Graphics Reference
In-Depth Information
In a
procedural animation,
the artist, who is usually a programmer, specifies an
explicit equation for the pose at all times. In a sense, all computer animation is
procedural. After all, to execute an animation on a computer a
procedure
must
be computing the new object positions. In the case of key pose animation that
procedure performs interpolation and in the case of dynamics it evaluates physi-
cal forces. However, it is useful to think of a separate case where we specify an
explicit, and typically not physically based, equation for the motion. Our cannon-
ball example straddled the line between dynamics and procedural animation. A
good example of complex procedural motion is Perlin's [Per95] dancer, whose
limbs move according to a noise function.
Procedural animations today are primarily used for very simple demonstra-
tions, like a planet orbiting a star, and for particle system effects in games. The lack
of general use is probably because of the challenges of encoding artist-specified
motion as an explicit equation and making such motion interact with other objects.
The current state of the art is to combine poses with dynamics. This combines
the expression of an actor's performance or artist's hand with the efficiency and
realism of physically based simulation. There are many ways to approach hybrid
control schemes. We outline only a few of the key ideas here.
An active research topic is adjusting or authoring poses using physics or phys-
ically inspired methods. For example, given a model of a chair and a human, a
system autonomously solves for the most stable and lowest-energy position for a
sitting person. A classic method in this category is
inverse kinematics
(IK). An
IK solver is given an initial pose, a set of constraints, and a goal. It then solves
for the intermediate poses that best satisfy these (see Figure 35.16), and the final
pose if it was underdetermined. For example, the pose may be a person standing
near a bookshelf. The goal may be to place the person's hand on a book that is
on the top of the shelf, above the character's head. The constraints may be that
the person must remain balanced, that all joints remain connected, and that no
joint exceeds a physical angular limit. IK systems are used extensively for small
modifications, such as ensuring that feet are properly planted when walking on
uneven terrain, and reaching for nearby objects. They must be supported by more
complex systems when the constraints are nontrivial.
It is also often desirable to insert a previously authored animation into a novel
scene with minor adjustment; for example, to adapt a walk animation recorded
on a flat floor to a character that is ascending a flight of stairs without the feet
penetrating the ground or the character losing balance. This is especially the case
for video games, where the character's motion is a combination of user input,
external forces, and preauthored content [MZS09, AFO05, AdSP07, WZ10].
As we said earlier, motion planning is an AI problem. But it is closely coupled
with dynamics. Getting dressed in the morning is more complex than reaching for
a bookâit cannot be satisfied by a single pose. Presented with a dresser full of
clothing, a virtual human would have to not only find the series of poses required
to step into a pair of pants while remaining balanced, but also realize that the
