[Radiance-general] Radiance Vs Viz

[Rob Guglielmetti]

On May 18, 2004, at 7:07 PM, Mark de la Fuente wrote:

> I've been having a discussion with the people at my office about the 
> differences between Radiance and 3D Studio Viz & Lightscape.  (They 
> are used to using Lightscape and don't know anything about Radiance)  
> And I was hoping some of you might have some comparison or contrasting 
> comments on this subject since I'm sure most of you have had some 
> experience with this sort of discussion.

Peter A-B's paper, mentioned in his reply to you, is an excellent 
resource.  Print out Section 5.4 and hand it out to your co-workers, 
and then point them toward the radiance-online.org website.  =8-)

I used Lightscape heavily for a number of years and beta tested both 
the last version of Lightscape and Viz4 (the first time Lightscape's 
code was used in Viz).  But I was always looking toward Radiance and 
wishing I had the time to learn its many nuances.  The demise of 
Lightscape (Viz is horrid) and the aquisition of a complex project 
finally conspired to push me into the Radiance learning mode full-time. 
  (P.S. I'm still in that mode, three years later!)  At this point, I 
use Radiance for all our calculations that require more than a simple 
zonal cavity or point calc.

> 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. 

Right.  Again, Peter & Kurt's paper is an excellent resource on all of 
this.

> 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. 

I think some confusion stems from the fact that Lightscape & Viz have a 
ray-tracing capability that you can apply, as a post-process.  When you 
apply that to a Lightscape/Viz model, the specular reflections are 
*rendered*, but not *calculated*.  So you have a rendering that looks 
somewhat accurate, but at that point that's all you have.  You cannot 
derive quantitative luminance/illuminance information from the image at 
that point.  That data always comes from the radiosity-calculated 
model, and that's fundamentally flawed because of the lack of specular 
reflections (and in Lightscape's case, no diffuse transmissions 
either).

=================
    Rob Guglielmetti
www.rumblestrip.org