Physically-based modeling: past, present, and future
Authors: Terzopoulos D. , Pltt J. , Barr A. , Zeltzer D. , Witkin A. , Blinn J. 
Published: November 2005

My name is Demetri Terzopoulos and my co-chair, John Platt, and
I would like to welcome you to the panel on Physically-Based
Modeling -- Past, Present and Future. I'll start by introducing the
panelists; the affiliations you see listed on the screen are
somewhat out of date.
I'm Program Leader of modeling and simulation at the
Schlumberger Laboratory for Computer Science in Austin, Texas, and
I was formerly at Schlumberger Palo Alto Research. I'll speak on
the subject of deformable models.
John Platt, formerly of Cal Tech, is now Principal Scientist at
Synaptics in San Jose, California. He will be concentrating on
constraints and control.
Alan Barr is Assistant Professor of computer science at Cal
Tech. Last year he received the computer graphics achievement
award. He'll speak about teleological modeling.
David Zeltzer is Associate Professor of computer graphics at the
MIT Media Laboratory. He will be speaking on interactive micro
worlds.
Andrew Witkin, formerly of Schlumberger Palo Alto Research, is
now Associate Professor of computer science at Carnegie Mellon
University. He will speak about interactive dynamics.
Last but not least, we have with us James Blinn, who of course
needs no introduction. Formerly of JPL, he is now Associate
Director of the Mathematics Project at Cal Tech. He says he'll have
several random comments to make against physically-based
modeling.
I was also asked by the SIGGRAPH organizers to remind the
audience that audio and video tape recording of this panel is not
permitted.
Many of you are already familiar with physically-based modeling,
so I will attempt only a very simple introduction to this, in my
opinion, very exciting paradigm. Physically-based techniques
facilitate the creation of models capable of automatically
synthesizing complex shapes and realistic motions that were, until
recently, attainable only by skilled animators, if at all.
Physically-based modeling adds new levels of representation to
graphics objects. In addition to geometry -- forces, torques,
velocities, accelerations, kinetic and potential energies, heat,
and other physical quantities are used to control the creation and
evolution of models. Simulated physical laws govern model behavior,
and animators can guide their models using physically-based control
systems. Physically-based models are responsive to one another and
to the simulated physical worlds that they inhabit.
We will review some past accomplishments in physically-based
modeling, look at what we are doing at present, and speculate about
what may happen in the near future. The best way to get a feel for
physically-based modeling is through animation, so we will be
showing you lots of animation as we go along.
I would like to talk about deformable models, which are
physically-based models of nonrigid objects. I have worked on
deformable models for graphics applications primarily with Kurt
Fleischer and also with John Platt and Andy Witkin. Deformable
models are based on the continuum mechanics of flexible materials.
Using deformable models, we can model the shapes of flexible
objects like cloth, plasticine, and skin, as well as their motions
through space under the action of forces and subject to
constraints.
Please roll my Betacam tape. Here is an early example of
deformable surfaces which are being dragged by invisible forces
through an invisible viscous fluid. Next we see a carpet falling in
gravity. It collides with two impenetrable geometric obstacles, a
sphere and a cylinder, and must deform around them. The next clip
shows another clastic model. It behaves like a cloth curtain that
is suspended at the upper corners, then released.
Here is a simulated physical world -- a very simple world
consisting of a room with walls and a floor. A spherical obstacle
rests in the middle of the floor. You're seeing the collision of an
elastically deformable solid with the sphere. Of course, we're also
simulating gravity.
We've developed inelastic models, such as the one you see here
which behaves like plasticine. When the model collides with the
sphere, there's a permanent deformation. By changing a physical
parameter, we obtain a fragile deformable model such as the one
here. This deformable solid breaks into pieces when it hits the
obstacle.
Deformable models can be computed efficiently in parallel. This
massively parallel simulation of a solid shattering over a sphere
was computed on a connection machine at Thinking Machines, with the
help of Carl Feynman.
Here is a cloth-like mesh capable of tearing. We're applying
shear forces to tear the mesh. The sound you're hearing has been
generated by an audio synthesizer which was programmed by Tony
Crossley so that it may be driven by the physical simulation of the
deformable model. Whenever a fiber breaks, the synthesizer makes a
pop. Keep watching the cloth; we get pretty vicious with it.
Deformable models are obviously useful in computer graphics, but
they are also useful for doing inverse graphics; that is to say,
computer vision.
For example, here we see an image of a garden variety squash.
Using a deformable tube model, we can reconstruct a three
dimensional model of the squash from its image, as shown. Once we
have reconstructed the model from the image, we can rotate the
model to view it from all sides. You can see, we have captured a
fully three dimensional model from that single, monocular image.
That's a basic goal of computer vision.
Kurt Fleischer, Andy Witkin, Michael Kass, and I used this
deformable model based vision technique to create an animation
called <i>Cooking with Kurt.</i> We wanted to mix live
video and physically-based animation in this production. You see
Kurt entering a kitchen carrying three vegetables. We captured
deformable squash models from a single video frame of the real
squashes sitting on the table -- this particular scene right here.
Now the reconstructed models are being animated using
physically-based techniques. The models behave like very primitive
actors; they have simple control mechanisms in them that make them
hop, maintain their balance, and follow choreographed paths. The
collisions and other interactions that you see are computed
automatically through the physical laws, and they look quite
realistic. It's difficult to do this sort of thing by hand, even if
you're a skilled animator.
This second tape will show you some of the physically-based
modeling we're up to now at the Schlumberger Laboratory for
Computer Science. Keith Waters and I are working on interactive
deformable models. We're now able to compute and render deformable
models in real time on our Silicon Graphics Iris 240 GTX computer.
For example, here is a simulation of a nonlinear membrane
constrained at the four corners and released in a gravitational
field. Watch it bounce and wiggle around.
Here you're seeing a physically-based model of flesh. It's a
three dimensional lattice of masses and springs with muscles
running through it. Again, this is computed and displayed in real
time. You can see the muscles underneath displayed as red lines.
They're fixed in space at one end and attached to certain nodes of
the lattice model at the other end. By contracting the muscles we
can produce deformations in this slab of -- whale blubber, if you
will. We did this simulation as an initial step towards animating
faces using deformable models as models of facial tissue. And of
course, the muscle models make good facial muscles.
The next clip will demonstrate real time, physically-based
facial animation on our SGI computer. Here we see the lattice
structure of the face. Let's not display all of the internal nodes
so that we can see the epidermis of the lattice more clearly.
There. Now we're contracting the zygomatic muscle attached to one
edge of the mouth -- now both zygomatics are contracting to create
a smile. The muscles inside the face model are producing forces
which deform the flesh to create facial expressions.
Now the epidermis polygons are displayed with flat shading. Next
we contract the brow muscles. Here the epidermis is being shaded
smoothly. Finally, we relax the muscles and the face returns to
normal.
An important reason for applying the physically-based modeling
approach to facial animation is realism. For instance, the facial
tissue model automatically produces physically realistic phenomena
such as the laugh lines around the mouth and the cheek bulges that
you see here.
Keith videotaped this animation off of our machine only last
week. Our next step will be to develop control processes to
coordinate the muscles so that the face model can create a wide
range of expressions in response to simple commands. Keith's prior
work on facial animation, published in SIGGRAPH 87, showed how one
can go about doing this using muscle model processes. Beyond muscle
control processes, we're also interested in incorporating vocoder
models -- that is, physically-based speech coding and generation
models, so that this face can talk to you.
The tape will end soon, so I'll release the podium to Dr. John
Platt, who will talk about constraint methods and control. Thank
you.