In Luxo Jr., there was an example of this problem of overshooting splines.
Jr.'s base was very heavy and when he hopped, we wanted the base to start
stationary, then pop up in the air from the momentum of his jump, arc
over, then land with a thud, suddenly stationary again. For the up
translation, there were three keyframes, the two stationary positions and the
highest point of his jump. The spline software forced continuity, so that his
base would move down under the surface of the floor just before and after
the jump. (figure 10a) The solution was to put two new extremes, equal to
the two stationary extremes, on the frames just before and just after the
extremes. This "locked" down the spline, so that the up translation stayed
the same value, popped up in the air, landed and then stayed the same value
again. This gave the desired feeling of weight to his little base, (figure 10b)
The same solution can be achieved by breaking the spline using its
continuity parameter [16] at the two stationary extremes. This solution
requires a graphical display of the spline so that the correct shape can be
achieved.
FIGURE 10a. This spline controls the Z (up) translation of Luxo Jr.
Dips in the spline cause him to intersect the floor.
FIGURE 10b. Two extra extremes are added to the spline
which removes the dips and prevents Jr. from going into the basement.
2.8 ARCS
The visual path of action from one extreme to another is always described
by an arc. Arcs in nature are the most economical routes by which a form
can move from one position to another. In animation, such arcs are used
extensively, for they make animation much smoother and less stiff than a
straight line for the path of action. In certain cases, an arc may resolve
itself into a straight path, as for a falling object, but usually,, even in a
straight line action, the object rotates. [12]
In most 3D keyframe computer animation systems, the path of action from
one extreme to another is controlled by the same spline that controls the
timing (slow in and out) of the inbetween values. This may simplify
computating the inbetweens but it has unfortunate effects. When a motion
is slow, with many inbetweens, the arc of the path of action is curved, as
desired. But when the action is fast, the arc flattens out: the faster the
action, the flatter the arc. Sometimes this is desirable, but more often, the
path of even a fast motion should be curved or arced. Straight inbetweens
can completely kill the essence of an action.
The spline that defines the path of action should be separate from the spline
that defines the timing or spacing of the inbetweens for several reasons: so
that the arc of a fast acdon doesn't flatten out; so that you can adjust the
timing of the inbetweens without effecting the path of action; so that you
can use different splines to define the path of action (where a B-spline is
appropriate for its smoothness) and the timing (a CatmuU - Rom spline so
you can adjust it's tension and direction controls to get slow in and out).
This technique is not common, but research is being done in this area. [15]
2.9 EXAGGERATION
The meaning of exaggeration is, in general, obvious. However, the
principle of exaggeration in animation does not mean arbitrarily distorting
shapes or objects or making an action more violent or unrealistic. The
animator must go to the heart of anything or any idea and develop its
essence, understanding the reason for it, so that the audience will also
In Luxo Jr., there was an example of this problem of overshooting splines. Jr.'s base was very heavy and when he hopped, we wanted the base to start stationary, then pop up in the air from the momentum of his jump, arc over, then land with a thud, suddenly stationary again. For the up translation, there were three keyframes, the two stationary positions and the highest point of his jump. The spline software forced continuity, so that his base would move down under the surface of the floor just before and after the jump. (figure 10a) The solution was to put two new extremes, equal to the two stationary extremes, on the frames just before and just after the extremes. This "locked" down the spline, so that the up translation stayed the same value, popped up in the air, landed and then stayed the same value again. This gave the desired feeling of weight to his little base, (figure 10b) The same solution can be achieved by breaking the spline using its continuity parameter [16] at the two stationary extremes. This solution requires a graphical display of the spline so that the correct shape can be achieved. FIGURE 10a. This spline controls the Z (up) translation of Luxo Jr. Dips in the spline cause him to intersect the floor. FIGURE 10b. Two extra extremes are added to the spline which removes the dips and prevents Jr. from going into the basement. 2.8 ARCS The visual path of action from one extreme to another is always described by an arc. Arcs in nature are the most economical routes by which a form can move from one position to another. In animation, such arcs are used extensively, for they make animation much smoother and less stiff than a straight line for the path of action. In certain cases, an arc may resolve itself into a straight path, as for a falling object, but usually,, even in a straight line action, the object rotates. [12] In most 3D keyframe computer animation systems, the path of action from one extreme to another is controlled by the same spline that controls the timing (slow in and out) of the inbetween values. This may simplify computating the inbetweens but it has unfortunate effects. When a motion is slow, with many inbetweens, the arc of the path of action is curved, as desired. But when the action is fast, the arc flattens out: the faster the action, the flatter the arc. Sometimes this is desirable, but more often, the path of even a fast motion should be curved or arced. Straight inbetweens can completely kill the essence of an action. The spline that defines the path of action should be separate from the spline that defines the timing or spacing of the inbetweens for several reasons: so that the arc of a fast acdon doesn't flatten out; so that you can adjust the timing of the inbetweens without effecting the path of action; so that you can use different splines to define the path of action (where a B-spline is appropriate for its smoothness) and the timing (a CatmuU - Rom spline so you can adjust it's tension and direction controls to get slow in and out). This technique is not common, but research is being done in this area. [15] 2.9 EXAGGERATION The meaning of exaggeration is, in general, obvious. However, the principle of exaggeration in animation does not mean arbitrarily distorting shapes or objects or making an action more violent or unrealistic. The animator must go to the heart of anything or any idea and develop its essence, understanding the reason for it, so that the audience will also
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