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770 Chapter 13: reactor
Fr act u re
Create panel > Helpers > reactor > Fractu re
Menu bar > reactor > Create Object > Fracture
reactor toolbar > Create Fracture button
TheFracturehelperobjectsimulatesthebreaking
of a rigid body into a number of smaller pieces
as the result of an impact. To do this, you need
to supply the pieces that are glued together
to create the whole object. This is known as
"pre-fracturing." reactor does not break the pieces
you supply into smaller pieces.
Tip: An excellent tool for pre-fracturing your
objects is the ProCutter compound object (page
1–388).
Rigid bodies t hat are par t of a Fracture helper
move together as a single body. When a rigid body
that belongs to a Fracture helper collides with
another body, the collision information is analyzed
and if a threshold is exceeded, the rigid body is
removed from the fracture helper. Once the rigid
body has been removed, it moves independently
of the fr ac ture object and is f ree to collide w ith
the rigid bodies that are still part of the fracture
object. This can often result in a chain reaction of
pieces breaking off.
You can optionally enable the use of connectivity.
Connectivit y analyzes the bodies belonging to the
Fracture helper to find bodies that are in contact
w ith each other b efore simulation occurs. This
builds up an internal graph of which bodies are
connected to which other bodies. Thus groups
of rigid b odies t hat are connected to each other,
but disconnected with other bodies in the fracture
helper, can move independently as separate
fracturable objects. As a result, you can get the
effect of multiple fracturable objects in a single
Fracture helper .
After a fracture event occurs, reactor once
again ana lyzes this connectivity gr aph to create
newchunksiftheyexist. Ifyouseefloating
disconnected bodies mov ing str angely because
they are invisibly still part of a larger fracture
object,besuretoturnonconnectivity.Abodyis
considered to be in contact with another body if
the closest distance between them is less than the
collision tolerance.
To allow the fracture feature to work reasonably
well, reactor uses a special collision detec t ion
technique: penetration depth calculation. Unlike
normal rigid bodies in reactor, bodies that are
part of a fracture helper are allowed to continue to
collidewitheachother,evenwhentheyareina
interpenetrating state. To see how the non-fracture
bodies are treated during collision detection, see
the Frac ture Tips (page 2 –773) topic. Penetration
depth calculation, especially in the context of
the s imulation paradigm us ed in reactor, i s
compu tationally expensive, so you might notice
slower performance when the fracture bodies are
penetrating each other. Stability in these cases can
also become an issue.
reactor provides a number of techniques for
getting your fracture s imulation to behave more
realistically If you find that it is unstable. For
instance, the fractured object might appear to