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

Post a reply to this message

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

Post a reply to this message

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

Post a reply to this message

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.

Post a reply to this message

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.

Post a reply to this message

Attachments:
Download 'us-ascii' (1 KB)

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.

Post a reply to this message

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.

Post a reply to this message

Attachments:
Download 'us-ascii' (1 KB)

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

Post a reply to this message

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

-- 
*******************************

* they'll tell you what can't * mailto:ber### [at] inamecom
* be done and why...          * http://www.enst.fr/~jberger
* Then do it.                 *
*******************************

Post a reply to this message