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

[Greg Ward]

Well, you have to convert an IES source to Radiance using ies2rad, right?  Just edit the output of that program to fit the described solution.  For example, you can use the "-i" option of ies2rad to generate a spherical light source, then change the "illum" to "glow" with the Dmax limit.

Cheers,
-Greg

> From: John Everist <john.everist at icloud.com>
> Date: July 17, 2017 2:15:33 PM PDT
> 
> 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...