Ponytail science & statistical physics

[Seze Mune]

Leonardo da Vinci mentioned it 500 years ago, and apparently scientists have been working at it since: how to predict the shape of a ponytail.  Why is this important?  Glad you asked!

For James Cameron, this may help him configure the way Na'vi hair moves, especially when it's loose and flowing like Neytiri wore in the Tsaheylu scene.  It will be useful in game updates.

For those interested in the particulars:

"Scientists from the University of Cambridge and the University of Warwick said they had devised a "Ponytail Shape Equation," which when calculated using the Rapunzel Number and a measure of the curliness of hair can be used to predict the shape of any ponytail.

"Cambridge's Professor Raymond Goldstein told Reuters that he and his colleagues took account of the stiffness of individual hairs, the effects of gravity and the average waviness of human hair to come up with their formula.

"The Rapunzel Number provides a key ratio needed to calculate the effects of gravity on hair relative to its length.

"That determines whether the ponytail looks like a fan or whether it arcs over and becomes nearly vertical at the bottom," Goldstein said in a telephone interview.

"The research also took into account how a bundle of hair is swelled by the outward pressure which arises from collisions between the component hairs.

"Scientists said the work has implications for understanding the structure of materials made up of random fibers, such as wool and fur and will have resonance with the computer graphics and animation industry, where the representation of hair has been a challenging problem.

"Our findings extend some central paradigms in statistical physics and show how they can be used to solve a problem that has puzzled scientists and artists ever since Leonardo da Vinci remarked on the fluid-like streamlines of hair in his notebooks 500 years ago," Goldstein said.

"The research was conducted by Goldstein, Professor Robin Ball from the University of Warwick and their colleagues. It will be presented to the American Physical Society in Boston on February 28.

(Reporting by Li-mei Hoang, editing by Paul Casciato)"

[Human No More]

Now that's detail

[`Eylan Ayfalulukanä]

This kind of thing has been a big deal for people trying to animate fur. If you watch any recent film that has animated furry animals (the Narnia films come into mind, especially Aslan, who they tried ot make as perfect as possible), they have the fur close, but not correct. A lot of computational resources are needed to render fur, and so this important for the film arts. It also may be one reason there are no furry animals on Pandora-- the fur algorithms are not yet up to JC's standards.

I wonder if this will also be applied to modeling the manes of lions. Mane fur is real wiry, and it is designed to puff out (although it also gets matted easily). In fact, you can make a lion's day by brushing out his mane, something he can't do well himself!

[Seze Mune]

This kind of thing has been a big deal for people trying to animate fur. If you watch any recent film that has animated furry animals (the Narnia films come into mind, especially Aslan, who they tried ot make as perfect as possible), they have the fur close, but not correct. A lot of computational resources are needed to render fur, and so this important for the film arts. It also may be one reason there are no furry animals on Pandora-- the fur algorithms are not yet up to JC's standards.

I wonder if this will also be applied to modeling the manes of lions. Mane fur is real wiry, and it is designed to puff out (although it also gets matted easily). In fact, you can make a lion's day by brushing out his mane, something he can't do well himself!

Wow, ma 'Eylan, that's the best speculation I've ever heard for why there is virtually no fur seen in the movie...and even Na'vi hair is easier to render when it is beaded and hanging in strands.  The only exception I remember is the tsaheylu scene with Neytiri.

[Irtaviš Ačankif]

Well, a simple tswìn would be relatively easy to simulate (though time-consuming because the method is slow): just model it as hundreds of pendulums attached to each other:

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o
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o
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o
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o
etc

However, a head full of such tiny pendulums would be a mess and that's why computer scientists apply approximations to thousands of pendulums which sometimes look fake. If they used the pendulum method, just waiting for the CGI to render could take months but it would look realistic.

[`Eylan Ayfalulukanä]

Well, a simple tswìn would be relatively easy to simulate (though time-consuming because the method is slow): just model it as hundreds of pendulums attached to each other:

|
o
|
o
|
o
|
o
etc

However, a head full of such tiny pendulums would be a mess and that's why computer scientists apply approximations to thousands of pendulums which sometimes look fake. If they used the pendulum method, just waiting for the CGI to render could take months but it would look realistic.

Computers are very good at this kind of thing, though, and a lot of problems are solved that way. Besides the pendulum-type motion (created, I am assuming by gravity), you need to take into account movements resulting from air currents or winds (along with the related motion caused by the entity wearing the hair). That part of the modeling still has a long ways to go.

[Irtaviš Ačankif]

Modeling a pendulum attached to the head of a moving object is very easy. The biggest problem now is 1. modeling ~millions of tiny pendulums and 2. taking care of air resistance and internal resistance. Currently strictly following the physical laws in a vacuum with ideal hair would result in hair tumbling faster and faster and never stopping as the object moves.

[Seze Mune]

Huh.  I just read that quaternions (which describes the rotation of 3D objects) were discovered in 1843 but were considered totally useless til computer scientists used them to render digital animation in 1985.  What're the odds they were used in Avatar?

[Irtaviš Ačankif]

Probably. Most 3D techniques were theoretically invented long ago (though most of them were only useful for pen-and-paper technical drawings at that time)