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672 Chapter 12: Animation
key of the keyframed "input motion" after the start
time of the simulation, or set a star t time b efore
the last key.
End Time—Specifies the last key considered for the
solution. This spinner is set to the last frame of
the
active segment (page 3–998) when you create
a new simulation. For example, if your active
segment ends at f r ame 200, when you click New to
create a new simulation, End Time is set to 200.
Calc Inter vals Per Frame—Specifies how many
calculations are performed for each frame of the
simulation time range. Range=1 to 160.
Finding the right number for this spinner is a
matter of experim entation. As a general rule, the
faster things are moving in the simulation, the
higher you should set this value.
Note: If you find that some objects aren’t colliding
properly with others (they’re going through them),
increase the Calc Intervals Per Frame value.
K eys Ever y N Frames —Specifies the frequency with
which keys are generated, per object. If this were
set to 2, keys would be generated in every ot her
frame.
Wa rn in g : When you reduce the key count by increasing
this setting, impor tant information can be lost. For
example, if a collision occurs on frame n, the Dyn amics
utility normally sets k e ys at frames n, n+1, and n-1. But if
you’ve set Keys Every N Frames to 2, a keyframe for the
impact itself might not be generated, while keys for the
sudden reversal of motion would be generated on either
side of the (missing) impact. Thus, the motion controller
is left to interpolate motion in a regi on where the
motion should be sharply d efined. When this happens,
motion can be incorrec t and the remainder of the
solve is affected. In the aforementioned example, a key
describing the impact is lost and the motion controller
interpolates motion so that objects that should collide
actually intersect, ruining the simulation.
Time Scale—
Slows down or speeds up the overall
effect of the simulation. It applies a linear s cale
factor to the outside forces affecting each object
(gravity, wind, and s o on).
Thedefaultvalueof1resultsinnormalspeed.You
can scale dow n the simulat ion (make it slower) by
usingvaluesbelow1(from0.1to1),andyoucan
scale up the simulation (make it faster) by using
values greater than 1 (from 1 to 100). If you speed
up your simulation and objects begin to behave
incorrectly (going through objects, for example),
increase the Calc Intervals Per Frame value to
compensate.
Simulation Controls group
Relates to IK settings and the transfer of
momentum.
Use IK J oint L i mits —Uses the current IK joint
limitsettingsasconstraintsforhierarchiesinthe
simulation.
Use I K Joint Damping—Uses the IK damping
settings as constraints fo r hierarchies in the
simulation.
Air Resistance g roup
Density percent—Sets the air density in t he
simulation. A setting of 100 is the air at sea level.
Asettingof0isatotalvacuum.
When anything moves, it hits air resistance (except
in space). T he faster it moves, the higher the
relative air resistance with the square of the speed.
Thus, air resistance imposes an upper limit on the
speed of things that are falling with gravity, and
also makes objects tumble due to the effect of air
resistance on each face of the object.
Clo se —Closes the Dy namics utility.