9

Simple Constraints 727
constraint space. So, for instance, if you
move the constraint space relative to the child,
youcanusethisoptiontomovetheparent’s
constraint space into alignment with it.
•
Align Spa ces To Parent Spa ce—Aligns the parent
and child constraint spaces with the parent’s
constraint space. So, for instance, if you move
the constraint space relative to the parent,
youcanusethisoptiontomovethechild’s
constraint space into alignment with it.
Certain constraints have additional alignment
options:
•
AlignSpacesToEachBody—(Spring and
Point-Point > Stiff Spring only) This option
aligns each loca l constraint space (and hence
the spring attachment point) with the relevant
body, so each body’s attachment point is at
its pivot. It also sets the spring length to the
current distance between the nodes. This is the
default alig nment for springs and stiff springs.
•
Align Parent Space To Path—(Point-Path
constraintonly)Thisoptionalignstheparent
space with the loc al space of the shape used for
the constraint’s path. This is the default parent
space a lignment for the Point-Path constraint.
Simple Constraints
Thesimpleconstraintsprovidedwithreactorare:
• Spring (page 2–727)
• Linear Dashpot (page 2–730)
• Angular Dashpot (page 2–732)
Unlike cooperative constraints, the simple
constraint does not require you to add it to a
Constraint Solver (page 2–736) helper. Instead,
reactor adds all valid simple constraints in a
scene to the simulation by default. A two-bodied
simple constraint is valid if it has two rigid bodies
attached, while a sing le-bo died simple constraint
is valid if it has one rigid body attached.
Spring
Create pa nel > Helpers > reactor > Spring
Menu bar > reactor > Create Object > Spring
reactor toolbar > Create Spring button
The Spring helper lets you create a spring-like
effect between two rigid bodies in the simulation,
or between a rigid body and a point in space.
Duringthesimulation,thespringexertsforces
on the attached bo dies in an attempt to maint a in
its rest length. So, for instance, if the objects are
pulledapartsothatthespringattachmentpoints
are further apart than the rest length, the spring
works to bring them back together again.
You can configure a spring’s behavior by specifying
its stiffness, damping, and rest length. reactor
also lets you choose whether the spring acts
under extension (when the attachment points are
pulled fur t her apart) or compression (when the
attachment points are pushed closer together)
or both (the default). A spring that acts only on
extension behaves as if the objects were joined
with a rubber band.
You do not have to add a S p ring constraint
explicitly to a simulation, as all valid springs in a
scene are added to the simulation by default. A
two-bodied spring is valid if it has two rigid b odies
attached, while a single-bodied spring is valid if it
has one rigid body attached. When not selected,
invalid springs will be colored red in the viewport.
Procedures
To create a spring:
• Chooseoneoftheaboveoptions,andthenclick
in any viewport to add the Spring.