On 13-6-2019 4:50, Mike Horvath wrote:
> On 6/12/2019 1:25 PM, Alain wrote:
>> For the Sun, don't bother with light fading : Use a light with the 
>> parallel option and you are good. Unless your scene scale is 
>> interplanetary, the fading over the extent of your scene is totally 
>> negligible.
>>
>> For the extent of the Sun when using an area_light, make it's diameter 
>> about 0.01 of the distance. Not exact, but close enough.
>>
>> For more normal lights, the fade_power must be 2. No other value is 
>> realistic.
>>
>> For fade_distance : Make it small. For an area_light, it should be 
>> about the width of the light. For point light, a value of about 0.1 is 
>> correct.
>>
> 
> Thanks. What about Lumens? Should Lumens always equal 1 for parallel 
> lights mimicking the Sun?
> 

With LightsysIV, I never bother with lumens. I use CIE which is part of 
the package:

#include "CIE.inc"
#declare SunCol  = Blackbody(6500)*Intensity; //Sun temperature: 
2800=red; 7500=blue white.

For position and diameter of the Sun, I use:

#declare SunPos  = vrotate(<0, 0, -2>*10e5, <Sun_alt, Sun_azm, 0.0>);
#declare SunDia  = vlength(SunPos)*2/215;  //apparent diameter of the Sun

The later is a formula by Cousin Ricky (somewhen in 2007 I think).

-- 
Thomas

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