Warp wrote:
>Neil Conway <nei### [at] yahoocom> wrote:
>> (I haven't figured out Warp's example very thoroughly yet.)
>
>  Which example?

Ahh.

Thanks, now I have figured it out.  I had trouble understanding the lack of
a radiosity block...  (But you did helpfully include an emission media
effect.)

Are those sigs random?

(Blush)

:-))

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"Neil Conway" <nei### [at] yahoocom> wrote in message
news:web.3e40df8bc7b69b7a1c4fca030@news.povray.org...
> >
> >  Which example?
>
> Ahh.
>

Heh-heh. I don't think even Warp would be so cruel to supply a sample scene
looking like *that* (iirc Ken is the only one around here who thinks newlines
are for wimps).

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Christopher James Huff wrote:
>You really need to define your problem more clearly: what do you mean by
>"proper reflections"? What is your goal, a picture that looks like the
>interior of the tokamak, or an actual simulation? If the latter is the
>case, a custom raytracer will probably be needed.

OK, here's an attempt at a reasonable definition:

The plasma is optically thin (at visible wavelengths anyway), so I don't
need to worry about absorption.  Scattering is also negligible.

Imagine if you will two types of toroidally symmetric emission regions: the
first is a simple filled torus inside which the emission is constant (and
let's say its minor radius is much smaller than the major radius); the
second type is a hollow torus, of which only a thin shell at its surface
emits light (and in this case let's the minor radius is nearly as large as
the major radius).

Now, a camera looking into the scene will see a very different result from
these two distributions, and one can take advantage of the toroidal
symmetry to convert the 2D camera picture into a pretty good estimate of
the emissivity profile of the toroids in question.

Then consider an enclosure around the plasma.  This will reflect light into
the camera view which would not otherwise be there.  Some of the light
reaching the camera will have been multiply reflected - in some cases from
parts of the enclosure which aren't even in the camera's view.  Some of the
reflecting surfaces are made of graphite (weak, mostly diffuse reflection)
while others are made of machined stainless steel (fairly strong, mostly
specular but the machining causes anisotropy of the specular part, possibly
averaging out to something close to diffuse).  These reflections will make
it harder to determine the light distribution within the plasma from camera
pictures.

What I'm hoping to do is model the amount of reflected light, and then study
the effects of adding absorbers - chiefly within certain limited regions of
the camera view, but also away from the direct view to minimise multiple
reflections.

Now, I'm not expecting to be able to model the processes perfectly - for
example to accurately model the specular reflections from the stainless
steel you'd need to know the direction of the machining for every part of
the surface.  (And then add to that the fact that POV doesn't do specular
reflections!)  Nonetheless it seems reasonable to hope that POV could do
usefully close approximations to reality...

Are multiple reflections handled by the radiosity & emitting media code?  (I
mean multiple reflections from regular objects btw.)

>POV is currently only capable of handling diffuse reflections of light
>emitted by media, such as light bouncing off a sheet of paper, or media
>seen directly. Specular reflections, like light bouncing off a mirror,
>are not handled.

I've read the FAQ's on this before, but I must admit I'm still somewhat
puzzled.  A perfect mirror (unlike machined metal) should be trivial to
handle - the direction of the ray is simply reflected about the normal to
the mirror, after which you track it until it hits something else.  Is it
that people want to handle imperfect mirrors?  Or is it simply that the
angular density of the rays becomes an issue?
(On re-reading your paragraph above, I realise you meant specular
reflections of light emitted from media.  However, I understood from the
docs that specular reflections aren't really handled at all - correct?)

thanks for your feedback...
Neil

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Neil Conway <nei### [at] yahoocom> wrote:
> Thanks, now I have figured it out.  I had trouble understanding the lack of
> a radiosity block...  (But you did helpfully include an emission media
> effect.)

  I started the boom of using <=4 lines of obfuscated pov-code as signature
(at first I used a 2-liner written in C). Some people use only one signature
while others use several (in such way that their newsreader chooses one
randomly).
  Some main ideas about the signatures is that they should be as short as
possible, and as obfuscated as possible (so that you simply have no idea
what it will render) and when rendered, they will show a nice image, preferably
with your name/initials/nick in it.

  The obfuscated signatures or some people are quite impressive.

-- 
plane{-x+y,-1pigment{bozo color_map{[0rgb x][1rgb x+y]}turbulence 1}}
sphere{0,2pigment{rgbt 1}interior{media{emission 1density{spherical
density_map{[0rgb 0][.5rgb<1,.5>][1rgb 1]}turbulence.9}}}scale
<1,1,3>hollow}text{ttf"timrom""Warp".1,0translate<-1,-.1,2>}//  - Warp -

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"Neil Conway" <nei### [at] yahoocom> wrote in message
news:web.3e4118dbc7b69b7a1c4fca030@news.povray.org...
> Christopher James Huff wrote:
>
> >POV is currently only capable of handling diffuse reflections of light
> >emitted by media, such as light bouncing off a sheet of paper, or media
> >seen directly. Specular reflections, like light bouncing off a mirror,
> >are not handled.
>
> I've read the FAQ's on this before, but I must admit I'm still somewhat
> puzzled.  A perfect mirror (unlike machined metal) should be trivial to
> handle - the direction of the ray is simply reflected about the normal to
> the mirror, after which you track it until it hits something else.  Is it
> that people want to handle imperfect mirrors?  Or is it simply that the
> angular density of the rays becomes an issue?
> (On re-reading your paragraph above, I realise you meant specular
> reflections of light emitted from media.  However, I understood from the
> docs that specular reflections aren't really handled at all - correct?)

Just to muddy the waters... specular reflection of light can be done in POV by
using photons (like many raytracers, POV doesn't handle specular reflection of
light easily, since rays are traced from the camera to the light source).

It sounds as though you need a combination of emitting media, radiosity and
photons - others will have to comment on whether those three components interact
properly together or not. Even if they do, it's going to be a hellishly long
trace. It does occur to me that, for a general impression, the pre-trace output
might be enough to indicate the bright and dark spots, and how they are affected
by changes to surface properties.

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Nathan Kopp <pov### [at] nkoppmailshellcom> wrote:
> 1) Lack of specular & phong highlights.

  Making the surfaces a bit reflective, and specially playing with the
reflection exponent should help a lot with this.

-- 
plane{-x+y,-1pigment{bozo color_map{[0rgb x][1rgb x+y]}turbulence 1}}
sphere{0,2pigment{rgbt 1}interior{media{emission 1density{spherical
density_map{[0rgb 0][.5rgb<1,.5>][1rgb 1]}turbulence.9}}}scale
<1,1,3>hollow}text{ttf"timrom""Warp".1,0translate<-1,-.1,2>}//  - Warp -

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On 5 Feb 2003 09:23:08 -0500, Warp <war### [at] tagpovrayorg> wrote:
>   Some main ideas about the signatures is that they should be as short as
> possible, and as obfuscated as possible (so that you simply have no idea
> what it will render) and when rendered, they will show a nice image, preferably
> with your name/initials/nick in it.

IMO POV signatures are also nice tutorials what SDL can and how to write some
simple effects without unnecessary overloading. Of course it requires usually
"unrolling" of script which can be another lesson of SDL. I consider sigs as
an optional stage in SDL experience.

ABX
-- 
disc{z,-z#macro O()asc(substr("-+((1*(,1,/.-,*/(,&.323/'1"e 1))/10-4#declare
e=e-1;#end#local e=26;5pigment{#local g=function(_){ceil(_)-_}function#local
k=function{pattern{object{sphere_sweep{linear_spline 13#while(e)<O()O()>.01#
end}}}}{k(g(atan2(x,y)),g(ln(pow(x+y,2)+1e-5)),0)}}finish{ambient 1}}//ABX35

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Warp wrote:
>Nathan Kopp <pov### [at] nkoppmailshellcom> wrote:
>> 1) Lack of specular & phong highlights.
>
>  Making the surfaces a bit reflective, and specially playing with the
>reflection exponent should help a lot with this.

Well, you've clearly had practice :-))

(Several other people have also added points about reflections - thanks
folks.)

To add more coal to the fire, I have just browsed the first few pictures in
your gallery (wow!).  I see your first picture (Juice.jpg) has what look
exactly like specular reflections from the surface of the glass sheet on
the table, not to mention highlights on the drinking vessels.  Then,
looking at the hourglass picture, I see a mirror !?  I had thought this was
exactly what the manual said wasn't possible...
The mirror shows a reflection of the rear side of the hourglass etc.  Was
this "real" or did you have to cheat by making additional objects (or a
composite picture)?

Nice one :-)

Neil

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Tom Melly wrote:
>Just to muddy the waters... specular reflection of light can be done in POV by
>using photons (like many raytracers, POV doesn't handle specular reflection of
>light easily, since rays are traced from the camera to the light source).

Hmm, I've been hopping to and fro in the docs - must read 'em properly - and
I did read about photons.  I will go back and try to figure out what they
can do for me (and what their limitations are).

In terms of CPU time, it doesn't necessarily represent a problem if this
takes weeks of CPU per picture.  Firstly, I don't need to do it very often,
and secondly, I can always beg some time on our Beowulf cluster (hmm, I
now remember reading that photons don't lend themselves well to clustered
tracing...).

thanks
Neil

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Neil Conway <nei### [at] yahoocom> wrote:
> Then,
> looking at the hourglass picture, I see a mirror !?  I had thought this was
> exactly what the manual said wasn't possible...

  Which part of the manual, exactly?

  Reflection is one of the most basic and simplest features of raytracing.
Where did you get the impression that it would be difficult?

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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