of a light
source on a varnished oak floor. If you increase -ad to a large number,
i.e., 5000 - 10,000, Radiance will consider the contribution of that
specular highlight. The luminance of this highlight is typically very
significant, even if the specular reflectance of the varnished floor is 1%
or less. A sun beam reflected off that floor can create an additional 200
Lux on the ceiling in the specular direction, which the radiosity
calculation  never catches. This error is very significant. 
 
3) You can combine Radiance plastic, metal and glass with the mirror
material and turn any Radiance surface that has a slight or large specular
component into a virtual light source. Again, the error is very significant
in radiosity since those specuar reflections are ignored. 
 
4) The important underlying assumption in using radiosity is that building
materials are mostly diffuse, and might only have a specularity of 0.1% or
so. Assume even a very low specularity of 0.1%, times the luminance of the
sun or an HID lamp reflected in that "matte" material. This is an extremely
bright virtual light source, no matter if the material is black or white.
That contribution cannot be ignored. If you have an aluminum ceiling,
aluminum lightshelf and some blinds, radiosity cannot be used at all, since
the specularity of most aluminum materials is 50% to 902%. 
 
 
 
 
 
 Like if I am trying to evaluate the performance of a light shelf, can I use
a radiosity based engine to compare the differences from one light shelf of
material A with the same light shelf made out of material B?  I mean if
material A is matte white paint and ! material B is chrome, how could you
compare the two scenarios if the software assumes lambertian reflections?
 
 
Not at all. You would have to do it by hand and it would be much more
accurate. The diffuse and specular reflectance of both materials is known,
and all you need is Excel to calculate the specular and diffuse light
transfer. Then add 15% indirect and you got it. 
 


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