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You are correct, but this power is applied within an exponential curve. I
think it's like exp(-x) vs exp(-pow(x,2)), IIRC. we were debating the power
part without mentioning the exp part... or at least that's what I think was
being discussed!
--
Tek
http://evilsuperbrain.com
"scott" <ask### [at] me com> wrote in message news:4528e243@news.povray.org...
>> > OMG... you created something 17,826% better than what I had in
>> > about 20 seconds!
>> Fade_power should be 1 to be physicaly correct. A value of 2 is
>> perfect for a light source, double the distance and get 1/4 the
>> lighting, not for fading trough some substance. Light absorbtion
>> trough a substance is linearly proportional to the thicknessm, double
>> the thickness and the effect double, it don't quadruple.
>
> Physically correct is exponential isn't it? ie a certain % of the light
> gets attenuated each unit distance. If a certain distance only lets 10%
> of the light through, then you'd expect 1% after double the distance,
> 0.01% after 4x etc.
>
>
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> You are correct, but this power is applied within an exponential
> curve.
I don't think it is, read section 3.6.1.6 of the documentation.
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"scott" <ask### [at] mecom> wrote in message news:452909e1@news.povray.org...
>> You are correct, but this power is applied within an exponential
>> curve.
>
> I don't think it is, read section 3.6.1.6 of the documentation.
>
Ah okay, well a 1/x curve...
Anyway, I dunno what's physically correct, but it looks good enough :-)
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Only because no-one has mentioned it yet:
Water will tend to look more realistic with some land underneath it.
Also, try using an image_map on a sky_sphere for your clouds.
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Orchid XP v3 <voi### [at] devnull> wrote:
>
> I only used the complex volumetric sky because I already had one from
> another scene that had the right colours, and I couldn't come up with a
> static pigment with anything approaching the right colours.
>
> For my trouble, I ended up with an entire scene that's bright blue. (Why
> does the real world not do this BTW? The real sky is blue...)
Tek wrote:
> > 4/ water material - a physically correct water is simply: pigment{rgbt
> > 1}finish{reflection{0,1 fresnel}}interior{ior 1.33 fade_colour ...
> > fade_power 2 fade_distance ...}, there's a lot of tweaks you can do but that
> > should get you something good. Basically it's transparent, refractive, the
> > reflections obey fresnel so there's no need to tweak them, and if you want
> > to colour it you should use fade_colour not pigment because the colour
> > should be throughout the material not just at the surface.
>
> The water *itself* is a fairly simple thing, as you say. It's
> transparent, refractive and reflective. So basically it doesn't look
> like anything by itself... gotta have something nice to reflect.
Actually, water really *is* blue. Sky reflection contributes little to the
color of the sea. As someone else mentioned, water reflects the sky color
significantly only at shallow angles, due to the Fresnel effect. It's only
a coincidence that the sky and water are similar colors.
The sky looks blue due to Reyleigh scattering; whatever light is *not*
scattered into the is still present in the direct sunlight. By contrast,
water absorbs (removes) light preferentially from the red end of the
spectrum, leaving blue and green to illuminate the scene. That's why
underwater scenes (without camera lights) look blue, whereas objects on
land do not. Also, shallow water looks greener than deep water because
water absorbs green faster than blue.
The reason a glass or a bathtub full of water looks completely clear is that
water absorbs very little light. If your bathtub were as big as a pond, the
water would be bluish.
In Real Life, bodies of water contain particulates, which often dominate the
color of the water. The slow-moving water at the lower ends of rivers looks
brown because of the silt mixed in. The water of cold ocean currents
contains a lot of plankton, giving it somewhat of a dark greenish cast.
Some rivers contain tannin, which literally dyes the water a reddish-brown
color.
BTW, from what little i know of these things, fade_power 1 seems to make
sense for absorption. (Point light sources, of course, are subject to the
inverse square law, i.e., fade_power 2. The formula POV uses isn't quite
inverse square, but i have a feeling that the Team did it that way to
compensate for the limited dynamic range of the output.)
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"Cousin Ricky" <ric### [at] yahoocom> wrote:
> BTW, from what little i know of these things, fade_power 1 seems to make
> sense for absorption. (Point light sources, of course, are subject to the
> inverse square law, i.e., fade_power 2. The formula POV uses isn't quite
> inverse square, but i have a feeling that the Team did it that way to
> compensate for the limited dynamic range of the output.)
It doesn't make a great deal of difference in rendering terms, but the
physically correct version is usually
fade_power 1000
which triggers the use of exp(-x/D). You tend to see more of the fade_colour
with it than with fade_power 1 or 2.
The exponential comes from Beer's law, which holds for simple homogeneous
materials that don't absorb incredibly strongly (like water).
Tom
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> The exponential comes from Beer's law, which holds for simple homogeneous
> materials that don't absorb incredibly strongly (like water).
OMG... water is blue due to *beer's law*? LMAO!
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>>>> I wouldn't have thought assumed_gamma 1 would have *any* effect...
>>>> surely that just means that each colour component is raised to
>>>> the power of 1 before being output?
>>> Yeah that confused the hell out of me when I first encountered it.
>> Here's a random question: does LCD require gamma correction?
>
> Yes, PC LCDs are designed specifically to match the response of CRTs.
Gah! >_<
Why don't they just make everything linear like it should be?
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>>> My main problem seems to be getting a decent sky. :-S
>>
>> Nope I disagree, if you have a realistic water material from this
>> angle it would be almost completely transparent, the reflection would
>> only show up on the top edge of the ripples (see attached picture).
>
> Mmm, OK. In that case, my water is going to be completely black...
OMG! It actually works... I put some stuff *under* the water and made
the water completely transparent to completely reflective with fresnel
reflection, and suddenly it looks cool! o__O
That's amazing! Heh.
The sky is just plain blue, and it still looks good...
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"Orchid XP v3" <voi### [at] devnull> wrote in message
news:452be6a1$1@news.povray.org...
>
> Why don't they just make everything linear like it should be?
Well the human eye isn't linear, so if you make a linear display device you
end up with horrible colour banding even in 24-bit colour.
And I could make that answer a lot longer if I wanted to... don't tempt
me... ;-)
--
Tek
http://evilsuperbrain.com
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