Hi,
can anybody tell me how to get the light incident direction in a function file?
In Digest v2n5 I found x,y,z. I tried this, but it doesn't work! 
Radiance doesn't know these variables.
Thank you very much for your help!
Best regards,
Frank
Hi Frank,
Only certain of the Radiance material types give access to the incident light direction. The BRDF types (plasfunc, metfunc, transfunc, plasdata, metdata, transdata, and BRTDfunc) allow you to define functions that pass the source direction (x,y,z). Since these are function parameters rather than system variables, you can only access them in the way described in the original Digests post (attached). The prism material types (prism1 and prism2) actually define a separate set of variables, DxA, DyA, and DzA that correspond to the target light source direction, but these variables are only defined when evaluating these primitives. The Radiance reference manual attempts to be complete it its obscure way in defining these things. (This may be found in HTML form in the latest distribution or directly from the website at <http://floyd.lbl.gov/radiance/refer/ray.html>\.\)
The reason the incident light direction is not defined in general is because there is usually no need for it, and typically it is not known during pattern/texture evaluation. What is your application for this information?
-Greg
USING BRDF DATA
From: [email protected]
Subject: Re: Question concerning BRTDfunc's
To: [email protected] (gregory ward)
Date: Fri, 19 Feb 93 8:30:55 MEZHi Greg,
thanks for your examples - they help. But as you assumed I have some questions
still. I believe I could most easily work with the data, eg transdata
material types. I do not see, however, so far the relationship between
the datafile and the functions eg in your reflector example. In order to
understand, it would be probably sufficient for me to know the meaning
of the data in the sae_refl.dat file. I could then figure out the rest.How can I relate to the direction of the incident light? There is no pre-
defined vector in rayinit.cal, is there? I read x,y,z in some places, but
they seem to be general variables.What exactly are coordinate inices and coordinate index functions?
I hope I do not take too much of your time. As feedback for you: There are more
and more people working with RADIANCE here in teh department. And the more
we find out about it and its proper use the better. I was just recently
invited to give a talk and paper (for publication in a little book) on
daylighting simulations. Ans as you can guess: RADIANCE and examples calculated
using it will make up the major part of the talk. So there is spreading of
the information.Best regards,
Fred
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----------------------------------------------------------------------------Date: Fri, 19 Feb 93 09:06:12 PST
From: greg (Gregory J. Ward)
To: [email protected]
Subject: Re: Question concerning BRTDfunc'sHi Fred,
OK, working just with the reflector example, we defined sae_red thusly:
void metdata sae_red
5 sae_refl sae_refl.dat reflector.cal sae_theta sae_phi
0
5 1 .01 .01 .9 .00258The first string argument above is a function that modifies the data value
in the file (correcting for the projected area of the object in this case).
The fourth and fifth string arguments are functions that for a given
(normalized) source ray direction, compute WHICH VALUES to look up in the
data file. This is a bit of a peculiar example, because we happen to have
data that gives reflectance as a function of the angle to the surface normal
(in degrees) and the angle between the reflected ray direction and the
source incident direction. Observe the definitions for these functions
given in reflector.cal:{ entrance angle (source to normal) }
sae_theta(x,y,z) = acos(x*Nx+y*Ny+z*Nz)*180/PI;
{ observation angle (view to source) }
sae_phi(x,y,z) = acos(-(x*Dx+y*Dy+z*Dz))*180/PI;Again, the x,y,z parameters to these functions, as supplied by the Radiance
renderer, are the normalized source ray direction. In sae_theta, this
vector is used in a dot-product against the surface normal (Nx,Ny,Nz) to
compute the polar angle. In sae_phi, a dot product with the incident
ray direction (Dx,Dy,Dz) (directed always towards the surface) to compute
the "observation angle" (ie. the angle between source ray and incident ray).
---- clip ----
路路路
From: Frank Eggenstein <[email protected]>
Date: Mon Aug 4, 2003 5:58:13 AM US/PacificHi,
can anybody tell me how to get the light incident direction in a function file?
In Digest v2n5 I found x,y,z. I tried this, but it doesn't work!
Radiance doesn't know these variables.Thank you very much for your help!
Best regards,
Frank
Frank Eggenstein wrote:
Hi Greg,
thanks for your fast response. The background of my question is, that I
want to simulate the behavior of holographic optical elements (HOE). They
operate on the principle of diffraction (showing a similar effect as
prisms). HOE's offer two possibilities:
Hi Frank,
Radiance doesn't provide enough spectral information, which makes
it nearly impossible to realistically simulate the coloring
effects you're after You already know how to simulate the
quantitative effects of those elements, of course.
If I understand you correctly, then you can't get the results
you'd like to see right now in an efficient manner with Radiance
alone at all. Ideally, you'd have a forward raytracer (photon
mapping), and a much finer spectral granularity than the RGB
color spaces offers.
-schorsch
路路路
--
Georg Mischler -- simulations developer -- schorsch at schorsch com
+schorsch.com+ -- lighting design tools -- http://www.schorsch.com/