Physics terminology and concepts

To better understand all the details of a physics engine you should understand the following terms and concepts:

Bodies

A PhysicsBody holds the physical properties of an object. These include mass, position, rotation, velocity and damping. PhysicsBody objects are the backbone for shapes. A PhysicsBody does not have a shape until you attach a shape to it.

Material

Materials describe material attributes:

-density: It is used to compute the mass properties of the parent body.

-restitution: It is used to make objects bounce. The restitution value is usually set to be between 0 and 1. 0 means no bouncing while 1 means perfect bouncing.

-friction: It is used to make objects slide along each other realistically.

Shapes

Shapes describe collision geometry. By attaching shapes to bodies, you define a body’s shape. You can attach as many shapes to a single body as you need in order to define a complex shape. Each shape relates to a PhysicsMaterial object and contains the following attributes: type, area, mass, moment, offset and tag. Some of these you might not be familiar with:

-type: describes the categories of shapes, such as circle, box, polygon, etc.

-area: used to compute the mass properties of the body. The density and area gives the mass.

-mass: the quantity of matter that a body contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field.

-moment: determines the torque needed for a desired angular acceleration.

-offset: offset from the body’s center of gravity in body local coordinates.

-tag: used to identify the shape easily for developers.​ You probably remember that you can assign all Node objects a tag for identification and easy access.

We describe the various shapes as:

-PhysicsShape: Shapes implement the PhysicsShape base class.

-PhysicsShapeCircle: Circles are solid. You cannot make a hollow circle using the circle shape.

-PhysicsShapePolygon: Polygon shapes are solid convex polygons.

-PhysicsShapeBox: Box shape is one kind of convex polygon.

-PhysicsShapeEdgeSegment: A segment shape.

-PhysicsShapeEdgePolygon: Hollow polygon shapes. A edge-polygon shape consists of multiple segment shapes.

-PhysicsShapeEdgeBox: Hollow box shapes. A edge-box shape consists of four segment shapes.

-PhysicsShapeEdgeChain: The chain shape provides an efficient way to connect many edges together.

Contacts/Joints

Contacts and joint objects describe how bodies are attached to each other.

World

A world container is what your physics bodies are added to and where they are simulated. You add bodies, shapes and constraints to a world and then update the world as a whole. The world controls how all of these items interact together. Much of the interaction with the physics API will be with a PhysicsWorld object.

There is a lot to remember here, keep these terms handy to refer back to them as needed.

Physics World and Physics Body

PhysicsWorld

A PhysicsWorld object is the core item used when simulating physics. Just like the world we live in, a PhysicsWorld has a lot of things happening at once. PhysicsWorld integrates deeply at the Scene level because of it's many facets. Let's use a simple example that we can all relate to. Does your residence have a kitchen? Think of this as your physics world! Now your world has PhysicsBody objects, like food, knives, appliances! These bodies interact with each other inside the world. These objects touch and also react to those touches. Example: use a knife to cut food and put it in an appliance. Does the knife cut the food? Maybe. Maybe not. Perhaps it isn't the correct type of knife for the job.

You can create a Scene that contains a PhysicsWorld just by calling the function initWithPhysics() in your Scene. Your init() function should have:

if( !Scene::initWithPhysics() )
{

}

Every PhysicsWorld has properties associated with it:

-gravity: Global gravity applied to the world. Defaults to Vec2(0.0f, -98.0f).

-speed: Set the speed of physics world, speed is the rate at which the simulation executes. Defaults to 1.0.

-updateRate: set the update rate of physics world, update rate is the value of EngineUpdateTimes/PhysicsWorldUpdateTimes.

-substeps: set the number of substeps in an update of the physics world.

The process of updating a PhysicsWorld is called stepping. By default, the PhysicsWorld updates through time automatically. This is called auto stepping. It automatically happens for you, each frame. You can disable auto steping of the PhysicsWorld by setting setAutoStep(false). If you do this, you would step the PhysicsWorld manually by setting step(time). Substeps are used to step the PhysicsWorld forward multiple times using a more precise time increment than a single frame. This allows for finer grained control of the stepping process including more fluid movements.

PhysicsBody

PhysicsBody objects have position and velocity. You can apply forces, movement, damping and impulses (as well as more) to PhysicsBody objects. PhysicsBody can be static or dynamic. A static body does not move under simulation and behaves as if it has infinite mass. A dynamic body is fully simulated. They can be moved manually by the user, but normally they move according to forces. A dynamic body can collide with all body types. Node provides setPhysicsBody() to associate a PhysicsBody to a Node object.

Lets create a static and 5 dynamic PhysicsBody objects that are a box shape:

auto visibleSize = Director::getInstance()->getVisibleSize();
s_centre = Vec2(visibleSize.width/2, visibleSize.height/2);

auto physicsBody = PhysicsBody::createBox(Size(65.0f, 81.0f),
                        PhysicsMaterial(0.1f, 1.0f, 0.0f));
physicsBody->setDynamic(false);

//create a sprite
auto sprite = Sprite::create("whiteSprite.png");
sprite->setPosition(s_centre);
addChild(sprite);

//apply physicsBody to the sprite
sprite->addComponent(physicsBody);

//add five dynamic bodies
for (int i = 0; i < 5; ++i)
{
    physicsBody = PhysicsBody::createBox(Size(65.0f, 81.0f),
                    PhysicsMaterial(0.1f, 1.0f, 0.0f));

    //set the body isn't affected by the physics world's gravitational force
    physicsBody->setGravityEnable(false);

    //set initial velocity of physicsBody
    physicsBody->setVelocity(Vec2(cocos2d::random(-500,500),
                cocos2d::random(-500,500)));
    physicsBody->setTag(10);

    sprite = Sprite::create("blueSprite.png");
    sprite->setPosition(Vec2(s_centre.x + cocos2d::random(-300,300),
                s_centre.y + cocos2d::random(-300,300)));
    sprite->addComponent(physicsBody);

    addChild(sprite);
}

The result is a stationary PhysicsBody with 5 additional PhysicsBody objects colliding around it.