Sunday, October 31, 2010

VIRTUAL CLOTHES

In pencil animation we just draw clothes on a character, then animate them.  But in CGI, costumes for the characters are more challenging.  Because the clothes, like the characters, will be seen in the round, they have to be designed like real clothes, cut from patterns and “sewn”, all in the computer.
And, like real clothes, it can start with a thread. 
A scientist builds a model of a thread in a computer.
Then s/he weaves the thread, with weft and woof, into a digital cloth, with the distinctive weave of the fabric desired: cotton (the most common thread), twill, satin, silk or wool, or whatever.  Finally, the cloth is lit in the computer to show off the texture.  You can see all the details here (click on the IM06.pdf file)  Thanks to Ian Hopkinson for this link.
These woven digital threads are used mainly in the fabric and textile industry.   If they were to be used in animation, strobing or a flickering visual effect might occur.  You can see this in Figures 12 through 14, in the above article.  This fabric in motion may strobe and flicker even more.


In CG animation, cloth simulation is used.  Here you can play with Andrew Hoyer’s simulated cloth and read his “little explanation” of how cloth is simulated: “…a collection of constraints and point masses in a never ending struggle.”
And here you can see how animator Jessica Hurst uses cloth simulation, including costumes for Alice in Wonderland and Harry Potter.
  (Thanks to Eloi Champagne for these and the Syflex links).

In the CGI animated feature "Stuart Little 2," very realistic costumes were required, even though Stuart Little didn't really have a good body being very short, with no neck and a belly.   
Costume Designer Mona May made actual tiny patterns for Stuart Little’s simple wardrobe of jeans and sweaters.  Everyday fabrics like denim and thermal knit were simulated, with dirt marks where Stuart wiped his paws. The patterns were scanned and "sewn" together in the computer so the fabric would exhibit surface tension. Stuart Little was "fitted" in the computer, as well.  And, like an actual fitting, adjustments had to be made, to accommodate Stuart's tail and the thickness of his fur.

Another challenge for virtual clothing, is animating it.
Virtual clothes naturally have to move with the character, so cloth software needs to calculate collisions between the cloth and itself as well as other objects, notably, the body wearing it.  Syflex is a widely used cloth simulator plug-in that can run inside most major 3D apps (Maya, Softimage, Houdini).  But most of these CGI (3D) apps are now also shipped with their own cloth simulation solver.


As amazing as these cloth simulators are, I still notice that the cloth doesn’t always move intelligently, i.e. heavy cloth like velvet should move more heavily, more slowly than lighter cloth like muslin.  But, in some simulations, the cloth seems to move like velvet in some places and in others, it bounces and flies around like muslin. 
Weight is always a problem in CGI animation              
*

6 comments:

SomeBeans said...

The problem with simulating cloth is that the mechanical model needs to be really sophisticated to look real for most fabrics - this tends to mean it is slow and to be honest the maths for a more complex model is hard and poorly understood.

Nora Lumiere said...

Is there any way scientists and animators could get together to solve this problem?

SomeBeans said...

@nora_lumiere they already are to a degree. For example, Steve Marschner at Cornell has links with some of the graphics houses. There is a gap in the sense that an animator is looking for intuitive tools to get the effect they want, whilst the physics end of computer graphics research is looking to do accurate, fast physical simulation. There is an issue in the sense that accurate physical simulation may run too slowly to facilitate a useful animators tool.

Nora Lumiere said...

I understand that an accurate physical simulation may run slowly right now, but tools evolve according to need. There’s just too much emphasis placed on the stunning graphic realism in CGI and not enough on realistic movement. Movement is the important thing in animation and it has been sorely neglected. Things float and flutter about too lightly in CGI. Scientists seem to prefer the sexy issues like textures, hair, skin and facial features, but more time should be spent on realistic movement.

John L said...

Great article. I think the limits of realistic cloth motion (and motion in general) are more apparent when using live actors or super-realistic animation, like Alice or Beowulf. In a Pixar-style film, since it is an imaginary cartoon world, everything can be exaggerated and it fits the style. But trying to pass it off as the "real" world is still tricky.

Nora Lumiere said...

Good point.
And Pixar folk are smart enough to avoid using flowing fabric in the first place. I love they way they dispensed with this problem in THE INCREDIBLES, by having the characters dislike capes so they didn't have to deal with the cloth simulation issue.