Graphics Reference
In-Depth Information
Actor Parameters, Boundaries, and Hull Types
Objects need to have several qualities in order to behave naturally in physics simulations. You've already come
across several of these as you've been setting up objects for Bullet simulations. Now I'll discuss what the terms
mean in a little more detail. The rigid body physics settings in the Physics properties area contain the following
options:
Actor Actor enables the rest of the buttons so that physics parameters can be set for an object. If this is not
selected, theobjectwillbeevaluated onlyasanobstacle.Youcanstillsetthecollisionboundsforanonactor.
Ghost Ghostenablesin-gameobjectsto not havephysicalinteractionswithothers,sothattheycanbepassed
through like a ghost. Because ordinary animation does not have physics calculated automatically, this option
is not usually needed in nongame simulations.
Dynamic Dynamic enables the object to be moved by physical forces. Only objects with this option selected
will have Ipos generated for them during a simulation.
Rigid Body Rigid Body enables spring-based collision behavior and rolling. This is a necessary option for
convincing physical simulation of multiobject interaction.
No Sleeping No Sleeping disables BGE's default behavior of turning off physics on objects that have not
been moved or touched for some time. If you find that an object is becoming unresponsive over the course of
a long simulation, select this option. Don't select it if it is not necessary.
In the next row in the panel are the Mass and Radius parameters. The Mass value determines the mass of
the object relative to other objects in the simulation. When an object with a greater mass collides with an object
with less mass, the trajectory of the object with less mass is more affected. As in the real world, objects with
greater mass have greater inertia. This means that they require more force to get moving and also that they are
more difficult to slow down when they are moving, making them less subject to damping in the physics engine.
BecauseofcurrentlimitationsintheBulletphysicsengine(andinfactallphysicsengines),simulationsbecome
unstable when objects with extremely different masses interact in certain ways. Objects in constraint chains
shouldbewithinafewkilogramsofeachother,andveryheavyobjectsshouldnotbeplacedontopofverylight
objects.Thesekindsofinteractionsbetweenobjectswithextremedifferencesinmassshouldprobablybekeyed
by hand or simulated in another way, such as with particles.
The Radius parameter indicates the size of the sphere collision bounds. When you enable an object as a dy-
namic actor, the sphere is displayed as a dotted line representing the default collision boundary for the object
(as shown in Figure 8-31 ) . This is accurate only if your object is a sphere. If not, you will need to select a
more-appropriate collision boundary type, and Radius is not pertinent (although the dotted circle will continue
to display).
IntheareabelowtheMassandRadiusfieldsarethedampingfields.DampingforbothTranslationandRota-
tion remove motion energy from the object over timeā€”Damp from Translation and RotDamp from Rotation.
When these values are zero, no damping occurs and the movement of the object is unimpeded. With higher val-
ues, movement and rotation are diminished to mimic the effects of air friction.
 
Search WWH ::




Custom Search