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Box2DFlash v2.0.2 User manual

About

Box2DFlash is a 2D rigid body simulation library for games. Programmers can use it in

their games to make objects move in believable ways and make the world seem more

interactive. From the game's point of view a physics engine is just a system for

procedural animation. Rather than paying (or begging) an animator to move your actors

around, you can let Sir Isaac Newton do the directing. Box2DFlash is written in AS3, and

resides in the Box2d namespace. Most of the types defined in the engine begin with the

b2 prefix, to match the C++ version.

Prerequisites

In this manual I'll assume you are familiar with basic physics concepts, such as mass,

force, torque ( 扭转力） , and impulses （推力） . If not, please first consult （翻阅） the many

tutorials provided by Chris Hecker and David Baraff (google these names). You do not

need to understand their tutorials in great detail, but they do a good job of laying out the

basic concepts that will help you use Box2D.

Wikipedia is also an excellent source of physics and mathematics knowledge. In some

ways it is more useful than Google, because it has carefully crafted content.

This is not a prerequisite, but if you are curious about the inner workings of Box2D, you

can look at these articles .

Since Box2DAS3 is written in Actionscript 3, you are expected to be experienced in this. If

you come from an AS2 background, you may find it easier to start on an easier project

before using Box2D, but it is not impossible.

Core Concepts

Box2D works with several fundamental objects. We briefly define these objects here and

more details are given later in this document.

rigid body （刚体）

A chunk of matter （物质） that is so strong that the distance between any two bits of

matter on the chunk is completely constant. They are hard like a diamond. In the

following discussion we use body

body

body interchangably with rigid body.

shape

A 2D piece of collision geometry that is rigidly attached to a body. Shapes have

material properties of friction and restitution. （依附于 body 的 2d 碰撞几何结构，具有

摩擦和恢复特性）

Constraint （约束）

A constraint is a physical connection that removes degrees of freedom from bodies. In

2D a body has 3 degrees of freedom. If we take a body and pin it to the wall (like a

pendulum) we have constrained

constrained

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 （接触约束）

A special constraint designed to prevent penetration （穿透） of rigid bodies and to

simulate （模仿） friction and restitution. You will never create a contact constraint, they

are created automatically by Box2D.

joint

This is a constraint used to hold two or more bodies together. Box2D supports these

joint types: revolute, prismatic, distance, and more. Joints may support limits

limits

limits and

motors

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, shapes, and constraints that interact together.

Box2D supports the creation of multiple worlds, but this is usually not necessary or

desirable.

Hello Box2D

This is a small example program that creates a large ground box and a small dynamic box.

This code does not contain any graphics, so prepare to be underwelmed. :). The

matching documents for this example have yet to be created.

Creating a World

Every Box2D program begins with the creation of a world object. This is the physics hub

that manages objects, and simulation.

create a world object, first we need to define a bounding box for the world. Box2D

uses the bounding box to accelerate collision detection. The size isn't critical, but a better

fit will improve performance. It is better to make the box too big than to make it too

small.

view source print ?

1 varworldAABB:b2AABB =newb2AABB();

2 worldAABB.lowerBound.Set(-100.0, -100.0);

3 worldAABB.upperBound.Set(100.0,100.0);

Caution

The world

world

world AABB

AABB

AABB should always be bigger then the region where your bodies are located.

It is better to make the world AABB too big than too small. If a body reaches the

boundary of the world AABB it will be frozen and will stop simulating.

Next we define the gravity vector . Yes, you can make gravity go sideways (or you could

just rotate your monitor). Also we tell the world to allow bodies to sleep when they come

to rest. A sleeping body doesn't require any simulation.

view source print ?

1 vargravity:b2Vec2 =newb2Vec2 (0.0, -10.0);

2 vardoSleep:Boolean=true;

Now we create the world object.

view source print ?

1 varworld:b2World =newb2World(worldAABB, gravity, doSleep);

So now we have our physics world, let's start adding some stuff to it.

Creating a Ground Box

Bodies are built using the following steps:

1 Define a body with a position, damping, etc.

2 Use the world object to create the body.

3 Define shapes with geometry, friction, density, etc.

4 Create shapes on the body.

5 Optionally adjust the body's mass to match the attached shapes.

For step 1 we create the ground body. For this we need a body

body

body definition

definition

definition . With the

body definition we specify the initial position of the ground body.

view source print ?

1 vargroundBodyDef:b2BodyDef =newb2BodyDef();

2 groundBodyDef.position.Set(0.0, -10.0);

For step 2 the body definition is passed to the world object to create the ground body.

The world object does not keep a reference to the body definition. The ground body is

created as a static body. Static bodies don't collide with other static bodies and are

immovable. Box2D determines that a body is static when it has zero mass. Bodies have

zero mass by default, therefore they are static by default.

view source print ?

1 vargroundBody:b2Body = world.CreateBody(groundBodyDef);

For step 3 we create a ground polygon definition. We use the

SetAsBox

shortcut to form

the ground polygon into a box shape, with the box centered on the origin of the parent

body.

view source print ?

1 vargroundShapeDef:b2PolygonDef =newb2PolygonDef();

2 groundShapeDef.SetAsBox(50.0,10.0);

The

SetAsBox

function takes the half-width and half-height. So in this case the ground

box is 100 units wide (x-axis) and 20 units tall (y-axis). Box2D is tuned for meters,

kilograms, and seconds. So you can consider the extents to be in meters. However, it is

possible to change unit systems, as discussed later in this document.

We finish the ground body in step 4 by creating the ground polygon shape on the ground

body.

view source print ?

1 groundBody.CreateShape(groundShapeDef);

Again, Box2D does not keep a reference to the shape or body definitions. It copies the

data into the

b2Body

structure.

Note that every shape must have a parent body, even shapes that are static. However,

you can attach all static shapes to a single static body. This need for static bodies is done

to make the Box2D code more uniform internally, reducing the number of potential bugs.

You might notice a pattern here. Most Box2D types are prefixed with

. This is done to

reduce the chance for naming conflicts with your code.

Creating a Dynamic Body

So now we have a ground body. We can use the same technique to create a dynamic

body. The main difference, besides dimensions, is that we must establish the dynamic

body's mass properties.

First we create the body using

CreateBody

view source print ?

1 varbodyDef:b2BodyDef =newb2BodyDef();

2 bodyDef.position.Set(0.0,4.0);

4 varbody:b2Body = world.CreateBody(bodyDef);

Next we create and attach a polygon shape. Notice that we set density to 1. The default

density is zero. Also, the friction on the shape is set to 0.3. Once the shape is attached,

we instruct the body to compute it's mass properties from the attached shapes using the

method

SetMassFromShapes

. This gives you a hint that you can attach more than one

shape per body. If the computed mass is zero, then the body becomes truly static. Bodies

have a mass of zero by default, that's why we didn't need to call

SetMassFromShapes

for

the ground body.

view source print ?

1 varshapeDef:b2PolygonDef =newb2PolygonDef();

2 shapeDef.SetAsBox(1.0,1.0);

3 shapeDef.density =1.0;

4 shapeDef.friction =0.3;

5 body.CreateShape(shapeDef);

6 body.SetMassFromShapes();

That's it for initialization. We are now ready to begin simulating.

Simulating the World (of Box2D)

So we have initialized the ground box and a dynamic box. Now we are ready to set

Newton loose to do his thing. We just have a couple more issues to consider.

Box2D uses a bit of numerical code called an integrator

integrator

integrator . Integrators simulate the

physics equations at discrete points of time. This goes along with the traditional game

loop where we essentially have a flip book of movement on the screen. So we need to

pick a time step for Box2D. Generally physics engines for games like a time step at least

as fast as 60Hz or 1/60 seconds. You can get away with larger time steps, but you will

have to be more careful about setting up the definitions for your world. We also don't like

the time step to change much. So don't tie the time step to your frame rate (unless you

really, really have to). Without further ado, here is the time step.

view source print ?

1 vartimeStep:Number=1.0/60.0;

In addition to the integrator, Box2D also uses a larger bit of code called a constraint

constraint

solver

solver . The constraint solver solves all the constraints in the simulation, one at a time. A

single constraint can be solved perfectly. However , when we solve one constraint, we

slightly disrupt other constraints.

get a good solution, we need to iterate over all

constraints a number of times. The suggested iteration count for Box2D is 10. You can

tune this number to your liking, just keep in mind that this has a trade-off between speed

and accuracy. Using fewer iterations increases performance but accuracy suffers.

Likewise, using more iterations decreases performance but improves the quality of your

simulation. Here is our chosen iteration count.

view source print ?

1 variterations:Number=10;

Note that the time step and the iteration count are completely unrelated. An iteration is

not a sub-step. One iteration is a single pass over all the constraints withing a time step.

You can have multiple passes over the constraints within a single time step.

We are now ready to begin the simulation loop. In your game the simulation loop can be

merged with your game loop. In each pass through your game loop you call

b2World.Step

. Just one call is usually enough, depending on your frame rate and your

physics time step.

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内容反馈

wutao_sc

2012-03-03

英文的，英语不好的就不用下了
shijiezhenqimiao

2012-11-13

还不错哦。就是英文的

西溪漫步

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