Physics

Your game is coming along nicely. You have Sprite objects, gameplay mechanics and your coding efforts are paying off. You are starting to feel like your game is playable. What do you do when you realize your game needs to simulate real world situations? You know, collision detection, gravity, elasticity and friction. Yes, you guessed it! This chapter is on physics and the use of a physics engine. Let's explore the when, wheres and whys of using a physics engine.

Physics is scary, do I really need it? Please tell me no!

Please don't run away there are no physics monsters under your bed! Your needs might be simple enough to not need to use a physics engine. Perhaps a combination of using a Node objects update() function, Rect objects and a combination of the containsPoint() or intersectsRect() functions might be enough for you? Example:

void update(float dt)
{
  auto p = touch->getLocation();
  auto rect = this->getBoundingBox();

  if(rect.containsPoint(p))
  {
      // do something, intersection
  }
}

This mechanism works for very simple needs, but doesn't scale. What if you had 100 Sprite objects all continuously updating to check for intersections with other objects? It could be done but the the CPU usage and framerate would suffer severely. Your game would be unplayable. A physics engine solves these concerns for us in a scalable and CPU friendly way. Even though this might look foreign, let's take a look at a simple example and then nut and bolt the example, terminology and best practice together.

// create a static PhysicsBody
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(Vec2(400, 400));

// sprite will use physicsBody
sprite->addComponent(physicsBody);

//add contact event listener
auto contactListener = EventListenerPhysicsContact::create();
contactListener->onContactBegin = CC_CALLBACK_1(onContactBegin, this);
_eventDispatcher->addEventListenerWithSceneGraphPriority(contactListener, this);

Even though this example is simple, it looks complicated and scary. It really isn't if we look closely. Here are the steps that are happening: A PhysicsBody object is created. A Sprite object is created. The Sprite object applies the properties of the PhysicsBody object. A listener is created to respond to an onContactBegin() event.

Once we look step by step the concept starts to make sense. To better understand all the details of a physics engine you should understand the following terms and concepts:

Physics terminology 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.

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

-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.

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 using:

auto scene = Scene::createWithPhysics();

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 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(DRAG_BODYS_TAG);

    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.

Collision

Have you ever been in a car accident? What did you collide with? Just like with cars, PhysicBody objects can come in contact. Collisions are what happens when PhysicBody objects come in contact with each other. When a collision takes place it can be ignored or it can trigger events to be fired.

Filtering Collisions

Collision filtering allows you to enable or prevent collisions between shapes. This physics engine supports collision filtering using category and group bitmasks.

There are 32 supported collision categories. For each shape you can specify which category it belongs to. You can also specify what other categories this shape can collide with. This is done with masking bits. For example:

auto sprite1 = addSpriteAtPosition(Vec2(s_centre.x - 150,s_centre.y));
sprite1->getPhysicsBody()->setCategoryBitmask(0x02);    // 0010
sprite1->getPhysicsBody()->setCollisionBitmask(0x01);   // 0001

sprite1 = addSpriteAtPosition(Vec2(s_centre.x - 150,s_centre.y + 100));
sprite1->getPhysicsBody()->setCategoryBitmask(0x02);    // 0010
sprite1->getPhysicsBody()->setCollisionBitmask(0x01);   // 0001

auto sprite2 = addSpriteAtPosition(Vec2(s_centre.x + 150,s_centre.y),1);
sprite2->getPhysicsBody()->setCategoryBitmask(0x01);    // 0001
sprite2->getPhysicsBody()->setCollisionBitmask(0x02);   // 0010

auto sprite3 = addSpriteAtPosition(Vec2(s_centre.x + 150,s_centre.y + 100),2);
sprite3->getPhysicsBody()->setCategoryBitmask(0x03);    // 0011
sprite3->getPhysicsBody()->setCollisionBitmask(0x03);   // 0011

You can check for collisions by checking and comparing category and collision bitmasks like:

if ((shapeA->getCategoryBitmask() & shapeB->getCollisionBitmask()) == 0
   || (shapeB->getCategoryBitmask() & shapeA->getCollisionBitmask()) == 0)
{
   // shapes can't collide
   ret = false;
}

Collision groups let you specify an integral group index. You can have all shapes with the same group index always collide (positive index) or never collide (negative index and zero index). Collisions between shapes of different group indices are filtered according the category and mask bits. In other words, group filtering has higher precedence than category filtering.

Contacts/Joints

Recall from the terminology above that joints are how contact points are connected to each other. Yes, you can think of it just like joints on your own body. Each joint type has a definition that derives from PhysicsJoint. All joints are connected between two different bodies. One body may be static. You can prevent the attached bodies from colliding with each other by joint->setCollisionEnable(false). Many joint definitions require that you provide some geometric data. Often a joint will be defined by anchor points. The rest of the joint definition data depends on the joint type.

-PhysicsJointFixed: A fixed joint fuses the two bodies together at a reference point. Fixed joints are useful for creating complex shapes that can be broken apart later.

-PhysicsJointLimit: A limit joint imposes a maximum distance between the two bodies, as if they were connected by a rope.

-PhysicsJointPin: A pin joint allows the two bodies to independently rotate around the anchor point as if pinned together.

-PhysicsJointDistance: Set the fixed distance with two bodies

-PhysicsJointSpring: Connecting two physics bodies together with a spring

-PhysicsJointGroove: Attach body a to a line, and attach body b to a dot

-PhysicsJointRotarySpring: Likes a spring joint, but works with rotary

-PhysicsJointRotaryLimit: Likes a limit joint, but works with rotary

-PhysicsJointRatchet: Works like a socket wrench

-PhysicsJointGear: Keeps the angular velocity ratio of a pair of bodies constant

-PhysicsJointMotor: Keeps the relative angular velocity of a pair of bodies constant

Collision detection

Contacts are objects created by the physics engine to manage the collision between two shapes. Contact objects are not created by the user, they are created automatically. There are a few terms associated with contacts.

-contact point: A contact point is a point where two shapes touch.

-contact normal: A contact normal is a unit vector that points from one shape to another.

You can get the PhysicsShape from a contact. From those you can get the bodies.

bool onContactBegin(PhysicsContact& contact)
{
    auto bodyA = contact.getShapeA()->getBody();
    auto bodyB = contact.getShapeB()->getBody();
    return true;
}

You can get access to contacts by implementing a contact listener. The contact listener supports several events: begin, pre-solve, post-solve and separate.

-begin: Two shapes just started touching for the first time this step. Return true from the callback to process the collision normally or false to cause physics engine to ignore the collision entirely. If you return false, the preSolve() and postSolve() callbacks will never be run, but you will still receive a separate event when the shapes stop overlapping.

-pre-solve: Two shapes are touching during this step. Return false from the callback to make physics engine ignore the collision this step or true to process it normally. Additionally, you may override collision values using setRestitution(), setFriction() or setSurfaceVelocity() to provide custom restitution, friction, or surface velocity values.

-post-solve: Two shapes are touching and their collision response has been processed.

-separate: Two shapes have just stopped touching for the first time this step.

You also can use EventListenerPhysicsContactWithBodies, EventListenerPhysicsContactWithShapes, EventListenerPhysicsContactWithGroup to listen for the event you're interested with bodies, shapes or groups. Besides this you also need to set the physics contact related bitmask value, as the contact event won't be received by default, even if you create the relative EventListener.

For example:

bool init()
{
    //create a static PhysicsBody
    auto sprite = addSpriteAtPosition(s_centre,1);
    sprite->setTag(10);
    sprite->getPhysicsBody()->setContactTestBitmask(0xFFFFFFFF);
    sprite->getPhysicsBody()->setDynamic(false);

    //adds contact event listener
    auto contactListener = EventListenerPhysicsContact::create();
    contactListener->onContactBegin = CC_CALLBACK_1(PhysicsDemoCollisionProcessing::onContactBegin, this);
    _eventDispatcher->addEventListenerWithSceneGraphPriority(contactListener, this);

    schedule(CC_SCHEDULE_SELECTOR(PhysicsDemoCollisionProcessing::tick), 0.3f);
    return true;

    return false;
}

void tick(float dt)
{
    auto sprite1 = addSpriteAtPosition(Vec2(s_centre.x + cocos2d::random(-300,300),
      s_centre.y + cocos2d::random(-300,300)));
    auto physicsBody = sprite1->getPhysicsBody();
    physicsBody->setVelocity(Vec2(cocos2d::random(-500,500),cocos2d::random(-500,500)));
    physicsBody->setContactTestBitmask(0xFFFFFFFF);
}

bool onContactBegin(PhysicsContact& contact)
{
    auto nodeA = contact.getShapeA()->getBody()->getNode();
    auto nodeB = contact.getShapeB()->getBody()->getNode();

    if (nodeA && nodeB)
    {
        if (nodeA->getTag() == 10)
        {
            nodeB->removeFromParentAndCleanup(true);
        }
        else if (nodeB->getTag() == 10)
        {
            nodeA->removeFromParentAndCleanup(true);
        }
    }

    //bodies can collide
    return true;
}

Queries

Have you ever stood in one position and looked around? You see things near to you and far from you. You can gauge how close things are to you. Physics engines provide this same type of spatial query. PhysicsWorld objects currently support point queryies, ray casts and rect queries.

Point Queries

When you touch something, say your desk, you can think of this as a point query. They allow you to check if there are shapes within a certain distance of a point. Point queries are useful for things like mouse picking and simple sensors. You can also find the closest point on a shape to a given point or find the closest shape to a point.

Ray Cast

If you are looking around, some object within your sight is bound to catch your attention. You have essentially performed a ray cast here. You scanned until you found something interesting to make you stop scanning. You can ray cast at a shape to get the point of first intersection. For example:

void tick(float dt)
{
    Vec2 d(300 * cosf(_angle), 300 * sinf(_angle));
    Vec2 point2 = s_centre + d;
    if (_drawNode)
    {
        removeChild(_drawNode);
    }
    _drawNode = DrawNode::create();

    Vec2 points[5];
    int num = 0;
    auto func = [&points, &num](PhysicsWorld& world,
        const PhysicsRayCastInfo& info, void* data)->bool
    {
        if (num < 5)
        {
            points[num++] = info.contact;
        }
        return true;
    };

    s_currScene->getPhysicsWorld()->rayCast(func, s_centre, point2, nullptr);

    _drawNode->drawSegment(s_centre, point2, 1, Color4F::RED);
    for (int i = 0; i < num; ++i)
    {
        _drawNode->drawDot(points[i], 3, Color4F(1.0f, 1.0f, 1.0f, 1.0f));
    }
    addChild(_drawNode);

    _angle += 1.5f * (float)M_PI / 180.0f;
}

Rect Queries

Rect queries provide a fast way to check roughly which shapes are in an area. It is pretty easy to implement:

auto func = [](PhysicsWorld& world, PhysicsShape& shape, void* userData)->bool
{
    //Return true from the callback to continue rect queries
    return true;
}

scene->getPhysicsWorld()->queryRect(func, Rect(0,0,200,200), nullptr);

A few examples of using a rect query while doing a logo smash:

Disabling Physics

Using the built-in physics engine is a good idea. It is solid and advanced. However, if you wish to use an alternative physics engine you can. All you need to do is disabling CC_USE_PHYSICS in base/ccConfig.h.