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### Box2D Flash v2.0.2 入门教程详解 #### 一、关于Box2D Flash Box2D Flash是一款专为游戏设计的2D刚体物理模拟库，适用于那些希望在游戏中实现真实物体运动效果及增强互动性的开发者。通过使用Box2D Flash，程序员可以让游戏中的对象按照牛顿力学的原理移动，从而无需过多的人工干预即可达到自然流畅的动画效果。 #### 二、预备知识在深入学习Box2D Flash之前，需要具备一定的预备知识： 1. **基本物理概念**：包括质量、力、扭矩和冲量等。如果这些概念对你来说较为陌生，建议首先查阅Chris Hecker和David Baraff提供的众多教程。虽然不需要完全理解这些教程的所有细节，但它们能够很好地帮助你了解使用Box2D所需的基本物理概念。 2. **数学与物理知识来源**：Wikipedia是一个非常优秀的资源，它提供了丰富的数学和物理知识，并且其内容经过精心编辑，相比简单的网络搜索更具针对性和准确性。 3. **ActionScript 3 (AS3) 编程经验**：由于Box2D AS3是用AS3编写的，因此需要掌握该编程语言。如果你之前主要是使用ActionScript 2 (AS2)，那么在接触Box2D之前可能需要先完成一些较简单的项目来熟悉AS3环境。 #### 三、核心概念 Box2D Flash基于以下几个核心对象进行工作： 1. **刚体 (Rigidbody)**：刚体是指一种物质块，其中任意两点之间的距离始终保持不变。在讨论中，“刚体”、“物体”或“body”这几个术语可以互换使用。 2. **形状 (Shape)**：形状是指刚性地附着于刚体上的2D碰撞几何结构。形状具有材料属性，如摩擦系数和恢复系数，这些属性决定了刚体之间的碰撞行为。 3. **约束 (Constraint)**：约束是一种物理连接，它可以限制刚体的自由度。例如，在2D空间中，一个刚体有三个自由度（平移和旋转）。如果我们把一个刚体固定在墙上（像钟摆一样），那么我们实际上就施加了一个约束，减少了其自由度。 #### 四、进一步学习资源为了更好地理解和应用Box2D Flash，以下是一些额外的学习资源推荐： 1. **Box2D官方文档**：Box2D的官方文档提供了详尽的技术说明和技术细节，是学习Box2D不可或缺的一部分。 2. **在线教程和视频**：互联网上有许多专门针对Box2D Flash的教程和视频课程，这些资源可以帮助你更快地掌握Box2D的使用技巧。 3. **社区和论坛**：加入Box2D的用户社区和论坛，与其他开发者交流心得，解决问题。社区中的高级用户往往能提供宝贵的建议和支持。 4. **示例项目**：通过研究现有的Box2D Flash项目，你可以更直观地了解如何将Box2D应用于实际的游戏开发中。尝试自己动手修改这些示例项目，加深对Box2D的理解。 #### 五、总结 Box2D Flash是一款功能强大的物理引擎，它为游戏开发者提供了一种简单有效的方式来创建逼真的物理交互效果。通过本文档介绍的核心概念和预备知识，你已经具备了开始使用Box2D Flash的基础。接下来，建议通过实践项目和深入学习来不断提升自己的技能水平。随着对Box2D Flash掌握的加深，你将能够在自己的游戏中实现更加生动、真实的物理效果。

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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.

剩余39页未读，继续阅读

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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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