Box2D.js full tutorial

Box2D is a physics engine that can be used in game development or other interactive applications. In this tutorial, we will cover the basics of using Box2D in Javascript.

Part 1: Setting Up Box2D

The first step to using Box2D in Javascript is to include the library in your project. You can download the library from the official website or use a CDN link.

<script src="https://cdnjs.cloudflare.com/ajax/libs/box2d.js/2.3.1/box2d.min.js"></script>

Once you have included the library in your project, you need to create a world object.

var gravity = new b2Vec2(0, -10);
var world = new b2World(gravity);

Here, we have created a world object with a gravity vector of (0, -10). You can adjust the gravity vector to suit your needs.

Part 2: Creating Bodies

In Box2D, bodies are objects that can move and interact with each other. To create a body, you need to define its properties such as position, shape, and density.

var bodyDef = new b2BodyDef;
bodyDef.type = b2Body.b2_dynamicBody;
bodyDef.position.x = 10;
bodyDef.position.y = 10;
var body = world.CreateBody(bodyDef);

var shape = new b2CircleShape(1);
var fixtureDef = new b2FixtureDef;
fixtureDef.shape = shape;
fixtureDef.density = 1;
fixtureDef.friction = 0.3;
fixtureDef.restitution = 0.5;
body.CreateFixture(fixtureDef);

Here, we have created a dynamic body at position (10, 10) with a circle shape of radius 1. We have also defined the density, friction, and restitution of the body.

Part 3: Updating the World

To simulate the physics in the world, you need to update the world at regular intervals.

var timeStep = 1 / 60;
var velocityIterations = 6;
var positionIterations = 2;

function update() {
  world.Step(timeStep, velocityIterations, positionIterations);
  world.ClearForces();
  // update the positions and rotations of the bodies in your game
  requestAnimationFrame(update);
}

requestAnimationFrame(update);

Here, we have defined the time step, velocity iterations, and position iterations for the world. We have also created a function called update that updates the world and clears the forces. Finally, we have used requestAnimationFrame to call the update function at regular intervals.

Part 4: Handling Collisions

In Box2D, collisions between bodies are handled using contact listeners.

var listener = new b2ContactListener;
listener.BeginContact = function(contact) {
  // handle the start of a collision
}
listener.EndContact = function(contact) {
  // handle the end of a collision
}
listener.PreSolve = function(contact, oldManifold) {
  // handle the collision before it happens
}
listener.PostSolve = function(contact, impulse) {
  // handle the collision after it happens
}
world.SetContactListener(listener);

Here, we have created a contact listener and defined four functions to handle different aspects of collisions. We have also set the contact listener for the world.

 

In Box2D, there are several types of shapes that can be used to define the geometry of a body. Here are some examples of each shape type:

1. Circle Shape - A circular shape defined by its radius. Example:

var shape = new b2CircleShape(2); // Creates a circle shape with radius 2

1. Polygon Shape - A convex polygon shape defined by its vertices. Example:

var shape = new b2PolygonShape();
shape.SetAsBox(2, 1); // Creates a rectangular shape with width 4 and height 2

1. Edge Shape - A line segment shape defined by its two end points. Example:

var shape = new b2EdgeShape();
shape.Set(new b2Vec2(-2, 0), new b2Vec2(2, 0)); // Creates a horizontal line segment of length 4

1. Chain Shape - A chain of line segments defined by its vertices. Example:

var shape = new b2ChainShape();
shape.CreateChain([
  new b2Vec2(-2, 0),
  new b2Vec2(-1, 1),
  new b2Vec2(1, 1),
  new b2Vec2(2, 0)
]); // Creates a chain of line segments that form a U shape

1. Multi-Shapes - You can also create bodies with multiple shapes, such as polygons or circles, combined together to create a more complex shape. Example:

var bodyDef = new b2BodyDef;
bodyDef.type = b2Body.b2_dynamicBody;
bodyDef.position.x = 10;
bodyDef.position.y = 10;
var body = world.CreateBody(bodyDef);

var circleShape = new b2CircleShape(1);
var polygonShape = new b2PolygonShape();
polygonShape.SetAsBox(1, 1);

var fixtureDef1 = new b2FixtureDef;
fixtureDef1.shape = circleShape;
fixtureDef1.density = 1;
fixtureDef1.friction = 0.3;
fixtureDef1.restitution = 0.5;

var fixtureDef2 = new b2FixtureDef;
fixtureDef2.shape = polygonShape;
fixtureDef2.density = 1;
fixtureDef2.friction = 0.3;
fixtureDef2.restitution = 0.5;

body.CreateFixture(fixtureDef1);
body.CreateFixture(fixtureDef2); // Creates a body with a circle and a rectangle shape

These are just a few examples of the types of shapes that can be used in Box2D. Depending on your needs, you may need to use other types of shapes or create more complex shapes by combining multiple shapes together.