[Radiance-general] Re: an invisible glow

Sun, 27 Oct 2002 12:25:11 +0000

Many thanks Greg - very nice trick!

I had completely forgotten that distance travelled T is available. I guess
the moral of the story is always to check the beginning of rayinit.cal!

Looks like just what we needed.

cheers
alex

> Hi Alex,
> 
> I'm a bit afraid to even dip into this conversation, as there are so
> many tricks and hacks you can play with Radiance before even delving
> into the source code, but I thought I'd offer at least one modest
> suggestion...
> 
> If you just want to substitute a material in the case of eye rays, you
> can use a mixfunc and pass it the world coordinates of the viewpoint
> (-vp Vx Vy Vz) in the real arguments like so:
> 
> void mixfunc view_switch_mat
> 4 view_mat normal_mat
> if(and(FTINY-abs(A1-Px+T*Dx),and(FTINY-abs(A2-Py+T*Dy),FTINY-abs(A3-
> Pz+T*Dz))),1,0)
> .
> 3 {Vx} {Vy} {Vz}
> 
> where of course {Vx} {Vy} and {Vz} are replaced by the appropriate real
> values.  The above works by testing to see if the ray that hit the
> surface originated at the viewpoint without being reflected or
> refracted along the way.  The FTINY values (equal to 1e-7) are
> necessary to avoid floating point roundoff errors in the equality
> tests.  I haven't tested this, but I believe it would work, and you
> wouldn't have to hack the source code.
> 
> -Greg
> 
>> From: alex summerfield <[email protected]>
> 
>> Is there a way to do this in general (with mixfunc?) so that a surface
>> appears from the viewpoint as one material, but as far as the lighting
>> calculation is concerned is another? Or more to the point how can i
>> test if
>> the ray being traced is a view ray or not?
>> 
>> To explain an example situation - suppose there is a complicated
>> existing
>> building that is simplest to represent with an image mapped onto its
>> facade
>> (and that only appears correct when viewed from the viewpoint), but in
>> terms
>> of its lighting contribution on the site in general is more accurately
>> described in some other way.
> 
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*******************************************************
A. J. Summerfield                 [email protected]
Faculty of Architecture, University of Sydney
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Outside of a dog, a book is man's best friend.
Inside a dog, it's too dark to read.       Groucho Marx
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