[Radiance-general] Remove attenuation and specify ray distance of an IES light source?

[Greg Ward]

Hi John,

If I understand your query, you just want to "see" what is within a certain distance from a point -- it that right?

To generate an image from the light source's perspective to some maximum distance, just use an rpict or rvu view with a fisheye view and aft clipping distance:

	-vta -vp Lx Ly Lz -vh 360 -vv 360 -va Dmax

Where (Lx,Ly,Lz) is the position of your light source (or just in front of it) and Dmax is how far you want to look.  You may also need the -vd and -vu specs to specify the central view direction and distance.  The above will generate a 360° fisheye view, which may be larger than you want, so adjust accordingly.

If instead you want to illuminate your scene, but only for points within some distance of a source, you can use the "glow" type:

void glow limited_light
0
0
4 1000 1000 1000 Dmax

limited_light sphere limited_bulb
0
0
5 Lx Ly Lz 1

Again, you'll need to insert the appropriate values for your source position, and this light will still have a 1/r^2 fall-off.  Also, make sure to set "-ab 0" in your rendering, so it doesn't try to count the glow source in the indirect calculation.

If you want to eliminate the fall-off, you can use a compensating pattern like so:

void brightfunc square_riseup
2 T*T .
0
0

square_riseup glow limited_light
0
0
4 1 1 1 Dmax

limited_light sphere limited_bulb
0
0
5 Lx Ly Lz 1

This will still have a cosine-orientation effect on your surfaces, but no fall-off.

Hope this helps.
-Greg

> From: John Everist <john.everist at icloud.com>
> Date: July 16, 2017 4:22:51 PM PDT
> 
> Hello community,
> 
> Is it possible to totally break the laws of physics in radiance? Can I make a calculation with an IES file where I can terminate the ray distance (at 100 metres for example) and remove any attenuation from the rays? 
> 
> In summary, i want to use radiance to identify 'line of sight' to a maximum distance - a  point was/was not 'seen' from the light source position.
> 
> Its properly an easy question but i cant find the answer. I have the rendering with radiance book if e answer is in there...
>