9

Working with Rigid Envelopes 1091
See also
Link Sub-Object (page 2–1135)
Bulge Sub-Object (page 2–1141)
Tendons Sub-Object (page 2–1147)
Working with R igid Envelopes
Vertices influenced by a rigid envelope follow the
link itself rather than the deformation spline, like
deformable vertices do. Low vertex-count models
used for games are ty pically connected to the
skeletons with rigid envelopes to simplify export
to game eng ines. In such a case, each vertex can be
simply described as assig ned to a specific link . It
canbelocatedbygivingalengthalongthelink,a
distance from the center of t he link, and an angle
around the link. As the link is transformed, all
the vertices assigned to the vertex are uniformly
transformed. This makes it simple for game
enginestomovethecharacter’sskin.
Theverticesmoveequallywiththecontrolling
link, so the skin appears rigid. For instance, in the
case of a flexing arm, the skin between the wrist
and elbow tightly follows the m ovement of the
forearm, with no change in the spacing between
vertices. The skin b etween the elbow and shoulder
tightly follows the upper arm. Vertices around the
elbowinfluencedbybothlinks,smoothlyblend
to an average position between the two changed
links.
Rigid and deformable envelopes can be mixed on
a given character. You mig ht, for instance, u se
deformable envelopes for most of the character,
but choose rigid for the head and for a suit of
armor around the torso.
Note: InLinksub-object,youcanturnontwisting
for rigid envelopes. As the wrist twists t he ver tices
on the forearm will twist also.
See also
Envelope Sub-Object (page 2–1130)
Worki ng with B ot h Defor ma ble
and R igid Envelopes
You can assign both Deformable and Rigid to a
given link. For example, you might assign both
to the shins, and size the envelop es differently to
gain the benefits of b oth.
First, size the deformable envelope to affect the
whole lower leg area and smoothly transition
into the thigh and ankle areas. Next, you define
asmallerrigidenvelopetoencompassthestiff
unbendable section of the shin. Increase the
weight of the rigid envelope to balance its effect
against that of the deformable envelope.
Some users mig ht use both envelopes as a way to
have a bone affect two completely separate regions
of a mesh. The envelopes are scaled so that the
deformable envelope influences certain vertices on
onesideofthelinkandtherigidenvelopeaffects
the other side.
See also
Envelope Sub-Object (page 2–1130)
Adjusting Link Par ameters
Physique creates a continuous spline through
eachofthejointsinthehierarchy. Thesplineis
representedbyanorangelinewithintheobject.
It maintains continuity through each joint: as a
joint angle changes, the spline passing through the
joint remains a smooth curve. Physique relies on
the deformation spline to obtain smooth bends of
the skin object. Physique offers bias and tension
controls to adjust the shape of the deformation
spline.