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

[John Everist]

Thank you greg - that's very kind of you to answer so quickly. You even managed to answer part 2 of my task before i asked it (relating to the rendered image).

Out of curiosity, is it possible to limit the ray distance of an IES light source?

Sent from my iPad

> On 17 Jul 2017, at 01:11, Greg Ward <gregoryjward at gmail.com> wrote:
> 
> 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...
>> 
> 
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