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694 Chapter 12: Animation
Timing group
Controls how keys are generated over time.
Star t Time—Specifies the first frame to generate
keys, which is the first frame to be considered for
the solution. Default=0.
Tip: If you set a start time that’s later than a
key frame-animated object’s last animation frame,
you may get unexpected motion during the
interim frames. For example, if you animate a
box’s position from frames 0 to 10, and then use
theboxinadynamicssimulationthatstartsat
frame 20, the box will move between f rames 10
and 20 b ecause of the Bezier controller’s default
interpolation. To avoid this, before solving the
simulation, set the last animation keyframe’s
Out tangent type to Linear or Step; see Bezier
Controllers (page 2–310).Alternatively,setthelast
key of the keyframed "input motion" after the sta rt
time of the simulation, or set a start time before
the last key.
End Time—Specifies the last key considere d for the
solution.Thisspinnerissettothelastframeofthe
active segment (page 3–904) when you create a new
simulation. For example, if your active segment
ends at f rame 200, w hen you click New to c reate a
new simulation, End Time is set to 200.
Calc Intervals Per Frame—Specifies how many
calculations are performed for each frame of the
simulation time r ange. Range=1 to 160.
Finding the right number for this spinner is a
matter of experiment ation. As a general rule, the
faster things are moving in the simulation, the
higher you should set th is value.
Note: If you find that some objects aren’t colliding
properly with others (they’re going through them),
increase the Calc Interva ls Per Frame value.
Keys Every N Frames—Specifies the frequency with
which keys are generated, per object. If this were
set to 2, keys would be generated in every other
frame.
Wa rn in g : When you reduce the key count by increasing
this setting, important in formation can be lost. For
example, if a collision occurs on frame n, the Dynamics
utility normally sets keys at frames n, n+1, and n-1. But if
you’vesetKeysEveryNFramesto2,akeyframeforthe
impact itself might not be generated, while keys for the
sudden reversal of motion would be gen erated on either
side of the (missing) impact. Thus, the motion controller
is left to interpolate motion in a region where the
motion should be sharply defined. When this happens,
motion can be incorrect and the remainder of the
solveisaffected.Intheaforementionedexample,akey
describing the impact is lost and the m otion controller
interpolates motion so that objects that should collide
actually intersect, rui nin g the simulation.
Time Scale—
Slowsdownorspeedsuptheoverall
effect of the simulation. It applies a linear scale