In article <3A3C43D0.9E91475C@faricy.net>, David Fontaine 
<dav### [at] faricynet> wrote:

> Does a suspension refract? It's a suspension of smoke particles in 
> air, and the ior of the air is the same inside the flame.

It is far more than a suspension of smoke particles in the air...there 
are also various vaporized compounds and ions that will also affect the 
density and ior. In addition, flames coming from a pressurized gas 
released through a small hole will have an even higher density at the 
beginning, flames from a pre-mixed gas will also be a bit different, 
etc...


> Heat also refracts light, like the puddle-on-the-road mirage. So 
> light would bend away from the center of the flame, no?

As mentioned, it isn't the heat, it is the varying density caused by the 
heat, and since the heat isn't constant and the composition of the flame 
at each point can also have an effect, the effect is more complex than 
simply bending light away from the center of the flame. Then you have 
things like turbulence, etc...

Of course, in just doing artistic images in POV, if you need the ior at 
all (and assuming someone writes the patch for variable ior) you could 
just do a spherical patterned ior...but the original question was about 
absorption.

-- 
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/

<><

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Rune wrote:
> 
> To create fire that looks realistic in POV-Ray the usual approach is
> to use both emitting and absorbing media. Absorbing media is used to
> make the fire visible on bright backgrounds.
> 
> But I think in reality fire doesn't absorb light (almost not anyway).

Hot gases *do* absorb light; in fact, they absorb at the exact same
frequencies at which they emit.  This is how astronomers can tell
which gases comprise the sun's corona, various nebulae, etc.

Regards,
John

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Lance Birch <-> wrote in message <3a3c4b9c@news.povray.org>...
>Ken wrote:
>>
>> Which will also vary depending on where you intersect the flame since
>> there are varying temperatures within the flame itself.
>
>That's right, which is why it's such a difficult thing to accurately model.
>The other thing that makes it hard is that because it isn't a solid object,
>the changes in density aren't as black and white (which means neither is
the
>normal)... I have no idea how someone would go about doing it perfectly.


The density of the flame at any given point is well-defined.  I think that
the overall process for tracing the ray would be to solve the appropriate
initial-value differential equation -- not the world's easiest process.

--
Mark

"The derivative of sin(2x) is cos(2x)"  - Matt Giwer
"I never said that" - Matt Giwer
"I completely agree a flywheel can store a million times more energy if its
rotational velocity is measured in microradians instead of radians." - Matt
Giwer

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Mark Wagner wrote:

> The density of the flame at any given point is well-defined.  I think that
> the overall process for tracing the ray would be to solve the appropriate
> initial-value differential equation -- not the world's easiest process.

Oh, that all? Should be easy. (j/k) ;)

--
David Fontaine  <dav### [at] faricynet>  ICQ 55354965
My raytracing gallery:  http://davidf.faricy.net/

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John VanSickle wrote:
> 
> Rune wrote:
> >
> > To create fire that looks realistic in POV-Ray the usual approach is
> > to use both emitting and absorbing media. Absorbing media is used to
> > make the fire visible on bright backgrounds.
> >
> > But I think in reality fire doesn't absorb light (almost not anyway).
> 
> Hot gases *do* absorb light; in fact, they absorb at the exact same
> frequencies at which they emit.  This is how astronomers can tell
> which gases comprise the sun's corona, various nebulae, etc.

	The moral of which is that - by default, and unless you _know_why_ you
want to do something else - you should set your emission and absorbtion
to be the same in any glowing media.  It will almost always look better.

	-Robert Dawson

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