Book Image

Box2D for Flash Games

Book Image

Box2D for Flash Games

Overview of this book

Physics games are getting more and more popular, and Box2D is the best choice if you are looking for a free, stable and robust library to handle physics. With Box2D you can create every kind of 2D physics game, only coding is not the fun part, but the game itself. "Box2D for Flash Games" will guide you through the process of making a Flash physics game starting from the bare bones and taking you by hand through complex features such as forces, joints and motors. As you are learning, your game will have more and more features, like the physics games you are used to playing. The book analyzes two of the most played physics games, and breaks them down to allow readers to build them from scratch in a step-by-step approach. By the end of the book, you will learn how to create basic primitive bodies as well as complex, compound bodies. Motors will give life to cars, catapults and siege machines firing bullets, while a complete collision management will make your game look even more realistic. If you want to make full Flash games with physics, then Box2D for Flash Games will guide you through the entire process of making a Flash physics game.

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Box2D for Flash Games
Nov, 2012 | 166 pages
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Table of Contents (15 chapters)
Box2D for Flash Games
Box2D for Flash Games
Credits
Credits
About the Author
About the Author
About the Reviewers
About the Reviewers
www.PacktPub.com
www.PacktPub.com
Preface
Preface
Free Chapter
1
Hello Box2D World
Hello Box2D World
Downloading and installing Box2D for Flash
Hello Box2D World
Defining the Box2D World
Running the simulation
Summary
2
Adding Bodies to the World
Adding Bodies to the World
Your first simulation – a ball falling on the floor
Creating a circular shape
Creating a fixture
Using debug draw to test your simulation
Creating a box shape
Different body types – static, dynamic, and kinematic
Density, friction, and restitution
Creating a Totem Destroyer level
Creating compound bodies
Creating an oriented box shape
Creating any kind of convex polygons
Summary
3
Interacting with Bodies
Interacting with Bodies
Selecting and destroying bodies with a mouse click
Assigning custom attributes to bodies
Looping through bodies and getting their properties
Summary
4
Applying Forces to Bodies
Applying Forces to Bodies
Falling apples, revamped
Force, impulse, and linear velocity
Applying an impulse to get a linear velocity
Applying a force to get a linear velocity
Forces in a real game
Physics games aren't just a matter of physics
Placing the physics bird
Shooting the physics bird
Summary
5
Handling Collisions
Handling Collisions
Checking for collisions
Box2D built-in collision listener
Trace the beginning and the end of a collision
Detect when you are about to solve a collision and when you have solved it
Detecting when the idol falls on the floor in Totem Destroyer
Destroying bricks and killing pigs in Angry Birds
Summary
6
Joints and Motors
Joints and Motors
Picking and dragging bodies – mouse joints
Keeping bodies at a given distance – distance joints
Making bodies rotate around a point – revolute joints
When Angry Birds meets Crush the Castle
Controlling joints with motors
Controlling motors with keyboard
Don't let some bodies collide – filtering collisions
Putting it all together
Summary
7
Skinning the Game
Skinning the Game
Replacing debug draw with your own graphic assets
Summary
8
Bullets and Sensors
Bullets and Sensors
Experiencing tunneling
Preventing tunneling – setting bodies as bullets
Allow bodies to overlap while detecting contacts with sensors
Summary
Index
Index

Force, impulse, and linear velocity

Let's jump straight to the point with some new lines in the Main function:

public function Main() {
  world=new b2World(new b2Vec2(0,10),true);
  debugDraw();
  floor();
  sphereVector=new Vector.<b2Body>();
  for (var i:int=0; i<3; i++) {
    sphereVector.push(sphere(170+i*150,410,40));
  }
  var force:b2Vec2=new b2Vec2(0,-15);
  var sphereCenter:b2Vec2=sphereVector[0].GetWorldCenter();
  sphereVector[0].ApplyForce(force,sphereCenter);
  sphereCenter=sphereVector[1].GetWorldCenter();
  sphereVector[1].ApplyImpulse(force,sphereCenter);
  sphereVector[2].SetLinearVelocity(force);
  addEventListener(Event.ENTER_FRAME,updateWorld);
}

This is the core of this chapter and it's a lot of new stuff, so let's look at it line by line:

var force:b2Vec2=new b2Vec2(0,-15);

First, we need to create a b2Vec2 variable that will represent the force we want to apply to all spheres. Setting it at (0,-15) means this is a vertical force, which should make the spheres...

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