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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). The
reason it is still visible is that it is many times brighter than the
surrounding objects.
I think if POV-Ray showed light intensities based on for example an
logarithmic function rather than linearly it would be possible to make more
realistic scenes, and it would be easier too. I think it would affect more
things than you would think; perhaps completely change the way we think of
colors in POV-Ray.
Rune
--
\ Include files, tutorials, 3D images, raytracing jokes,
/ The POV Desktop Theme, and The POV-Ray Logo Contest can
\ all be found at http://rsj.mobilixnet.dk (updated October 9)
/ Also visit http://www.povrayusers.org
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In article <3a3a48b2@news.povray.org>, "Rune" <run### [at] iname com>
wrote:
> But I think in reality fire doesn't absorb light (almost not anyway). The
> reason it is still visible is that it is many times brighter than the
> surrounding objects.
Actually, it does absorb and scatter light, sometimes to a very
significant degree. Especially yellow flames, such as a candle flame or
wood fire, where there are many incandescent particles of carbon and
other smoke particles, and a lot of complex organic substances. It might
be an interesting experiment to shine laser beams through various types
of flames to measure the effects...
However, you are right that it usually just overwhelms the background
more than blocking it. Because of the limited, linear dynamic range, POV
will just make extremely bright flames look white.
> I think if POV-Ray showed light intensities based on for example an
> logarithmic function rather than linearly it would be possible to
> make more realistic scenes, and it would be easier too. I think it
> would affect more things than you would think; perhaps completely
> change the way we think of colors in POV-Ray.
This is possible to do as a post_process filter(*), in fact, the
"curves" filter could be used with a "raw_color" filter to do
this...then you could set up an include file with macros to imitate the
response curves of various film types or the human eye. Or maybe a
"limit_range" post_process that scales colors with components >1 down so
their brightest component is equal to one...not exactly realistic, but
maybe useful.
*the post_process patch is in MegaPOV, I have released the code for the
curves filter in these groups, and the raw_color and limit_range filters
aren't implemented yet, though they will be easy to do.
--
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/
<><
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Chris Huff wrote:
> Actually, it does absorb and scatter light, sometimes to a very
> significant degree. Especially yellow flames, such as a candle flame or
> wood fire, where there are many incandescent particles of carbon and
> other smoke particles, and a lot of complex organic substances. It might
> be an interesting experiment to shine laser beams through various types
> of flames to measure the effects...
Doesn't it also bend light away from the center of the flame? IIRC
--
David Fontaine <dav### [at] faricynet> ICQ 55354965
My raytracing gallery: http://davidf.faricy.net/
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In article <3A3A978C.C48AA29C@faricy.net>, David Fontaine
<dav### [at] faricynet> wrote:
> Doesn't it also bend light away from the center of the flame? IIRC
The different densities will cause the ior to be different, refracting
light...maybe not always away from the center. However, this is variable
ior, a pretty difficult and processor-intensive effect.
Also, I don't know how much of an effect it would have...it could easily
be barely noticeable. You can often see shadows from the "turbulence" of
a flame, but they aren't very distinct.
Back to the laser-beam experiment(I wish I had my laser pointer!), to be
thorough, you would need to use lasers of several different wavelengths
to get the actual color of scattering...actually, it might be better to
use a focused, high intensity white light instead of a monochromatic
laser...
--
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/
<><
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Chris Huff wrote:
> > Doesn't it also bend light away from the center of the flame? IIRC
>
> The different densities will cause the ior to be different, refracting
> light...maybe not always away from the center. However, this is variable
> ior, a pretty difficult and processor-intensive effect.
give me a second to think about this...ok
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.
Heat also refracts light, like the puddle-on-the-road mirage. So light would
bend away from the center of the flame, no?
--
David Fontaine <dav### [at] faricynet> ICQ 55354965
My raytracing gallery: http://davidf.faricy.net/
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David Fontaine wrote:
> Heat also refracts light, like the puddle-on-the-road mirage. So light
would
> bend away from the center of the flame, no?
Well, it's not really the heat that does it, just the density change caused
by the heat, but yes you're right, it will. Because the air in the flame is
hotter, it will be less dense, so a ray of light going in to it should bend
away from the normal (if you were to draw the normal right through that is).
--
Lance.
http://come.to/the.zone
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Lance Birch wrote:
>
> David Fontaine wrote:
> > Heat also refracts light, like the puddle-on-the-road mirage. So light
> would
> > bend away from the center of the flame, no?
>
> Well, it's not really the heat that does it, just the density change caused
> by the heat, but yes you're right, it will. Because the air in the flame is
> hotter, it will be less dense, so a ray of light going in to it should bend
> away from the normal (if you were to draw the normal right through that is).
Which will also vary depending on where you intersect the flame since
there are varying temperatures within the flame itself.
--
Ken Tyler - 1400+ POV-Ray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html http://www.povray.org/links/
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Ken wrote:
>
> Lance Birch wrote:
> >
> > David Fontaine wrote:
> > > Heat also refracts light, like the puddle-on-the-road mirage. So light
> > would
> > > bend away from the center of the flame, no?
> >
> > Well, it's not really the heat that does it, just the density change
caused
> > by the heat, but yes you're right, it will. Because the air in the
flame is
> > hotter, it will be less dense, so a ray of light going in to it should
bend
> > away from the normal (if you were to draw the normal right through that
is).
>
> 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.
--
Lance.
http://come.to/the.zone
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Lance Birch wrote:
> normal)... I have no idea how someone would go about doing it perfectly.
As it is with many things in the 3D graphic industry. If you can't model
it with real world physical attributes you fake it so that it at least
appears like it was. Unless you goal is academic it probably doesn't
matter anyway since the viewer is more concerned about the results than
the methods used to achieve it.
--
Ken Tyler - 1400+ POV-Ray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html http://www.povray.org/links/
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Ken wrote:
> Lance Birch wrote:
> > normal)... I have no idea how someone would go about doing it perfectly.
>
> As it is with many things in the 3D graphic industry. If you can't model
> it with real world physical attributes you fake it so that it at least
> appears like it was. Unless you goal is academic it probably doesn't
> matter anyway since the viewer is more concerned about the results than
> the methods used to achieve it.
Yes that's right. I made an image today that is a bit along those lines...
--
Lance.
http://come.to/the.zone
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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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