I have been wondering how accurate the angle dependent reflection
patch is. Since I live near a lake, I went there to look for answers.
I came away confused. I couldn't see *any* difference in amount or
quality of the water's reflections based upon viewing angle. What is
the supposed physical property that this patch is trying to simulate?
Is it based in physics at all, or is it just an estheticly pleasing
modification of the image? 

Does anyone know the answers?

Later,
Glen

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The effect is real.  It's more noticable if you are on higher ground
looking down on the water.  I put an example in binaries.images.

-Mike

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On Fri, 30 Jul 1999 03:54:49 -0500, Mike <Ama### [at] aolcom> wrote:

>The effect is real.  It's more noticable if you are on higher ground
>looking down on the water.  I put an example in binaries.images.
>
>-Mike

I will say that I have noticed that it's harder to see into a body of
water when viewing at a small angle. As for the brightness of the
surface reflections themselves, they always seemed relatively constant
to me. 

Now the proportion of surface reflection intensity vs. the intensity
of light transmitted through of a body of water will certainly vary,
but the only variance that I can see is in the transmitted rays of
light. I might be wrong, but I believe a similar principle is at work
here as in fiber optics. At some point, there is probably total
internal reflection of the light from under the surface and images
from the bottom don't reach the viewer's eye, at least in the case of
very small angles of view. POV-Ray might already handle total internal
reflection effects, but I don't know for sure.

As for the image you have posted, I am left wondering what sort of
material is on the hillside above the rocks. By any chance is the
hillside a darker color than the rocks? That would explain the darker
reflections closest to the viewer. As for the stick's reflection being
dimmer, it seems the stick is not as bright as the rocks to begin
with. I also think that since the stick is so slender an object, that
it's reflection in the perturbed surface of the water would have some
"holes" in the reflection. That is to say, since the reflecting
surface is perturbed, some of the areas where you would expect to see
the reflection of the stick, are actually reflecting some other nearby
object instead. In the case of the rocks, they are a much larger area.
It would take much more surface distortion to "lose" the image of the
rocky area. Instead of parts of the warped surface reflecting an
object other than a rock, it is simply reflecting light from a
different rock instead and the overall brightness is about the same. 

Later,

Glen Berry

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Mike wrote:

> The effect is real.  It's more noticable if you are on higher ground
> looking down on the water.  I put an example in binaries.images.

Better yet, look at a puddle.

When it's in the distance, it is very shiny, like a mirror. You cannot
see the ground beneath the water. As you approach it, and view it at
steeper and steeper angles, the reflection seems to go down, and you can
see through it more easily. When you are directly above the puddle, it
seems to be almost completely transparant, and hardly reflective at all.

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When I read this statement of yours I immediately thought of mirages. I
wonder if mirages are caused in part to this natural effect?

Samuel benge


Simon de Vet wrote:

> Mike wrote:
>
> > The effect is real.  It's more noticable if you are on higher ground
> > looking down on the water.  I put an example in binaries.images.
>
> Better yet, look at a puddle.
>
> When it's in the distance, it is very shiny, like a mirror. You cannot
> see the ground beneath the water. As you approach it, and view it at
> steeper and steeper angles, the reflection seems to go down, and you can
> see through it more easily. When you are directly above the puddle, it
> seems to be almost completely transparant, and hardly reflective at all.

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I'd say "yes".

I mentioned it in a different thread here, wondering the same thing myself.

Section 7 in
http://www.islandnet.com/~see/weather/eyes/wxwatch2.htm


And then here's a nice paper with lots of math and such
http://www.ncas.org/condon/s6chap04.htm

And then a page on rendering mirages:
http://www.cs.man.ac.uk/aig/mash/MSc_stuff/render.html


"SamuelT." wrote:

> When I read this statement of yours I immediately thought of mirages. I
> wonder if mirages are caused in part to this natural effect?
>
> Samuel benge
>
> Simon de Vet wrote:
>
> > Mike wrote:
> >
> > > The effect is real.  It's more noticable if you are on higher ground
> > > looking down on the water.  I put an example in binaries.images.
> >
> > Better yet, look at a puddle.
> >
> > When it's in the distance, it is very shiny, like a mirror. You cannot
> > see the ground beneath the water. As you approach it, and view it at
> > steeper and steeper angles, the reflection seems to go down, and you can
> > see through it more easily. When you are directly above the puddle, it
> > seems to be almost completely transparant, and hardly reflective at all.

--
"My new computer's got the clocks, it rocks
But it was obsolete before I opened the box" - W.A.Y.

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I believe so.  Air of different tempuratures have slighter different IORs.
That's why hot air rising casts shadows like ripples on the ground and
distorts the view.  Hot air has the lower density, so the viewing ray is going
from a higher IOR to a lower one on the ground.  At a shallow enough angle,
TIR should occur.

Mostly speculation on my part. :)

-Mike

SamuelT. wrote:

> When I read this statement of yours I immediately thought of mirages. I
> wonder if mirages are caused in part to this natural effect?
>

Post a reply to this message

"SamuelT." wrote:

> When I read this statement of yours I immediately thought of mirages. I
> wonder if mirages are caused in part to this natural effect?

	Yes. It is a total reflection angle that is dependent upon the
IOR of what is reflecting it. Air near the ground is hotter,
decreasing its IOR.

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Matt Giwer wrote:
> 
> "SamuelT." wrote:
> 
> > When I read this statement of yours I immediately thought of mirages. I
> > wonder if mirages are caused in part to this natural effect?
> 
>         Yes. It is a total reflection angle that is dependent upon the
> IOR of what is reflecting it. Air near the ground is hotter,
> decreasing its IOR.

 I meant to bring this up with a couple of the other comments made to this
thread and that is that it's not a reflection action that occurs rather
it is a refraction through the differing IOR's that makes the phenomenon
happen. Reflection and refraction are two different physical occurrences
and they are in most cases mutually exclusive.

-- 
Ken Tyler

See my 700+ Povray and 3D Rendering and Raytracing Links at:
http://home.pacbell.net/tylereng/index.html

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Ken wrote:
> 
> Matt Giwer wrote:
> >
> > "SamuelT." wrote:
> >
> > > When I read this statement of yours I immediately thought of mirages. I
> > > wonder if mirages are caused in part to this natural effect?
> >
> >         Yes. It is a total reflection angle that is dependent upon the
> > IOR of what is reflecting it. Air near the ground is hotter,
> > decreasing its IOR.
> 
>  I meant to bring this up with a couple of the other comments made to this
> thread and that is that it's not a reflection action that occurs rather
> it is a refraction through the differing IOR's that makes the phenomenon
> happen. Reflection and refraction are two different physical occurrences
> and they are in most cases mutually exclusive.
> 
	??? What do you mean "mutually exclusive"? In most cases, both occur
simultaneously (each time you change medium: air/water, air/glass, ...).
The only cases I can think of where you can have one without the other
is the mirage (or anytime the IOR changes continuously) where you only
have refraction and the metallic surface where you have no transmitted
light (and therefore no refraction...)

		Jerome

-- 
*******************************
* Always listen to experts,   * Jérôme M. BERGER
* they'll tell you what can't * mailto:ber### [at] inamecom
* be done and why...          * http://www.enst.fr/~jberger
* Then do it.                 *
*******************************

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I believe Ken means light only enters the medium and is refracted if it is not
reflected. Light can be partially reflected or refracted, but only one of the
two can happen on a photon by photon basis.

Margus

"Jerome M. BERGER" wrote:
> 
>         ??? What do you mean "mutually exclusive"? In most cases, both occur
> simultaneously (each time you change medium: air/water, air/glass, ...).
> The only cases I can think of where you can have one without the other
> is the mirage (or anytime the IOR changes continuously) where you only
> have refraction and the metallic surface where you have no transmitted
> light (and therefore no refraction...)
> 
>                 Jerome
> 
> --
> *******************************
> * Always listen to experts,   * Jérôme M. BERGER
> * they'll tell you what can't * mailto:ber### [at] inamecom
> * be done and why...          * http://www.enst.fr/~jberger
> * Then do it.                 *
> *******************************

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Margus Ramst wrote:
> 
> I believe Ken means light only enters the medium and is refracted if it is not
> reflected. Light can be partially reflected or refracted, but only one of the
> two can happen on a photon by photon basis.
> 
> Margus

Ior is a property of a material. Reflection is a property of a surface. At least
this was the rational for moving ior into it's own property wrapper i.e. interior.
I know that surfaces can be considered to have an ior value but this is generaly
a very minor property where roughness and color have a much greater impact than
does a surfaces ior value will.

  To really clarify my point air does not generaly reflect light. It will transmit
and refract light in varying amounts however. Shiny steel will reflect light redily
but has extremely poor light transmittance.


-- 
Ken Tyler

See my 700+ Povray and 3D Rendering and Raytracing Links at:
http://home.pacbell.net/tylereng/index.html

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Margus Ramst wrote in message <37B0DFCB.4BB5598C@peak.edu.ee>...
>I believe Ken means light only enters the medium and is refracted if it is
not
>reflected. Light can be partially reflected or refracted, but only one of
the
>two can happen on a photon by photon basis.


Take this one up with the quantum physicists -- they will tell you that a
photon can be simultaneously reflected and refracted. :-)

Mark

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>Ior is a property of a material. Reflection is a property of a
surface.<
Hmm, actually, they can both be considered properties of the boundaries
between two materials(and the same substance at a different temperature
could be considered a different material.) And gaseous materials do
reflect light, although usually in small amounts.(although clouds, fog,
etc are not examples of this because they are droplets of liquid
suspended in the air.)
Of course, this has nothing to do with the way POV considers it...

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Ken wrote:
> 
> Margus Ramst wrote:
> >
> > I believe Ken means light only enters the medium and is refracted if it is not
> > reflected. Light can be partially reflected or refracted, but only one of the
> > two can happen on a photon by photon basis.
> >
> > Margus
> 
> Ior is a property of a material. Reflection is a property of a surface. At least
> this was the rational for moving ior into it's own property wrapper i.e. interior.
> I know that surfaces can be considered to have an ior value but this is generaly
> a very minor property where roughness and color have a much greater impact than
> does a surfaces ior value will.
> 
>   To really clarify my point air does not generaly reflect light. It will transmit
> and refract light in varying amounts however. Shiny steel will reflect light redily
> but has extremely poor light transmittance.
> 
> --
> Ken Tyler
> 
> See my 700+ Povray and 3D Rendering and Raytracing Links at:
> http://home.pacbell.net/tylereng/index.html

	Well, actually I think that reflection depends on the ior (or rather on
the difference of iors between two materials) which in turn depends on
its electrical conductivity (is that the right word?). At least, that's
what happens when an electromagnetic wave hits a metal (please don't
think in photons as they are governed by quantic laws, which I'm not
familiar with): the electric field in the wave agitates the metal's
surface electrons creating an eletric current (since the metal is a
conductor) which in turn generates a new electromagnetic wave perfectly
in sync with the incoming one but of opposite sign. This wave is of
course emitted in all directions which means it *cancels* the incoming
wave inside the metal and creates what we call "reflection" outside.
When the material isn't as good an electrical conductor, the new wave
isn't as strong as the incoming one and that's what causes partial
reflection and refraction. Moreover the exact effect depends on the
angle between the *eletrical field* (not the direction of travel) and
the surface, therefore: angle dependant effects and polarization
dependant effects. This same effect causes absorption and scattering: as
the wave travels through a slightly conductive material, it will agitate
more and more electrons along the way and each time a small fraction of
the wave gets cancelled out and reemitted in other directions.

	Okay, there's just one problem with this model: I don't remember how it
accounts for materials like wood that neither reflect nor transmit any
light...

	Just my 2c,
		Jerome

-- 
*******************************
* Always listen to experts,   * Jérôme M. BERGER
* they'll tell you what can't * mailto:ber### [at] inamecom
* be done and why...          * http://www.enst.fr/~jberger
* Then do it.                 *
*******************************

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Jerome M. BERGER <jbe### [at] enstfr> wrote:
: 	Okay, there's just one problem with this model: I don't remember how it
: accounts for materials like wood that neither reflect nor transmit any
: light...

  Actually wood reflects light. If it didn't, you would see it as black.
And a white object in a room with wooden walls will certainly be colored
differently than in a room with white walls.
  I think that the color of the wood (or any material) depends on the
frequency of the electrons the light affects.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Nieminen Mika wrote:
> 
> Jerome M. BERGER <jbe### [at] enstfr> wrote:
> :       Okay, there's just one problem with this model: I don't remember how it
> : accounts for materials like wood that neither reflect nor transmit any
> : light...
> 
>   Actually wood reflects light. If it didn't, you would see it as black.
> And a white object in a room with wooden walls will certainly be colored
> differently than in a room with white walls.
>   I think that the color of the wood (or any material) depends on the
> frequency of the electrons the light affects.

  Yeah don't get me started on the properties of "blackbody" objects. I got
into a lengthy argument not to long ago in one of sci.electronics groups
about the thermal radiation and emmisivity characteristics of variously
colored heat sink materials and don't want to relive that again for a while.
I was right of course but they just wouldn't listen to my common sense:)

-- 
Ken Tyler

See my 700+ Povray and 3D Rendering and Raytracing Links at:
http://home.pacbell.net/tylereng/index.html

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No, thanks. They could well be BS-ing and I'd have no way of finding out :)

Margus

Mark Wagner wrote:
> 
> Margus Ramst wrote in message <37B0DFCB.4BB5598C@peak.edu.ee>...
> >I believe Ken means light only enters the medium and is refracted if it is
> not
> >reflected. Light can be partially reflected or refracted, but only one of
> the
> >two can happen on a photon by photon basis.
> 
> Take this one up with the quantum physicists -- they will tell you that a
> photon can be simultaneously reflected and refracted. :-)
> 
> Mark

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Nieminen Mika wrote:
> 
> Jerome M. BERGER <jbe### [at] enstfr> wrote:
> :       Okay, there's just one problem with this model: I don't remember how it
> : accounts for materials like wood that neither reflect nor transmit any
> : light...
> 
>   Actually wood reflects light. If it didn't, you would see it as black.
> And a white object in a room with wooden walls will certainly be colored
> differently than in a room with white walls.
>   I think that the color of the wood (or any material) depends on the
> frequency of the electrons the light affects.
> 
> --
> main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
> ):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

	Yes, what I meant was that you can't see any reflections on a wood
surface (unless it is higly polished and varnished). The effect that
causes wood to become visible is probably different from the one I
described since it is independant on the viewing direction...

		Jerome

-- 
*******************************
* Always listen to experts,   * Jérôme M. BERGER
* they'll tell you what can't * mailto:ber### [at] inamecom
* be done and why...          * http://www.enst.fr/~jberger
* Then do it.                 *
*******************************

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"Jerome M. BERGER" wrote:
> 
> Nieminen Mika wrote:

> 
>         Yes, what I meant was that you can't see any reflections on a wood
> surface (unless it is higly polished and varnished). The effect that
> causes wood to become visible is probably different from the one I
> described since it is independant on the viewing direction...
> 
>                 Jerome

  Actually wood cellulose is highly reflective. This cam be seen when
viewed with a simple magnifying glass. It's surface properties are so
porous and broken into so many angles that it is not redily apparent
from casual inspection.

-- 
Ken Tyler

See my 700+ Povray and 3D Rendering and Raytracing Links at:
http://home.pacbell.net/tylereng/index.html

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Jerome M. BERGER <jbe### [at] enstfr> wrote:
: 	Yes, what I meant was that you can't see any reflections on a wood
: surface (unless it is higly polished and varnished). The effect that
: causes wood to become visible is probably different from the one I
: described since it is independant on the viewing direction...

  I think it is angle dependant. That's why we see specular highlights.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

Post a reply to this message

Matt Giwer wrote:
> 
> "SamuelT." wrote:
> 
> > When I read this statement of yours I immediately thought of mirages. I
> > wonder if mirages are caused in part to this natural effect?
> 
>         Yes. It is a total reflection angle that is dependent upon the
> IOR of what is reflecting it. Air near the ground is hotter,
> decreasing its IOR. 

	I forgot about this discussion. 

	At some critical (shallow) angle there is "total" reflection.
Debating total is left to the purists. There is also refraction
agreed. 

	Now as to the mirage case ...

	The total reflection plane is the closest to the ground. That is
why it look blue because it is reflecting the sky. Above that is
the refraction area. As it is rising heat, there is turbulence.
That is what makes the reflected sky appear to ripple and thus
appear to be water -- a fashion only a person desperate for water
would imagine. Also note the critical angle surface is also
rising heat so it is also turbulent and will contribute to the
water-like appearance. 

	Generally we are familiar with a critical angle for water and
glass. The critical angle is dependent upon the difference in
IORs for the media. The less the difference, the shallower the
angle and the further away the mirage water appears to be. 

	In the mirage case we are not dealing with our usual experience
with water and glass but with a material with a turbulent
boundary. We have both refraction and reflection at work. What we
can say is that refraction is in play higher than the reflection
height but we can not see refraction effects below that
reflection height. 

	This is the sort of thing that makes physics fun. It is
separating two phenomena and they seeing how they interact.
Newton's first law of motion (stay in motion) would have been
known to cavemen were it not for friction. Once motion and
friction were split there was progress. 

-- 
http://www.giwersworld.org/artiii/

Oh my God! They've rendered Kenny!

How to profit from the end of civilization as know available here 
soon.

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Ken wrote:

> Ior is a property of a material. Reflection is a property of a surface. At least
> this was the rational for moving ior into it's own property wrapper i.e. interior.
> I know that surfaces can be considered to have an ior value but this is generaly
> a very minor property where roughness and color have a much greater impact than
> does a surfaces ior value will.

	Yes but ... a surface can be defined as a constant IOR from any
cause. In the case of air the isosurfaces for temperature and IOR
would be the same. You can go through any perturbed volume and
define a surface as all points with the same perturbation value,
be it temperature, IOR, electric or magnetic potential. 

>   To really clarify my point air does not generaly reflect light. It will transmit
> and refract light in varying amounts however. Shiny steel will reflect light redily
> but has extremely poor light transmittance.

	However, when the IOR varies there is an angle at which incident
light is reflected. The smaller that difference, the shallower
that angle. 

-- 
http://www.giwersworld.org/artiii/

Oh my God! They've rendered Kenny!

How to profit from the end of civilization as know available here 
soon.

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Nieminen Mika wrote:

>   Actually wood reflects light. If it didn't, you would see it as black.
> And a white object in a room with wooden walls will certainly be colored
> differently than in a room with white walls.
>   I think that the color of the wood (or any material) depends on the
> frequency of the electrons the light affects.

	I think here we are getting into coherent (mirror, waxed wood)
and incoherent (regular wood) reflection. If we could smooth
regular wood to a fraction of a wavelength of light it would be a
mirror also. To the mirage case, were there no turbulence people
would clearly see any clouds in the sky. If there were more
turbulence such as a wind the it would not be smooth enough to
reflect an image. 

	As to color, yes it is a matter of what is reflected and
absorbed. Due to the wavelength of light it is most always only
chemical bond lengths that affect the color rather than
monatomic. (I think it is always rather than most always but I'd
have to think about it.) 

-- 
http://www.giwersworld.org/artiii/

Oh my God! They've rendered Kenny!

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Mark Wagner wrote:

> Take this one up with the quantum physicists -- they will tell you that a
> photon can be simultaneously reflected and refracted. :-)

	Yes but that is just the way we have to make it fit the macro
world models we have. We could always solve the quantum equations
instead after we break 4096 bit PGP in a year to test the
processor's speed. 

-- 
http://www.giwersworld.org/artiii/

Oh my God! They've rendered Kenny!

How to profit from the end of civilization as know available here 
soon.

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