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> If I remember my physics correctly, fade_power should be 2 in order to
> reassemble reality.
But doesn't that depend on ones measuring of distance? I personally use 1
unit = 1 centimeter but others do something else.
Hugo
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> > If I remember my physics correctly, fade_power should be 2 in order to
> > reassemble reality.
>
> But doesn't that depend on ones measuring of distance? I personally use 1
> unit = 1 centimeter but others do something else.
No. The fade_power doesn't depend on the measuring of distance.
fade_distance takes care of that, I believe.
- Slime
[ http://www.slimeland.com/ ]
[ http://www.slimeland.com/images/ ]
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For ideal diffuse source (POV 3.5):
fade_power 2 fade_distance sqrt(area-of-emitting-surface)
spotlight radius -90 falloff 90 tightness 1
Dividing the source to multiple smaller sources improves
the intensity simulation for objects close to to the source.
Same applies when the shape of the source differs much from
circle or square.
When the source is not diffuse but directs light like a spot light
fade_distance should be increased. The modifier depends on the
spotlight parameters. For spotlight radius 0 falloff FO tightness 0
my guess for the modifier would be 90^2/FO^2, until I find
out how lighting engineers calculate spotlights.
_____________
Kari Kivisalo
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From: Tim Nikias
Subject: Re: Question on realistic lighting (using fade_power)
Date: 22 Jan 2002 00:34:33
Message: <3C4CF9C4.63E31DA5@gmx.de>
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>
> fade_power 2 fade_distance sqrt(area-of-emitting-surface)
> spotlight radius -90 falloff 90 tightness 1
>
radius -90 ?! shouldn't that normally be something like radius >0?
But good explanation nontheless...
--
Tim Nikias
Homepage: http://www.digitaltwilight.de/no_lights/index.html
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Tim Nikias wrote:
>
> radius -90 ?! shouldn't that normally be something like radius >0?
Yes, normally. This is advanced stuff :) It's in the manual
but -90 is needed to enable cosine distribution for the spot light.
It's needed to simulate how the brightness and projected area of
planar diffuse source change as a fuction of viewing angle.
_____________
Kari Kivisalo
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From: Hugo
Subject: Re: Question on realistic lighting (using fade_power)
Date: 22 Jan 2002 08:08:50
Message: <3c4d6462@news.povray.org>
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> > > If I remember my physics correctly, fade_power should be 2 in order to
> > > reassemble reality.
> >
> > But doesn't that depend on ones measuring of distance? I personally use
1
> > unit = 1 centimeter but others do something else.
>
> No. The fade_power doesn't depend on the measuring of distance.
> fade_distance takes care of that, I believe.
I don't understand that. According to the manual and to my experiences,
"fade_distance" is the distance from the light source at which the light
keeps a constant intensity.. After this distance has been reached, intensity
decreases with the speed of "fade_power".. I can't see this doesn't relate
to ones measuring of distance..
Hugo
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Hugo wrote:
>
> > The fade_power doesn't depend on the measuring of distance.
>
> I don't understand that.
intensity = 2/( 1 + (d/fade_distance)^fade_power )
At the surface the intensity is 2 and at fade_distance the intensity
is 1. It's a continous smooth function. Take a scene and scale it by
10 but don't modify fade_power. You will see that fade_power doesn't
depend on the measuring of distance. 1/d^2 for intensity holds true
for point sources in RL, why not in povray.
The more complicated equation in povray simulates how light sources
with non-zero dimensions behave at close range. It's the result of
integrating multiple point sources over an area, I believe. When d
is large enough the equation simplifies to 1/d^2 (in a broad sense).
I verified that this equation works for planar diffuse sources:
http://news.povray.org/povray.binaries.images/19212/
I assumed of course, that povray radiosity works :)
_____________
Kari Kivisalo
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On Tue, 22 Jan 2002 14:14:26 +0100, "Hugo" <hua### [at] post3 teledk> wrote:
>I don't understand that. According to the manual and to my experiences,
>"fade_distance" is the distance from the light source at which the light
>keeps a constant intensity..
It is IIRC the distance at which the light is at half its nominal
intensity.
Peter Popov ICQ : 15002700
Personal e-mail : pet### [at] vipbg
TAG e-mail : pet### [at] tagpovrayorg
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From: Hugo
Subject: Re: Question on realistic lighting (using fade_power)
Date: 23 Jan 2002 06:57:32
Message: <3c4ea52c@news.povray.org>
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Thanks for your explanation! This time I understood. I checked the POV docs
again to see what used to confuse me, and find *two* sections explaining
the same thing:
6.6.6 Attenuation
6.5.7.9 Light Fading
I only knew of 6.5.7.9 until now, which has a very confusing sound compared
to the other...
Hugo
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From: Jaime Vives Piqueres
Subject: Re: Question on realistic lighting (using fade_power)
Date: 24 Jan 2002 04:34:32
Message: <3c4fd528@news.povray.org>
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Kari Kivisalo wrote:
> my guess for the modifier would be 90^2/FO^2, until I find
> out how lighting engineers calculate spotlights.
I've not still tried to understand this:
http://www.escape.ca/~williams/lxspot11.htm
but I think it contains interesting data.
--
Jaime Vives Piqueres
La Persistencia de la Ignorancia
http://www.ignorancia.org
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