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lpv### [at] gmx de news:412dbf92$1@news.povray.org
> Since reflections/refractions can only occur in rays between camera
> and object, but not between object/lightsource (without photon
> mapping, photon mapping should get the fading automatically correct,
> just like radiosity) it isn't necessary to handle light fading there.
>
But what about shadow/light-test rays (not camera-rays) that got refracted?
--
http://www.raf256.com/3d/
Rafal Maj 'Raf256', home page - http://www.raf256.com/me/
Computer Graphics
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Rafal 'Raf256' Maj nous apporta ses lumieres ainsi en ce 26/08/2004
07:35... :
>sev### [at] NOT_THISsibafi news:412dca8d$1@news.povray.org
>
>
>
>Hmm yes. (Btw - what about old "caustics" finish?).
>So applying fading to photons should work all right.
>
>
>
The old caustics are still there and fully useable. The problem, is that
those are distance independent. So, you take a refractive sphere, shine
light thrue it, and the bright spot stay the same size 1 unit away and
1000 units away. It's OK for a water surface casting caustics under it,
but that's all.
Alain
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"Ross" <rli### [at] everestkcnet> wrote:
> from the documentation "Reflected or refracted light is not attenuated by
> distance." is this because it is computationally too expensive, or is this
> just how light works in nature?
To do so, simply add media to the object (iirc, this is one of the things
media was introduced for).
....Chambers
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Among other things, wrote:
> refracted light is not governed by fade_distance/fade_power is it? i think
> i recall reading that in the docs of 3.5. (opening docs for 3.6...) i
> always wondered why. for instance if you have a candle (point light) in a
> glass candle holder, nearly all the light would be defracted, and
> therefore, not fade with distance.
>
> from the documentation "Reflected or refracted light is not attenuated by
> distance." is this because it is computationally too expensive, or is this
> just how light works in nature?
I will guess an explanation.
The "physical" fade_power=2 comes, as explained, from a geometric/topologic
fact and, ultimately, from the light traveling in (almost) straight lines.
If light travels in straight lines, in all directions, the "amount" of
light per unit area (the perceived lighting intensity) diminishes with the
square of distance, just because the surface (of a sphere, for example)
grows with the square of distance (the sphere's radius).
This effect could be obtained by using forward raytracing: Shoot light rays
in every direction, then count how many rays reach a given
object/area/surface. Using photons in POV-Ray would, I guess, reproduce the
"inverse square law" without need of any kind of fade_power, because it's
intrinsic to the way light propagates (and it's correctly simulated in this
case).
With backward raytracing, however, there is no easy way to know how many of
the "original" light rays will have reached a given object, all you know is
it is possible for a ray to reach the object. But, assuming the light
travels in straight line, it is safe (and realistic) to just use a
fade_power=2 to simulate the light fading. As soon as the light doesn't
travel in a straight line, which happens when it is "reflacted" or
"refrected" the assumption is not valid and the "fade_power=2" could be
plain wrong, you have to take into account all the possible ways a ray
could reach the object.
Summing it up: fade_power=2 is a quick and useful way to simulate light when
it travels in straight lines, but it cannot be used when it doesn't!
Now, this doesn't mean that's the actual reason it works like that in
POV-Ray. I haven't thought quite deeply about the physics and the "POV-way"
in this phenomenon and I could be wrong.
--
light_source{9+9*x,1}camera{orthographic look_at(1-y)/4angle 30location
9/4-z*4}light_source{-9*z,1}union{box{.9-z.1+x clipped_by{plane{2+y-4*x
0}}}box{z-y-.1.1+z}box{-.1.1+x}box{.1z-.1}pigment{rgb<.8.2,1>}}//Jellby
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