In article <3fac85b4@news.povray.org>, "gonzo" <rgo### [at] lansetcom> wrote:

> That's the best explanation I've seen yet of light_ source output, thank
> you.  My question is, what does fade_power do?
> 
> If I have a light_source with <7.5,5,1> it's output is white,  however using
> a light_source of <.75,.5,.1> with a fade_power of 10 & fade_distance >
> distance_to_camera, it's output still looks amber.

Your < 7.5, 5, 1> light source only *looks* white when shining on a 
surface where the reflected color is >= < 1, 1, 1>. Internally, it is 
really < 7.5, 5, 1>, and avery dark surface, thick media, or other 
circumstances can give visible effects from these high light 
intensities. Distance fading is pretty self-explanatory: it makes the 
light fade with distance.
http://povray.org/documentation/view/172/#s06_05_07_09

So with a fade_distance of 1 and fade_power 2, the light intensity from 
your < 7.5, 5, 1> light source at a distance of 5 units will be:
< 0.576923077, 0.384615385, 0.0769230769>

A fade_power of 2 is most realistic (approximating the inverse-square 
law), 10 doesn't correspond to any real-world effect. And fade_distance 
is just used to control the falloff rate, it has nothing to do with the 
camera position. It is really the same as adjusting the brightness of 
the light, though it is not implemented that way internally. The 
distance given to fade_distance is the distance where the light is at 
the specified brightness, it decreases with distance from there, and 
increases as the distance drops below fade_distance. (the documentation 
appears to be slightly incorrect about this: only the ratio of 
distance/fade_distance matters, not the size of fade_distance in 
relation to 1)

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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