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dmitry-rizshkov Blog

Send Private Message Posted January 15, 2011 1:25 pm

Box2D is an Open Source 2D physics library for game makers. This library take care about friction, gravity, body mass, colissions and much more. Using this library you can create your own phisics based games, shooting games like Angry Birds, for example.

I will show you how it works on the small, basic example, writting with Qt. Why Qt - because this is a crossplatform library, write once use anywere, said on Qt main page. And this is a true.

Here is you can download Box2D library for your platform.

Let see how it works. First we need to create the "world".

b2World* m_Box2DWorld;

// Gravity vector. X and Y gravity could be setup here
b2Vec2 gravity(0.0f, -10.0f);

// Do we want to let bodies sleep? This is CPU saving.
bool doSleep = true;

// Create the world
m_Box2DWorld = new b2World(gravity, doSleep);

World is done. Now we need to add some game objects. Let add the ground object. This object will be a static object (we do not need to move it or recalculate it position).

// Gound body definition
b2BodyDef groundBodyDef;
// Ground position, in this example - the bottom part of the screen
groundBodyDef.position.Set(0.0f, 0.0f);

// World is create the body, you do not need to care about memory allocation
// for Box2D objects, library takes care about it
b2Body* groundBody = m_Box2DWorld->CreateBody(&groundBodyDef);

// Every body must be linked to some shape
// in this case this is a rectangle
b2PolygonShape groundBox;
groundBox.SetAsBox(640.0f, 20.0f);
// This fuction will create our share and link it
// to our body definition
groundBody->CreateFixture(&groundBox, 0.0f);

We have the ground, now lets add the cannon body

// Cannon
b2BodyDef cannonDef;
cannonDef.position.Set(40.0f, 45.0f);
// Rotation angle of cannon
cannonDef.angle = (-45 * (2 * PI)) / 360.0;

b2Body* cannonBody = m_Box2DWorld->CreateBody(&cannonDef);

b2PolygonShape cannonBox;
cannonBox.SetAsBox(10.0f, 20.0f);
cannonBody->CreateFixture(&cannonBox, 0.0f);

All of the object above (ground and cannon) is the static objects. Lets add some dynamic objects. This will be a row of the 5 blocks.

for(int i = 0; i < 5; ++i)
{
  b2BodyDef boxDef;
    // Object is a dynamic object
  boxDef.type = b2_dynamicBody;
  boxDef.position.Set(400.0, 30.0f + i * 20);

  b2Body* boxBody = m_Box2DWorld->CreateBody(&boxDef);

  b2PolygonShape boxBox;
  boxBox.SetAsBox(10.0f, 10.0f);

  b2FixtureDef fixtureDef;
  fixtureDef.shape = &boxBox;
    // Mass of the object
  fixtureDef.density = -0.1f;
    // Friction powr of the object
    // Object with high friction will slide more easily than
    // object with less friction
  fixtureDef.friction = 0.1f;

  boxBody->CreateFixture(&fixtureDef);
}

The world is done. Now we going to add the bullets. In the key down handler we will add code for shoot the bullets.

// Bullet
b2BodyDef bulletDef;
bulletDef.type = b2_dynamicBody;
bulletDef.position.Set(63.0f, 68.0f);

b2Body* bulletBody = m_Box2DWorld->CreateBody(&bulletDef);

b2PolygonShape bulletBox;
bulletBox.SetAsBox(10.0f, 10.0f);

b2FixtureDef fixtureDef;
fixtureDef.shape = &bulletBox;
fixtureDef.density = -0.9f;
fixtureDef.friction = 0.1f;

bulletBody->CreateFixture(&fixtureDef);

// Power of the initial impulse
float power = 100.0f;
// Alngel of the impulse
float angle = (-45 * (2 * PI)) / 360.0;

bulletBody->ApplyLinearImpulse(b2Vec2(cos(angle) * power, -sin(angle) * power), bulletBody->GetWorldCenter());

Use this sample app as a playground for your experiments. The potential of the Box2D library is a huge.

Want to see more examples? Check out the world construction kit. This is a really amazing. Or play Angry Birds game (available in the AppUp Store). Did you know - this game is uses Box2D library too.

All source code for this article available here.