Hi ALL,
I have been using the "Turbulence" function to add noise to the color in the
media. But as we know turbulence is coherent in space. But to generate a
sequence of renderings (e.g. an explosion cloud evolving in time) a noise
function needs to be temporally coherent. Otherwise, if the same noise is
applied to a point at all the steps in time, then the animation will have
the classical "shower curtain effect" cited in literature. So, can anyone
please suggest if there is a way to generate such a noise in POVRAY,
somethings that looks spatially as well as temporally coherent?
Thanks in advance,
Maurya
http://www-scf.usc.edu/~mauryash

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In article <402891e8$1@news.povray.org>,
 "Maurya Shah" <mau### [at] hotmailcom> wrote:

> I have been using the "Turbulence" function to add noise to the color in the
> media. But as we know turbulence is coherent in space. But to generate a
> sequence of renderings (e.g. an explosion cloud evolving in time) a noise
> function needs to be temporally coherent. Otherwise, if the same noise is
> applied to a point at all the steps in time, then the animation will have
> the classical "shower curtain effect" cited in literature. So, can anyone
> please suggest if there is a way to generate such a noise in POVRAY,
> somethings that looks spatially as well as temporally coherent?

I'm not familiar with the "shower curtain" effect. However, noise is 
temporally coherent in that it is constant and deterministic, you always 
get the same results for a given 3D point. It sounds like what you want 
is a 4D pattern, a noise function that varies smoothly with time. This 
is possible, I even once saw a recursive implementation of Perlin noise 
that could handle any number of dimensions, but slower to compute and 
more complex, and I don't know if it has been done in POV.

If your use of the pattern is constrained to two dimensions, a floor 
plane for example, then you can just move the pattern along the 
remaining dimension. Or you could combine several noise functions in 
various ways, and translate them in different directions.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

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Hi Christopher,
Thanks for your comments.
I tried to review some literature about Perlin noise and it seems that if
the noise has to varied smoothly both in space and time then I feel the use
of advected textures is the only way out. I found the paper called "Advected
Textures" by Fabric Neyret to be quite interesting and claims to have solved
this problem successfully. The basic idea is to combine multiple texture
noise together and vary them over time so that the statistical property of
noise is maintained as well as the noise follows the local distortion of the
media.
However, implementing this is quite a bit of work.
Maurya

"Christopher James Huff" <cja### [at] earthlinknet> wrote in message
news:cjameshuff-5C2251.07190510022004@news.povray.org...
> In article <402891e8$1@news.povray.org>,
>  "Maurya Shah" <mau### [at] hotmailcom> wrote:
>
> > I have been using the "Turbulence" function to add noise to the color in
the
> > media. But as we know turbulence is coherent in space. But to generate a
> > sequence of renderings (e.g. an explosion cloud evolving in time) a
noise
> > function needs to be temporally coherent. Otherwise, if the same noise
is
> > applied to a point at all the steps in time, then the animation will
have
> > the classical "shower curtain effect" cited in literature. So, can
anyone
> > please suggest if there is a way to generate such a noise in POVRAY,
> > somethings that looks spatially as well as temporally coherent?
>
> I'm not familiar with the "shower curtain" effect. However, noise is
> temporally coherent in that it is constant and deterministic, you always
> get the same results for a given 3D point. It sounds like what you want
> is a 4D pattern, a noise function that varies smoothly with time. This
> is possible, I even once saw a recursive implementation of Perlin noise
> that could handle any number of dimensions, but slower to compute and
> more complex, and I don't know if it has been done in POV.
>
> If your use of the pattern is constrained to two dimensions, a floor
> plane for example, then you can just move the pattern along the
> remaining dimension. Or you could combine several noise functions in
> various ways, and translate them in different directions.
>
> --
> Christopher James Huff <cja### [at] earthlinknet>
> http://home.earthlink.net/~cjameshuff/
> POV-Ray TAG: <chr### [at] tagpovrayorg>
> http://tag.povray.org/

Post a reply to this message

In article <402b0c92@news.povray.org>,
 "Maurya Shah" <mau### [at] hotmailcom> wrote:

> I tried to review some literature about Perlin noise and it seems that if
> the noise has to varied smoothly both in space and time then I feel the use
> of advected textures is the only way out.

This is not so. There is a 4D variant of Perlin noise which does exactly 
that, varying in time as it varies along x, y, and z. In fact, Perlin 
noise can be extended to any number of dimensions, though I doubt more 
than 4 is very useful. Ah, SlimePOV has an implementation:
http://www.slimeland.com/slimepov/

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

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