3D Animation: Actors and Hierarchies of Motion

Just like in the production of a movie, an actor in animation is someone (or something) that plays a role in the movie sequence. In animation, actors are simply objects or groups of objects that have a single identifiable name. A single box could be an actor named "Box" or the entire composition of a door, including the wood slab, a window, door knobs, etc., can be grouped together and simply called door. From that point forth in the animation, those objects will operate collectively as "Door". Assemblies of this sort can be very simple, as in the case of the door, or can be very complex, such as a human figure with independant arms, legs, hands, fingers, and toes. We'll work with an actor named Ergoman a bit later.

Animation actors can also have specific types of mobility. Any object in MicroStation can be keyframed in any condition without contraint. These objects can be moved, stretched, and scaled around without anything preventing you from doing this. There are many cases, however, when we need, or desire, to constrain the mobility of a given actor to a specific set of motions. Without constraints, an actor has a full nine degrees of freedom: 3 axes of linear motion, 3 axes of revolution, and 3 axes of scaling. (Non-Actors add a 10th "degree" of freedom, as they can be warped and distorted, sometimes called morphing, into a different shape). Objects like a door, for instance, do not need all nine of these freedoms. A door, in fact, only needs one: rotation about its vertical axis. Because of this, we have the ability in MicroStation to establish constraint properties for a given actor, and only allow for its motion along the freedoms that we choose. This keeps our understanding of an object's motion simpler, and the process by which we manipulate it cleaner and less prone to mistakes.

Use MicroStation Link to work with this door as an example.

A door may not be the best example of when we would choose to constrain an actor, as a door is a simple enough object to work with. a human figure, on the other hand, is not. Arms and legs twist only one way, and I doubt that you would want anyone pulling your head off of the top of your body, sliding it along the vertical axis... Actors give us the ability to identify these parts, then to establish their individual rules of motion.

Hierarchies of Motion
Taking the example of a human figure a bit farther, we need in animation, the capability to link these unique actor parts together into some meaningful, and practical, organization. Typically, an arm would be a separate actor from a torso, as the two have very different constraints on their motion, but if we move the torso along an axis, we would more than likely want the arms to follow along. Linkages of this sort are accomplished through actor hierarchies.

To create a hierarchy of motion in MicroStation, we must simply 'Attach' one actor to another. Their separate motions will then be linked, and motion of the upper parts of the hierarchy (the torso) will effect motion of any lower parts (the arm, then hand, then fingers). Once actors are attached, we can manipulate them individually at will, but all motions will carry throughout the assembly. We can then keyframe a condition of the complete hiererchical assembly (you cannot keyframe individual parts separately), and create a sequence of motion.

MicroStation Link file ERGOMAN2.DGN