[Radiance-general] RE: Light pipe

[Giulio Antonutto]

the simplest way to proceed is to create an equation that links a
coordinate to the radiance of the surface in form of a multiplier (hence
the equation is between [0,1]).
you may use a cal file or directly express it into the brightfunc
definition.

As an example:
Supposing the relation between light output and distance is this:
(you may use the proper equation here)

y = a*x*x + b   
and 
y= 0 when x = 1000m 
y = 1 when x = 0

it would be:

y = -0.000001*x*x+1

that becomes an 'if' sentence to avoid negative values...
y = if(-0.000001*x*x+1, -0.000001*x*x+1,0)

Putting all together I would define a light material this way:

void light max
0
0
3 100 100 100

max brightfunc light
2 'if(-0.000001*Px*Px+1,-0.000001*Px*Px+1,0)' .
0
0

light cylinder pipe
0
0
7
0 0 0
2000 0 0
.125

void plastic white
0
0
5 1 1 1 0 0

white polygon base
0
0
12
0 -10 -3
0 10 -3
2000 10 -3
2000 -10 -3

if it is ok you should see half cylinder black and the other half with a
gradient. Remember to use really low -ds 0.01.
Ciao,
Giulio


NOTE: Unfortunately the uv trick doesn't work here, since you need light
and you cannot nest it into instances (and meshes).
Hopefully your light pipe follows a simple direction like Px, Py or
Pz... otherwise you need to change the equation a little (you may also
find another equation...).