Physicals

【Physicals】

　　The physics simulation in Sprite Kit is performed by adding physics bodies to scenes.

【Type of Physics Body】

　　An SKPhysicsBody object defines the shape and simulation parameters for a physics body in the system.　　

　　There are three kinds of physics bodies:

• A dynamic volume simulates a physical object with volume and mass that can be affected by forces and collisions in the system. Use dynamic volumes to represent items in the scene that need to move around and collide with each other.

• A static volume is similar to a dynamic volume, but its velocity is ignored and it is unaffected by forces or collisions. However, because it still has volume, other objects can bounce off it or interact with it. Use static volumes to represent items that take up space in the scene, but that should not be moved by the simulation. For example, you might use static volumes to represent the walls of a maze.

  While it is useful to think of static and dynamic volumes as distinct entities, in practice these are two different modes you can apply to any volume-based physics body. This can be useful because you can selectively enable or disable effects for a body.

• An edge is a static volume-less body. Edges are never moved by the simulation and their mass doesn’t matter. Edges are used to represent negative space within a scene (such as a hollow spot inside another entity) or an uncrossable, invisibly thin boundary. For example, edges are frequently used to represent the boundaries of your scene.

  The main difference between a edge and a volume is that an edge permits movement inside its own boundaries, while a volume is considered a solid object. If edges are moved through other means, they only interact with volumes, not with other edges.

【Physics Shape】

　When choosing a shape for your physics body, do not be overly precise. More complex shapes require more work to be properly simulated. For volume-based bodies, use the following guidelines:

• A circle is the most efficient shape.

• A path-based polygon is the least efficient shape, and the computational work scales with the complexity of the polygon.

• An edge-based body is more expensive to compute than a volume-based body.

• Lines and rectangles are the most efficient edge-based bodies.

• Edge loops and edge chains are the most expensive edge-based bodies, and the computational work scales with the complexity of the path.

【Configuring the Physical Properties of a Physics Body】

　　The SKPhysicsBody class defines properties that determine how the physics body is simulated.

• The mass property determines how forces affect the body, as well as how much momentum the body has when it is involved in a collision.
• The friction property determines the roughness of the body’s surface. It is used to calculate the frictional force that a body applies to other bodies moving along its surface.
• The linearDamping and angularDamping properties are used to calculate friction on the body as it moves through the world. For example, this might be used to simulate air or water friction.
• The restitution property determines how much energy a body maintains during a collision—its bounciness.

Other properties are used to determine how the simulation is performed on the body itself:

• The dynamic property determines whether the body is simulated by the physics subsystem.
• The affectedByGravity property determines whether the simulation exerts a gravitational force on the body. For more information on the physics world, see “Configuring the Physics World.”
• The allowsRotation property determines whether forces can impart angular velocity on the body.

【Mass Determines a Body’s Resistance to Acceleration】

　　You should set the mass on every volume-based body in your scene so that it properly reacts to forces applied to it.

　　A physics body’s mass, area, and density properties are all interrelated. When you first create a body, the body’s area is calculated, and never changes afterwards. The other two properties change values at the same time, based on the following formula:

　　mass = density x area

When you configure a physics body, you have two options:

• Set the mass property of the body. The density property is then automatically recalculated. This approach is most useful when you want to precisely control each body’s mass.
• Set the density property of the body. The mass property is then automatically recalculated. This approach is most useful when you have a collection of similar bodies created with different sizes. For example, if your physics bodies were used to simulate asteroids, you might give all asteroids the same density, and then set an appropriate bounding polygon for each. Each body automatically computes an appropriate mass based on its size on the screen.

【Physics World】

　　All physics bodies in a scene are part of the physics world, which is represented in Sprite Kit by an SKPhysicsWorld object attached to the scene. The physics world defines two important characteristics of the simulation:

• The gravity property applies an acceleration to volume-based bodies in the simulation. Static volumes and physics bodies that have set the affectedByGravity property to NO are unaffected.
• The speed property determines the rate at which the simulation runs.

【Making Physics Bodies Move】

　　you can control a physics body’s velocity directly, by setting its velocity and angularVelocity properties.

　　The default collection of forces that apply to a body include:

• The gravitational force applied by the physics world
• The damping forces applied by the body’s own properties
• A frictional force based on contact with another body in the system

　　You can also apply your own forces and impulses to physics bodies.

• A force is applied for a length of time based on the amount of simulation time that passes between when you apply the force and when the next frame of the simulation is processed. So, to apply a continuous force to an body, you need to make the appropriate method calls each time a new frame is processed. Forces are usually used for continuous effects
• An impulse makes an instantaneous change to the body’s velocity that is independent of the amount of simulation time that has passed. Impulses are usually used for immediate changes to a body’s velocity.

　　For example, assume for a moment you are making a space-based game where a rocket ship can fire missiles. When the ship fires a missile, the missile should have a starting velocity of the ship plus an additional vector in the direction of the launch. Listing 8-3 shows one implementation for calculating the launch velocity.

　　Forces and impulses are modeling the same concept—adjusting a body’s velocity.

　　You can apply a force to a body in one of three ways:

• A linear force that only affects the body’s linear velocity.

• An angular force that only affects the body’s angular velocity.

• A force applied to a point on the body. The physics simulation calculates separate changes to the body’s angular and linear velocity, based on the shape of the object and the point where the force was applied.

【Working with Collisions and Contacts】

　　Sprite Kit uses two kinds of interactions between physics bodies:

• A contact is used when you need to know that two bodies are touching each other. In most cases, you use contacts when you need to make gameplay changes when a collision occurs.
• A collision is used to prevent two objects from interpenetrating each other. When one body strikes another body, Sprite Kit automatically computes the results of the collision and applies impulse to the bodies in the collision.

　　It isn’t necessary for these interactions to be symmetrical, because Sprite Kit only calls your delegate once per frame for each contact. Either body can specify that it is interested in the contact. Because a missile already requests a contact message when it strikes a ship, the ship does not need to ask for the same contact message.

　　When Sprite Kit performs collision detection, it first determines the locations of all of the physics bodies in the scene. Then it determines whether collisions or contacts occurred. This computational method is fast, but can sometimes result in missed collisions. A small body might move so fast that it completely passes through another physics body without ever having a frame of animation where the two touch each other.

　　If you have physics bodies that must collide, you can hint to Sprite Kit to use a more precise collision model to check for interactions. This model is more expensive, so it should be used sparingly. When either body uses precise collisions, multiple movement positions are contacted and tested to ensure that all contacts are detected.