Each human joint has a range of motion that is (at least somewhat) independent of the joints to which it is connected. While the geometry of a hand is fairly fixed, to know where the hand is in space y ou have to know the position of the torso, the shoulder joint, the elbow, and the wrist. Further, in normal motion, each of these moves along a trajectory involving momentum, purpose, and such subtle arts as walking, running, turning, etc. "Difficult" doesn't begin to describe it!
A normal body has a rigid skeleton which moves in response to muscle contraction. To a great extent, the limbs are fixed geometry, but the skin (and clothing) surface rearranges itself to remain continuous over the sketon. By defining a skeleton and the skin-to-skeleton relationship, the very complex problem of body motion can be reduced to a less-complex question of skeletal motion. That's called "rigging".
One of the work-arounds for the problem of motion is to use the power of film to provide data for animation. By marking a live human body with dots or other markers and filming the individual performing the desired motion, it is possible to digitize the trajectories of the joints and then apply this data to the motion of a digital model. This process is generally referred to as "motion capture".
Synthetic motion & Synthetic actors
It would be great if you could generate an animated human (or ORC--think Lord of the Rings battle scenes) by scripting "runs from upper-left to lower-right while screaming" or similar. The ability to turn such a compact representation into believable motion is being developed in animation studios and computer-science departments (including UW's) around the world. Soon.
2D Synthetic actors
The 2D texture-mapped actor we met earlier might be extended by mapping frames from an animation onto a flat plane. By filming the original from multiple view-points we could even have an actor that we could walk around and get a reasonable facsimile of 3D. Several 'consumer quality' rendering tools, including ones used in this class, have added this ability in recent years, though it may require an added-cost plugin to do this.
Last updated: April, 2014