Package org.jbox2d
Core Concepts
Box2D works with several fundamental concepts and objects. We briefly define these objects here and more details are given later in this document.
shape
A shape is 2D geometrical object, such as a circle or polygon.
rigid body
A chunk of matter that is so strong that the distance between any two bits of matter on the chunk is constant. They are hard like a diamond. In the following discussion we use body interchangeably with rigid body.
fixture
A fixture binds a shape to a body and adds material properties such as density, friction, and restitution. A fixture puts a shape into the collision system (broad-phase) so that it can collide with other shapes.
constraint
A constraint is a physical connection that removes degrees of freedom from bodies. A 2D body has 3 degrees of freedom (two translation coordinates and one rotation coordinate). If we take a body and pin it to the wall (like a pendulum) we have constrained the body to the wall. At this point the body can only rotate about the pin, so the constraint has removed 2 degrees of freedom.
contact constraint
A special constraint designed to prevent penetration of rigid bodies and to simulate friction and restitution. You do not create contact constraints; they are created automatically by Box2D.
joint
This is a constraint used to hold two or more bodies together. Box2D supports several joint types: revolute, prismatic, distance, and more. Some joints may have limits and motors.
joint limit
A joint limit restricts the range of motion of a joint. For example, the human elbow only allows a certain range of angles.
joint motor
A joint motor drives the motion of the connected bodies according to the joint's degrees of freedom. For example, you can use a motor to drive the rotation of an elbow.
world
A physics world is a collection of bodies, fixtures, and constraints that interact together. Box2D supports the creation of multiple worlds, but this is usually not necessary or desirable.
solver
The physics world has a solver that is used to advance time and to resolve contact and joint constraints. The Box2D solver is a high performance iterative solver that operates in order N time, where N is the number of constraints.
continuous collision
The solver advances bodies in time using discrete time steps. Without intervention this can lead to tunneling.
Box2D contains specialized algorithms to deal with tunneling. First, the collision algorithms can interpolate the motion of two bodies to find the first time of impact (TOI). Second, there is a sub-stepping solver that moves bodies to their first time of impact and then resolves the collision.
Modules
jbox2d is composed of three main modules:common,
collision , and dynamics. The Common module has code for allocation, math, and settings. The
Collision module defines shapes, a broad-phase, and collision
functions/queries. Finally the Dynamics module provides the simulation world,
bodies, fixtures, and joints.
Units
jbox2d works with floating point numbers and tolerances have to be used to make jbox2d perform well. These tolerances have been tuned to work well with meters-kilogram-second (MKS) units. In particular, jbox2d has been tuned to work well with moving shapes between 0.1 and 10 meters. So this means objects between soup cans and buses in size should work well. Static shapes may be up to 50 meters long without trouble.
Being a 2D physics engine, it is tempting to use pixels as your units. Unfortunately this will lead to a poor simulation and possibly weird behavior. An object of length 200 pixels would be seen by jbox2d as the size of a 45 story building.
Caution: jbox2d is tuned for MKS units. Keep the size of moving objects roughly between 0.1 and 10 meters. You'll need to use some scaling system when you render your environment and actors. The jbox2d testbed does this by using an OpenGL viewport transform. DO NOT USE PIXELS.
It is best to think of jbox2d bodies as moving billboards upon which you attach your artwork. The billboard may move in a unit system of meters, but you can convert that to pixel coordinates with a simple scaling factor. You can then use those pixel coordinates to place your sprites, etc. You can also account for flipped coordinate axes.
Another limitation to consider is overall world size. If your world units become larger than 2 kilometers or so, then the lost precision can affect stability.
Caution: jbox2d works best with world sizes less than 2 kilometers.
jbox2d uses radians for angles. The body rotation is stored in radians and
may grow unbounded. Consider normalizing the angle of your bodies if the
magnitude of the angle becomes too large
Body.setTransform(org.jbox2d.common.Vec2, float).
Caution: jbox2d uses radians, not degrees.
Source: Box2D-Website