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To follow up on this I posted two images in p.b.i showing how it would work.
In one image I used standard light attenuation, and in the second I have
added a black fog with the distance set the same as the attenuation distance
of the light sources, note that the light sources have a white sphere with a
high ambient value applied to them, so they do not fade as much as the red
spheres.
-tgq
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On Mon, 22 Oct 2001 14:35:57 -0400, Trevor Quayle wrote:
> work that way). As Ron shows, one way to simulate this is with absorbing
> media. As a quicker method, you could use constant fog with a black
> pigment, as the features of media aren't really required here. Adding this
You have greatly misunderstood the point of my demonstration.
That absorbing media is funny. Notice how it doesn't do anything to the
first five green cylinders, but the other five (the reflections of the
first five in the surface of the sphere) fade out? There's something special
going on there that you can't do with fog.
For more fun, try adding another reflective sphere to the scene somewhere and
notice how it doesn't behave the same as the one that's there now. So
there's something about that absorbing media that's not only strange from
the camera's point of view, but also from that one sphere's point of view.
The answer, of course, lies in the two spheres that are differenced out of
the media, and in the no_image keyword. It's hard to state the exact rules
that dictate when the media appears and when it doesn't, but I'll give it
a shot:
- a camera ray that originates inside the media container is affected. This
might very well be a bug, depending on whether you believe that the no_image
keyword should affect the media in the interior of the object. Luckily,
even if this bug is fixed it won't matter for this technique.
- a camera ray that originates outside the media container is not affected,
because its intersections with the media container are ignored due to the
no_image.
- When the camera is outside the media container, a reflected ray that
originates inside the media container is not affected. This is because
we only add the media to the list of things to consider when the ray
enters the media container, which doesn't happen in this case unless
the ray happens to intersect a "bubble" in the container. This fact
is probably also a bug. If it's fixed, it will limit the technique
considerably (requiring all reflective objects in your scene to fade,
and requiring them all to fade to the same color.) It probably should
be fixed, but fixing it seems like it would cause a performance hit,
and it's not as though it's really a real-world feature anyway. The
part about the "bubbles" has some effect on what sort of objects and
scene geometry you can use, but not too much.
- When a reflected ray originates outside the media container, it is
always affected by the media. This is the only part that really makes
sense about the whole thing. :)
--
#macro R(L P)sphere{L __}cylinder{L P __}#end#macro P(_1)union{R(z+_ z)R(-z _-z)
R(_-z*3_+z)torus{1__ clipped_by{plane{_ 0}}}translate z+_1}#end#macro S(_)9-(_1-
_)*(_1-_)#end#macro Z(_1 _ __)union{P(_)P(-_)R(y-z-1_)translate.1*_1-y*8pigment{
rgb<S(7)S(5)S(3)>}}#if(_1)Z(_1-__,_,__)#end#end Z(10x*-2,.2)camera{rotate x*90}
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On 22 Oct 2001 16:23:35 -0400, Ron Parker wrote:
> On Mon, 22 Oct 2001 14:35:57 -0400, Trevor Quayle wrote:
>> work that way). As Ron shows, one way to simulate this is with absorbing
>> media. As a quicker method, you could use constant fog with a black
>> pigment, as the features of media aren't really required here. Adding this
>
> You have greatly misunderstood the point of my demonstration.
>
> That absorbing media is funny. Notice how it doesn't do anything to the
> first five green cylinders, but the other five (the reflections of the
> first five in the surface of the sphere) fade out? There's something special
> going on there that you can't do with fog.
Oh, and just for fun, try adding a sky_sphere. One that's not black. I'll
wait here while you try that...
sky_sphere {pigment {color blue 1}}
So, uh... shouldn't the background still be black, or at least darker than
blue 1? There really is something funny going on here.
--
#macro R(L P)sphere{L __}cylinder{L P __}#end#macro P(_1)union{R(z+_ z)R(-z _-z)
R(_-z*3_+z)torus{1__ clipped_by{plane{_ 0}}}translate z+_1}#end#macro S(_)9-(_1-
_)*(_1-_)#end#macro Z(_1 _ __)union{P(_)P(-_)R(y-z-1_)translate.1*_1-y*8pigment{
rgb<S(7)S(5)S(3)>}}#if(_1)Z(_1-__,_,__)#end#end Z(10x*-2,.2)camera{rotate x*90}
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Actually I was commenting on how to fade reflections, not the funniness with
media you pointed out, I had just mentioned your post in regards to the
using of media for this.
-tgq
"Ron Parker" <ron### [at] povray org> wrote in message
news:slr### [at] fwicom...
> On Mon, 22 Oct 2001 14:35:57 -0400, Trevor Quayle wrote:
> > work that way). As Ron shows, one way to simulate this is with absorbing
> > media. As a quicker method, you could use constant fog with a black
> > pigment, as the features of media aren't really required here. Adding
this
>
> You have greatly misunderstood the point of my demonstration.
>
> That absorbing media is funny. Notice how it doesn't do anything to the
> first five green cylinders, but the other five (the reflections of the
> first five in the surface of the sphere) fade out? There's something
special
> going on there that you can't do with fog.
>
> For more fun, try adding another reflective sphere to the scene somewhere
and
> notice how it doesn't behave the same as the one that's there now. So
> there's something about that absorbing media that's not only strange from
> the camera's point of view, but also from that one sphere's point of view.
>
> The answer, of course, lies in the two spheres that are differenced out of
> the media, and in the no_image keyword. It's hard to state the exact
rules
> that dictate when the media appears and when it doesn't, but I'll give it
> a shot:
>
> - a camera ray that originates inside the media container is affected.
This
> might very well be a bug, depending on whether you believe that the
no_image
> keyword should affect the media in the interior of the object. Luckily,
> even if this bug is fixed it won't matter for this technique.
>
> - a camera ray that originates outside the media container is not
affected,
> because its intersections with the media container are ignored due to
the
> no_image.
>
> - When the camera is outside the media container, a reflected ray that
> originates inside the media container is not affected. This is because
> we only add the media to the list of things to consider when the ray
> enters the media container, which doesn't happen in this case unless
> the ray happens to intersect a "bubble" in the container. This fact
> is probably also a bug. If it's fixed, it will limit the technique
> considerably (requiring all reflective objects in your scene to fade,
> and requiring them all to fade to the same color.) It probably should
> be fixed, but fixing it seems like it would cause a performance hit,
> and it's not as though it's really a real-world feature anyway. The
> part about the "bubbles" has some effect on what sort of objects and
> scene geometry you can use, but not too much.
>
> - When a reflected ray originates outside the media container, it is
> always affected by the media. This is the only part that really makes
> sense about the whole thing. :)
>
> --
> #macro R(L P)sphere{L __}cylinder{L P __}#end#macro P(_1)union{R(z+_
z)R(-z _-z)
> R(_-z*3_+z)torus{1__ clipped_by{plane{_ 0}}}translate z+_1}#end#macro
S(_)9-(_1-
> _)*(_1-_)#end#macro Z(_1 _
__)union{P(_)P(-_)R(y-z-1_)translate.1*_1-y*8pigment{
> rgb<S(7)S(5)S(3)>}}#if(_1)Z(_1-__,_,__)#end#end Z(10x*-2,.2)camera{rotate
x*90}
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On Mon, 22 Oct 2001 16:50:28 -0400, Trevor Quayle wrote:
> Actually I was commenting on how to fade reflections, not the funniness with
> media you pointed out, I had just mentioned your post in regards to the
> using of media for this.
But the point is, what you suggested fades everything, not just reflections.
--
#local R=rgb 99;#local P=R-R;#local F=pigment{gradient x}box{0,1pigment{gradient
y pigment_map{[.5F pigment_map{[.3R][.3F color_map{[.15red 99][.15P]}rotate z*45
translate x]}]#local H=pigment{gradient y color_map{[.5P][.5R]}scale 1/3}[.5F
pigment_map{[.3R][.3H][.7H][.7R]}]}}}camera{location.5-3*z}//only my opinions
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Trevor Quayle wrote:
>
> as an equally illuminated object gets further from
> the camera, it does not fade (which in reality, it should if direct light
> gets attenuated
No, this is not what happens in reality. The energy arriving
from an object when viewed through a constant space angle stays
the same because the observed area (and energy) on the object
increases when viewed from greater distance. The 1/r^2 attenuation
and increase of observed area cancel each other out. The brightness
(pixel values) of surfaces is not a function of distance.
_____________
Kari Kivisalo
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What I was trying to show here was that reflections don't fade as a result
of reflective properties (not counting variable reflection, whereby the
reflection fades with the viewing angle). The intensity of light does fade
over distance (1/r^2), the energy stays the same but it is spread over a
larger area:
http://imagine.gsfc.nasa.gov/YBA/M31-velocity/1overR2-more.html,
http://www2.ncsu.edu/ncsu/pams/science_house/CBLable/resources.html
Thus, making objects further away appear less bright. POVRay can do light
fading, but, as the docs state, it is only applied to direct illumination
from light sources, reflected or diffuse light does not get attenuated. If
there is any fading of a reflection as the objects distance increases, this
is why, it is not a reflective property, but a light property and should be
extended to the whole scene, not just reflected images. I was trying to use
a black fog to simulate this effect, I haven't put too much thought into it
yet as to the realism, but perhaps I will and post some of my results and
conclusions at a later time.
But, as always, feel free to comment on or dispute this if you feel I am
mistaken in my assumptions, as I promote the spreading of knowledge, not
ignorance and I do appreciate any input or illumination on the matter, pun
intended ;).
-tgq
"Kari Kivisalo" <ray### [at] engineercom> wrote in message
news:3BD488F1.AA0F3F21@engineer.com...
> Trevor Quayle wrote:
> >
> > as an equally illuminated object gets further from
> > the camera, it does not fade (which in reality, it should if direct
light
> > gets attenuated
>
> No, this is not what happens in reality. The energy arriving
> from an object when viewed through a constant space angle stays
> the same because the observed area (and energy) on the object
> increases when viewed from greater distance. The 1/r^2 attenuation
> and increase of observed area cancel each other out. The brightness
> (pixel values) of surfaces is not a function of distance.
>
>
> _____________
> Kari Kivisalo
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Ron Parker wrote:
> > What physical property of materials and light would this model?
>
> It's supposed to resemble a poor man's reflection blur.
Actually I was thinking more along the lines of linoleum floors,
where you only have a few inches of reflection.
--
Tim Cook
http://empyrean.scifi-fantasy.com
-----BEGIN GEEK CODE BLOCK-----
Version: 3.12
GFA dpu- s: a?-- C++(++++) U P? L E--- W++(+++)>$
N++ o? K- w(+) O? M-(--) V? PS+(+++) PE(--) Y(--)
PGP-(--) t* 5++>+++++ X+ R* tv+ b++(+++) DI
D++(---) G(++) e*>++ h+ !r--- !y--
------END GEEK CODE BLOCK------
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Trevor Quayle wrote:
>
> over distance (1/r^2), the energy stays the same but it is spread over a
> larger area:
Yes, but the observed area increases by r^2 so they cancel out.
How the energy got on or leaves the surface (direct, radiosity, transmission,
reflection) doesn't matter, the emitted energy to observer's fixed
space angle is not a function of distance.
Do your own experiment. Look at a wall from 1 m distance. Note the
brightness. Go to 3 m distance and compare the brightness. It's
the same!
A pixel gathers light from fixed space angle so the area it gathers
the energy from increases by r^2. The energy arriving at the pixel
from a fixed area on the surface decreases by 1/r^2. You don't have
to be a rocket scientist to figure this one out.
_____________
Kari Kivisalo
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>
> No, this is not what happens in reality. The energy arriving
> from an object when viewed through a constant space angle stays
> the same because the observed area (and energy) on the object
> increases when viewed from greater distance. The 1/r^2 attenuation
> and increase of observed area cancel each other out. The brightness
> (pixel values) of surfaces is not a function of distance.
Yes, you are absolutely correct. But we are missing one point here.
That's for theorically perfect surfaces! We were talking about fading on
blurred reflection, and that's another thing...
When some reflective object shows blurred reflections, it's due to the
very small irregularity of the surface, which spreads out the rays. So,
as
greater is the distance to the surface from the light source (or
object),
and less rays hit the surface, then smaller is the chance to get rays
reflected on that small irregularities towards the camera. The visible
effect is that blurred reflections seems to disapear with distance. Note
I said "seems". Because in fact the reflections of distant objects are
highly and very softly distributed along all the surface irregularities.
Isn't?
So, I agree that a faked reflection fading will make some artistic
tasks easier to do... like the linoleum floor example.
--
Jaime Vives Piqueres
La Persistencia de la Ignorancia
http://www.ignorancia.org/
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