Box2D¶

Box2D is an open source C++ engine to simulate rigid bodies in 2D, it is also, AngryBirds’ motor engine. You can check wikipedia Box2D explanation.

Box2D library is stored at erincatto/box2d, which is generated from this github repo.

Bounces of a circle falling¶

In this example you will calculate whenever a circle falls from a certain height and bounces at a defined lower limit in the created world.

Creating a new project¶

Create a new project and a main.cpp file:

$ bii init box2d_example -L
$ cd box2d_example
$ # Create main.cpp

Now place the following code inside main.cpp:

main.cpp

#include "Box2D/Box2D.h"
#include <iostream>

using namespace std;

int main(int argc, char** argv)
{
    B2_NOT_USED(argc);
    B2_NOT_USED(argv);

    //**************************************************//
    //                 Creating a World                        //
    //**************************************************//

    // Define the gravity vector.
    b2Vec2 gravity(0.0f,-10.0f);

    // Construct a world object, which will hold and simulate the rigid bodies.
    b2World world(gravity);


    //**************************************************//
    //                 Creating a Ground Box               //
    //**************************************************//

    // Define the ground body.
    b2BodyDef groundBodyDef;
    groundBodyDef.position.Set(0.0f,-10.0f);

    // Call the body factory which allocates memory for the ground body
    // from a pool and creates the ground box shape (also from a pool).
    // The body is also added to the world.
    b2Body* groundBody = world.CreateBody(&groundBodyDef);

    // Define the ground box shape.
    b2PolygonShape groundBox;

    // The extents are the half-widths of the box.
    groundBox.SetAsBox(50.0f, 10.0f);

    // Add the ground fixture to the ground body.
    groundBody->CreateFixture(&groundBox, 0.0f);

    //**************************************************//
    //            Creating a Circle Shape                   //
    //**************************************************//

    b2BodyDef BodyDef;
    BodyDef.type = b2_dynamicBody;
    BodyDef.position = b2Vec2(0.0f, 4.0f);
    BodyDef.userData = (void *) "Circle";
    b2Body* body = world.CreateBody(&BodyDef);

    b2CircleShape circle;
    circle.m_radius = 1.0f;

    b2FixtureDef fixtureDef;
    fixtureDef.density = 1.0f;
    fixtureDef.friction = 2.0f;
    fixtureDef.restitution = 0.5f;

    fixtureDef.shape = &circle;

    body->CreateFixture(&fixtureDef);

    //**************************************************//
    //            Simulating the World (of Box2D)     //
    //**************************************************//

    float32 timeStep = 1.0f / 60.0f;
    int32 velocityIterations = 6;
    int32 positionIterations = 2;

    // This is our little game loop.
    for (int32 i = 0; i < 100; ++i)
    {
        // Instruct the world to perform a single step of simulation.
        // It is generally best to keep the time step and iterations fixed.
        world.Step(timeStep, velocityIterations, positionIterations);

        // Now print the position and angle of the body.
        b2Vec2 position = body->GetPosition();
        float32 angle = body->GetAngle();

        if (position.y - 1.00 <= 0.001)
            cout<< "Ball hits the ground!!" << endl;
        else
            cout<<"X = " << position.x << " Y = " << position.y << endl;
    }
}

Manage your dependencies¶

Check the dependencies of the project with bii deps:

$ bii deps
your_user/box2d_example depends on:
      system:
         iostream
      unresolved:
         Box2D/Box2D.h

Now, edit the biicode.conf file generated in the project folder. Add your [requirements] depending on the version you want and map your [includes]:

[requirements]
    erincatto/box2d: 10

[includes]
    Box2D/Box2d.h: erincatto/box2d

Check again with bii deps and now all dependencies are resolved.

Build the project¶

Next, the only thing left is building the project:

$ bii build

Execute the binary placed in bin directory and this is how output looks like:

~/box2d$ bin/myuser_box2d_example_main
X = 0 Y = 3.99722
X = 0 Y = 3.99167
X = 0 Y = 3.98333
X = 0 Y = 3.97222
X = 0 Y = 3.95833
X = 0 Y = 3.94167
X = 0 Y = 3.92222
Ball hits the ground!!

That’s it!

Open and build¶

This example is already in biicode: examples/box2d.

To give it a try, create a new project and open the block:

$ bii init box2d_example
$ cd box2d_example
$ bii open examples/box2d

Build the example and execute it:

$ bii build
$ cd bin
$ # Execute it
...
 X = 0 Y = 3.95833
 X = 0 Y = 3.94167
 X = 0 Y = 3.92222
 Ball hits the ground!!

Got any doubts? Do not hesitate to contact us, visit our forum and feel free to ask any questions.