In the 1960s Buckminster Fuller coined the word tensegrity as a combination of tension and integrity, to describe a structure which holds its form through the balance of tension between its parts. It’s a great metaphor when thinking about how energy is distributed in a visual effects simulation at rest and I’ll misuse it as a shorthand for just that.
Let’s take clothing dynamics for example. Look at the clothes you’re wearing. Every fold and wrinkle is an expression or the outcome, of a complex set of interconnected forces – friction, tensile strength, elasticity, etc. The shape exists as it does, solely because of the physical forces of the pieces of fabric, how they’re attached to each other, and the mutual exertion between the cloth and its environment. This is its tensegrity, and the folds of a shirt are a system of balanced tensions, momentarily stabilized.
So what use is this to simulating clothing in animation? Well, because it explains why it’s such a pain in the ass. The cloth’s tensegrity is essentially a lot of forces to keep in check with one another. Let’s look at it backwards.
When a fold is modeled into a shirt for instance, to get an approvable geometric model, and that model is then used as the basis of a simulation – what’s modeled is not actually a fold, but a complex interconnected web of physical tensions and exertions, held together in a sate by, and according to, its tensegrity. When the forces to that matrix of physical interplays change, the system must rebalance, and since the original balance was not based on anything resembling the physics of cloth, it’s efforts to rebalance are not very cloth-like.
Traditional clothes start out as really weird flat shapes. What materials these shapes are made of and how these shapes are attached to one another establish their tensegrity. That results in specific shapes, folds, draping and motion in response to environmental forces – shape is motion and motion is shape. Most CG modeled clothing could never be “unstitched” to lie flat – it would have odd warping, buckling and distortions. The simulated forces act on those structural malformations as input, and the simulation math tries to make sense of it all, as if the warpings were intentional distributions of mass in space.
The visual results are weird, “bubbly”, oozing, and overreactive motions that fold unexpectedly, and keep crawling after the character stops. Those are the simulation engine’s efforts to reestablish balance in the energy of the clothing mesh. It’s just the simulation version of a computer never does anything you don’t tell it to do.
for an actual, more scholarly explanation of tensegrity start here (please) http://en.wikipedia.org/wiki/Tensegrity
an interesting example of a 3D printed dress that uses modeled forms rather that flat patterns (and moves more like a sim)
take a peek at pattern making
#simulation #clothsim #cfx #tensegrity #moviephysics #patterndrafting #3dprinting