Skin/Flesh Simulation

Ideas, enhancements, feature requests and development related discussion.

Skin/Flesh Simulation

Postby John VanSickle » Fri Jul 18, 2008 5:56 pm

Ideas for the realistic simulation of semi-rigid structures (skin, muscle, fat, sinew, hair, and cloth) keep coming to mind as I work on my far less capable modeler. There are advantages and drawbacks to starting work on a modeler than can handle this stuff:

Advantages: It's an excuse to start a new project, and if it works, it will be a good time to park my old modeler in a bad neighborhood with the keys in the ignition and walk away from it. If it's really good, I can get a better job than the non-IT job I have now.

Disadvantages: Much of the math hasn't been figured out yet by anybody.

Issue #1: The data structures involved will of course have to be more complex than what we're dealing with for two-dimensional manifolds. I suspect that I will be venturing into areas where there isn't even a Wikipedia page yet, although I did read a PDF about subdivision volumes some time back...

Issue #2: Finding a way to efficiently deal with collisions is going to be a major computative challenge.

Issue #3: I should probably learn Java so that the results can be easily shared across platforms.
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Re: Skin/Flesh Simulation

Postby dcuny » Fri Jul 18, 2008 9:29 pm

So you're potentially giving up on LionSnake? :?

Muscle, hair and cloth simulation is already part of most major 3D packages, such as Maya. One thing that Pixar found out was that for cloth to behave correctly, you actually need to tailor the clothing the same way that clothing is made. Similarly, to create a convincing musculature, you need to know the underlying anatomy. It's challenging enough to come create a model, without having to create the underlying muscle system. Pixar got around this by having a standard rig that all characters inherited.

As you noted, dealing efficiently with the problem is a different matter. In The Incredibles, Pixar pre-calculated the deformations:
Essentially, they put a character through a set of exercises— a representative sample of poses — and then used the muscle positions for those poses to train a mathematical representation of the internal coefficients; they’d create a compressed memory of what the muscles looked liked and implement it as an algorithm. "The system would know that when a character looks like this, the muscles look like that," says Rick Sayre, supervising technical director. Thus, the animators could see the result of a dynamic muscle-and-skin system without running a simulation.

Then the tools group spun that idea into clothing. "We trained a statistical model of the cloth," says Anderson. An evolution of the simulation engine developed by Andy Witkin, David Baraff, and Michael Kass for Monsters, Inc. moved the cloth during the training exercise. Once trained, the statistical model took over. That made it possible not only to do cloth sims for multiple characters who appear throughout the film, but also to do so while giving animators control of each major character’s silhouette.
I didn't see a specific paper for it on the Pixar paper page, but I suspect they are using harmonic coordinates to compress the information.
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Re: Skin/Flesh Simulation

Postby John VanSickle » Sat Jul 19, 2008 6:34 am

dcuny wrote:So you're potentially giving up on LionSnake? :?

I often think about the stuff that needs doing and say to myself, "man I'm getting tired of this..." It generally happens when I can't think of elegant or efficient solutions to the problem at hand...

Muscle, hair and cloth simulation is already part of most major 3D packages...

None of which, to my knowledge, are free. That's the gap I wanted to fill.
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