Hi
I'm wondering if anyone is working on improving the radiosity of POV-ray.
It's ok when set properly and I admire the developers but it doesn't have
the reputation of being great. Being a firm believer in POV I would like to
see it give Lightscape 'a run for it's money'.
Thanks
jarobertson

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On Sun, 4 Oct 1998 20:59:15 -0400, "Jeff Robertson"
<jar### [at] odysseyonca> wrote:

>Hi
>I'm wondering if anyone is working on improving the radiosity of POV-ray.
>It's ok when set properly and I admire the developers but it doesn't have
>the reputation of being great. Being a firm believer in POV I would like to
>see it give Lightscape 'a run for it's money'.

I'm looking at it.  I think Stephen Horn is, too.  I don't have
anything good to report yet, though, so I've kept quiet here.

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Jeff Robertson wrote in message <36181af0.0@news.povray.org>...
>Hi
>I'm wondering if anyone is working on improving the radiosity of POV-ray.
>It's ok when set properly and I admire the developers but it doesn't have
>the reputation of being great. Being a firm believer in POV I would like to
>see it give Lightscape 'a run for it's money'.
>Thanks
>jarobertson


This rises a question I've often wanted to ask and always forgot to. How
does Lightscape work? What is radiosity tracing anyway? If it is a method of
a completely different nature than raytracing should we expect POV to be as
good as Lightwave in radiosity (or vice-versa: LW be as good in glasses,
caustics, etc.)?

Peter

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You know, you can find some tips on how radiosity works in PoV's doc. Roughly
(please, feel free to correct me if I do any mistake due to loss of memory...)
It's a way to achieve a better result than ambient light, by calculating first a
"normaliz (s ?)ed" ambient value for each of a lot of subdividings in the final
image, and then using it to render with a correct look and feel. Quite time
consuming, but really worth if you have a powerful enough cpu.

And again, sorry if some of my explanations here were wrong, I haven't the doc
here...

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My mistake. I really means Radiance here, not Light-whatever. Sorry.

Peter

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Marc SCHEFFER wrote in message <361BDC68.F842C77A@wanadoo.fr>...
>You know, you can find some tips on how radiosity works in PoV's doc.
Roughly
>(please, feel free to correct me if I do any mistake due to loss of
memory...)
>It's a way to achieve a better result than ambient light, by calculating
first a
>"normaliz (s ?)ed" ambient value for each of a lot of subdividings in the
final
>image, and then using it to render with a correct look and feel. Quite time
>consuming, but really worth if you have a powerful enough cpu.
>
>And again, sorry if some of my explanations here were wrong, I haven't the
doc
>here...

Actually, my question was a little bit different. Yes, I know how radiosity
is implemented in POV, but it's still raytracing, since it relies on
shooting rays, intersecting with (mostly polynomial) objects etc. I think
Radiance uses a completely different approach. For example, in the c.g.r.r.
faq it is said that rendering speed in Radiance is not affected by the
number of objects in the scene (?!?!?). I am just curios, that's all.

Peter

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Peter Popov wrote:
> 
> Marc SCHEFFER wrote in message <361BDC68.F842C77A@wanadoo.fr>...
> >You know, you can find some tips on how radiosity works in PoV's doc.
> Roughly
> >(please, feel free to correct me if I do any mistake due to loss of
> memory...)
> >It's a way to achieve a better result than ambient light, by
> calculating
> first a
> >"normaliz (s ?)ed" ambient value for each of a lot of subdividings in
> the
> final
> >image, and then using it to render with a correct look and feel.
> Quite time
> >consuming, but really worth if you have a powerful enough cpu.
> >
> >And again, sorry if some of my explanations here were wrong, I
> haven't the
> doc
> >here...
> 
> Actually, my question was a little bit different. Yes, I know how
> radiosity
> is implemented in POV, but it's still raytracing, since it relies on
> shooting rays, intersecting with (mostly polynomial) objects etc. I
> think
> Radiance uses a completely different approach. For example, in the
> c.g.r.r.
> faq it is said that rendering speed in Radiance is not affected by the
> number of objects in the scene (?!?!?). I am just curios, that's all.
> 
> Peter

Different programs calculate global illumination differently.  Radiance
uses Monte Carlo raytracing to determine the indirect lighting
correctly.  It calculates additional rays in much the same way as
PovRay's radiosity part.  One difference is that it is not limited to
one level of indirection.  A ray can be bounced off many objects before
it reaches the light source.  Another difference is that Radiance uses
the gradient of the surface in its interpolation calculation.  Because
of all the work that has gone into Radiance and testing it, it is the
benchmark which most global illumination models are compared against.

Radiance: http://radsite.lbl.gov/radiance/HOME.html
Radiance lighting model: http://radsite.lbl.gov/radiance/refer/long.html

-Eric Brown

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On Mon, 5 Oct 1998 07:13:12 +0300, "Peter Popov" <pet### [at] usanet> wrote:
>
>This rises a question I've often wanted to ask and always forgot to. How
>does Lightscape work? What is radiosity tracing anyway? If it is a method of
>a completely different nature than raytracing should we expect POV to be as
>good as Lightwave in radiosity (or vice-versa: LW be as good in glasses,
>caustics, etc.)?
>
>Peter
>

The so-called "radiosity" in POV is badly named.  POV actually uses an
algorithm more properly called _distributive ray tracing_.  The radiosity
algorithm is something entirely different than what POV does.

From thermodynamics in physics, there is a problem where you have a bunch of
flat surfaces in space that all begin with an initial heat.  The heat will
then begin to transfer to the other surfaces where it will eventually reach an
equilibrium.  If all the surfaces are flat and emitting heat in all
directions, you can solve for the equilibrium with a system of linear
equations set in a matrix.   Someone thought that the same technique could be
used for 3-dimensional computer scenes, wherein light was substituted for the
heat.  Radiosity was born....

Radiosity does not trace any rays, so it tends to be enormously faster than
other algorithms. But it requires some things that aren't true to life or
helpful for modelling. 

I. All the surfaces diffuse light homogeneously in all directions.  This is
not true for glossy surfaces or mirrors, or any refracting object for that
matter.

II. Every object in the scene must be "tessalated", i.e. divided into a patch
of small triangles.  We already know about objects that this won't work on.
Blobs, fractals, implicit surfaces...the list goes on.  This tessalation
burdens memory, because it multiplies the number of objects in the scene by
unpredictable amounts.  Some scenes are even IMPOSSIBLE to tessalate on a PC,
because they require more than say, 64Mb of storage.

III. The light emmitted by any very small, bright patch will not shade objects
or make them cast shadows.  This is extremely unrealistic.  The avoid this,
you must then do a ray-tracing pass, but then you are back to tracing rays
again!  In general, if an important bright patch begins to reach the size of a
point, rather than a large luminaire, radiosity breaks down, and ray-tracing
steps in as the victor.

The bottom line is that ray-tracing and radiosity _seem_ to go hand in hand.
To each is its own solution.  We know light moves in specific directions. But
we also know that it globally illuminates scenes.  Here are the strong points
of each rendering algorithm. So....

Radiosity     ------> Global Illumination.
Ray Tracing -----> Any phenomenon involving the direction of light.

Its not to say that ray-tracing is _doomed_ in calculating indirect/global
illumination.  There are algorithms such as monte-carlo, photon mapping, etc,
that use the tracing of rays, that are arguably more powerful than radiosity
could ever hope to be.

Any questions?
---------------------------------------
Steve

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On Mon, 5 Oct 1998 07:13:12 +0300, "Peter Popov" <pet### [at] usanet> wrote:

>>
>This rises a question I've often wanted to ask and always forgot to. How
>does Lightscape work? What is radiosity tracing anyway? If it is a method of
>a completely different nature than raytracing should we expect POV to be as
>good as Lightwave in radiosity (or vice-versa: LW be as good in glasses,
>caustics, etc.)?
>
>Peter
>

Lightscape uses a radiosity pass, then a ray-tracing pass.  Viola! 

------
Steve

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