What is the correct method of calculating and specifying Lumens in 
LightSysIV?

     Light_Color(light_temp, light_lumens)

Do I need to know the distance and diameter of the Sun beforehand? What 
about fade_power and fade_distance?

Thanks.


Michael

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Le 19-06-11 à 21:32, Mike Horvath a écrit :
> What is the correct method of calculating and specifying Lumens in 
> LightSysIV?
> 
>      Light_Color(light_temp, light_lumens)
> 
> Do I need to know the distance and diameter of the Sun beforehand? What 
> about fade_power and fade_distance?
> 
> Thanks.
> 
> 
> Michael

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.

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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?


Michael

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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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