9

Adjusting Vertical Motion 879
is based on t he current gravity setting , the height
of the Body Vertical key at lif t-off and touchdown,
and the amount of time in the air.
By default, there is no B ody Vertical key at the peak
of the biped’s trajectory; the biped’s p ea k airborne
heightiscalculatedandenforcedautomatically.
You cannot, for example, set a Body Vert ical key
at the peak of an airborne trajectory and move
thebipedupordown.Ifyouattempttodoso,
the biped will snap back to its orig inal airborne
height. A lthough this will c ause a Body Vertical
key (page 2–867)to appear on the Track Bar and
in Track View, the key will have no effect on the
biped’s airborne heigh t.
Grav ity and T iming
Inreality,thelengthoftimeaperson,animalor
insect stays in the air during a jump is based on
two fac tors:
• Howhighthecreaturejumps,whichinturnis
based on how hard the creature pushes with
its legs at the st art of the jump. T he creature’s
weight has no bearing on the height of the
jump,excepttoaffectitsabilitytogiveagood
push at the start. A very light character might
be taken by the wind and thus stay in the air
longer, but that circumstance is not part of the
gravitational equation.
• Thegravitationalpulloftheplanetfromwhich
thecreaturejumps.
From these two factors, you can calculate how
longthecreaturewillstayintheair. Youcan
also perform this calculation backward; if you
know how long the creature was in the air and
the gravitational equation for the planet, you can
figure out how high it jumped.
Jump height increases w ith time in air
character studio uses the latter method for
calculating the heig ht of a biped’s jump. It k nows
from footstep timing how long the biped will be in
the air, and it has a metho d for determining the
“gr av itational pull” in your scene.
The default gravity setting in character studio is
based on the standard equation for calculating t he
Earth’s gravitational pull, which is an acceleration
of approximately 32 feet/sec/sec. B ecause this
equat ion depends on an accurate measure of
distance, the biped’s height is used as a guide to
actual distances in the scene. For the purpose of
gravitational computation, the biped is considered
to be about 5’10” tall, the average height for a male
human being.
Of course, some of your characters will not be
thesameheightastheaverageman,soairborne
periods between footsteps for these characters will
appear inaccurate, with the biped jumping too
highortoolowforyourpurposes.
There are two solutions to this problem. You
can change the amount of time the character is
airb orne between footsteps (see Editing Footstep
Timing (page 2–869)), or you can accelerate gravity
w ith the GravAccel parameter.