Physics System Overview

By 'northWind'

Major Classes

Physics

iPhysicsFrame
iRigidBody
RBDynamics

Physics Coordinator

iPhysicsScene

Collision Information

iCollisionGeometry
iCSphere
iCOBB

Collision Notification

iCollisionListener

Collision Space

iCollisionSpace
iSimpleCollisionSpace

How to Use

Sample Code To Be Placed In Design

iPhysicsFrame* f = CreatePhysicsFrame(); // Create base physics frame
RBDynamics* d = f->getDynamics(); // Obtain handle to physics frame

iObject* o = CreateSphere(…..); o->attachTo(f); // Create graphical object and attach to pframe

d.velocity += Vector(0, 0, 500); // Set some of the physics properties

// OPTIONAL - Attach collision primitive
iCollisionGeometry* c = CreateCSphere(f->getRB(), 50);

// At this point the object is simulating in the world and no further action is *necessary*
// Other dynamics properties can also be changed for further effects
// Although the physics simulator keeps track of collision geometries and rigid bodies and will dispose of
// them upon shutdown (it does not track PhysicsFrame objects), it is good practice to clear bodies that
// are out of use since physics computations, especially collision, are quite costly.

// To release, call delete on the allocated elements. Generally, the elements can be disallocated in any
// order. Note here that I did NOT need to delete the Object created but I am deleting it since I attached
// it to my PhysicsFrame and I do not want to delete the parent of the object prior to deleting the
// object itself.
o->Delete(); f->Delete(); c->Delete();

Framework Breakdown

The framework consists of a physics simulator (PhysicsScene) and a collision space (SimpleCollisionSpace). They are designed to work relatively independently using a set a of joint interfaces.

Let's first look at the physics-only components:

Physics

PhysicsScene

#include "iPhysicsScene.h"

PhysicsScene is the coordinator responsible for frame by frame physics simulation. It should be created in Engine's constructor by calling:

CreatePhysicsScene(iContext* c)

The collision space, if any, should be set in Engine's setup() function. We will look at this later.

Let's take a look at PhysicsScene's interface:

/* Physics Scene Interface - Scene Component - Model Branch
 *
 * iPhysicsScene.h
 * November 6 2010
 */

//--------------------------- iPhysicsScene -----------------------------------
//
// iPhysicsScene is the interface to the physics scene coordinator
//
class iRigidBody;
class iContext;
class iCollisionSpace;

class iPhysicsScene {
public:
    // initialization functions
    virtual bool add(iRigidBody* o)                     = 0;
    virtual void setNewGlobalCollisionSpace(iCollisionSpace* cs) = 0;
    virtual void restore(int now)                       = 0;
    // execution functions
    virtual void update(int now)                        = 0;
    // termination functions
    virtual void reset(int now)                         = 0;
    virtual void suspend() const                        = 0;
    virtual void release() const                        = 0;
    virtual void remove(const iRigidBody* o)            = 0;
    virtual void Delete() const                         = 0;
};

extern "C"
iPhysicsScene* CreatePhysicsScene(iContext* c);

The standard controller functions should be called in the usual places in Engine.cpp (Delete(), release(), suspend(), reset(), restore()), please refer to Engine.cpp in the reference implementation for more details.

In order to advance the simulation forward, update(int now) should be called every frame. Naturally, PhysicsScene's update() should be called from within Engine's run. Physics should be updated before design is updated. Let's look at how this is accomplished in the reference implementation:

now = rightNow;

// retrieve user input, if any
keyboard->retrieveInput();
mouse->retrieveInput();
joystick->retrieveInput();

// update the model components
physicsScene->update(rightNow);
design->update(rightNow);
scene->updateEmitters(rightNow);
viewing->update(rightNow);
audio->update(rightNow);
lighting->update();
hud->update(rightNow);

This is sufficient to add physics simulation support to Chris' framework. This won't magically cause your objects to suddenly start moving around and reacting to gravity however. To do that, each object that should be simulated by the simulator must be associated with a RigidBody. It is also possible to use a PhysicsFrame in a similar role, more on that later.

RigidBody

#include "iRigidBody.h"

A RigidBody provides access to a physics object's state in the world at any given point in time. Each RigidBody instance holds its physics properties in an RBDynamics struct. In order to access a RigidBody instance's properties, the method getDynamics() is called on the instance. This method returns a reference to an RBDynamics struct. Let's look at the interface:

class iRigidBody {
public:
    // initialization functions
    virtual iRigidBody* clone() const                       = 0;
    virtual void attach(iFrame* o)                          = 0;
    virtual void attachListener(iCollisionListener* l)      = 0;
    virtual void detachListener(const iCollisionListener* l)= 0;
    virtual void restore(int now)                           = 0;
    // execution functions
    virtual std::vector<iCollisionListener*>& getListeners()= 0;
    virtual iFrame* getFrame  ()                            = 0;
    virtual RBDynamics& getDynamics()                       = 0;
    virtual const RBDynamics& getDynamics() const           = 0;
    virtual void setDynamics(const RBDynamics& d)           = 0;
    // termination functions
    virtual void reset(int now)                             = 0;
    virtual void suspend() const                            = 0;
    virtual void release() const                            = 0;
    virtual void Delete() const                             = 0;
};

extern "C"
iRigidBody* CreateRigidBody(iFrame* o);

Aside from the usual suspects, a few functions are of particular importance:

    iRigidBody* CreateRigidBody(iFrame* o);
    virtual void attach(iFrame* o)                          = 0;
    virtual RBDynamics& getDynamics()                       = 0;
    virtual void Delete() const                             = 0;

iRigidBody* CreateRigidBody(iFrame* o): Creates a RigidBody and associates it with the active PhysicsScene coordinator. This function accepts an iFrame* o parameter. This parameter can be NULL if desired but should usually be the address of a valid iFrame.

The passed iFrame will be bound to this RigidBody. Every frame, PhysicsScene will update the frame's homogenous transformation matrix (T) to follow the location and rotation of the RigidBody. This is done at the end of the physics update step.
virtual void attach(iFrame* o): Associates an iFrame instance with the RigidBody. Can also be used to associate the RigidBody with a new iFrame or to detach it from its currently associated iFrame by passing in NULL.
virtual RBDynamics& getDynamics(): Returns a reference to the RBDynamics struct. The public properties of the struct can be directly edited in this way. The physics simulator will update these properties every frame.
virtual void Delete() const: Deletes this RigidBody and its RBDynamics instance. Does not delete the associated object or any associated collision listeners.

Collision

CollisionSpace

#include "iCollisionSpace.h"

A CollisionSpace is not a coordinator and does not need to be updated every frame. A collision space is responsible for holding a list of collision geometries (CollisionGeometry) and for generating a list of all collisions between them. There are two ways to add collision geometries to a collision space:

By manually adding each CollisionGeometry to to the space by calling virtual bool add(iCollisionGeometry* o) on the CollisionSpace instance.
By setting a particular CollisionSpace instance to be the default collision space for all collision geometries. This is done by calling CollisionGeometry::setGlobalCollisionSpace(iCollisionSpace* cs)

Let's look at the interface:

class iCollisionSpace {
public:
    // execution functions
    virtual bool add(iCollisionGeometry* o)             = 0;
    virtual void remove(iCollisionGeometry* o)          = 0;
    virtual const std::list<iCollisionGeometry*>& getAttachedGeometry() = 0;
    virtual void populateContactList(float delta)       = 0;
    virtual const CollisionContact* getContactList() const  = 0;
    virtual size_t getNumContacts() const               = 0;
    // termination functions
    virtual void suspend() const                        = 0;
    virtual void release() const                        = 0;
    virtual void Delete() const                         = 0;
};

Of these functions, three are of particular importance:

    virtual void populateContactList(float delta)       = 0;
    virtual const CollisionContact* getContactList() const  = 0;
    virtual size_t getNumContacts() const               = 0;

virtual void populateContactList(float delta): Collides all attached collision geometries with each other, clears then builds the internal contact list. This function requires a delta time amount to be passed in that represents the last time populateContactList() was called. Valid values for delta are inclusively between 0.0f and FLOAT_INFINITE.
virtual size_t getNumContacts() const: Returns the number of CollisionContacts in the contact list array.
virtual const CollisionContact* getContactList(): Returns a pointer to the first element in the internal CollisionContact array.

Currently, only a simple collision space is implemented. It performs N^2 collision checks when populateContactList is called where N is the number of collision geometries attached to the space.

In the very near future, an octree collision space will be implemented. This space will be updated when this happens :)

SimpleCollisionSpace

#include "iSimpleCollisionSpace.h"

Creating a simple collision space is as easy as cake. Let's look at the interface:

/* Simple Collision Space Interface - Physics Scene Component - Model Branch
 *
 * iSimpleCollisionSpace.h
 * November 17 2010
 */

//--------------------------- iSimpleCollisionSpace ---------------------------
//
// A simple collision space. Performance is N^2 rigid bodies.
//

class iCollisionSpace;

extern "C"
iCollisionSpace* CreateSimpleCollisionSpace(size_t samplesPerSecond=60);

Surprise! There is no interface! iSimpleCollisionSpace.h simply holds the header for the creation function. To create a simple collision space, simply call:

iCollisionSpace* CreateSimpleCollisionSpace()

size_t samplesPerSecond=60: The number of collision samples to perform per second; used in contact determination. At the moment, this feature is simply disabled as the implementation is borked.... HOWEVER! If the implementation were not borked, then under most circumstances the samplesPerSecond parameter should not have to be changed.

CollisionContact

#include "iCollisionSpace.h"

A CollisionContact represents a single collision at a point in world space between two collision geometries. Let's look at the definition:

struct CollisionContact{
    Vector pos;         // contact position
    Vector normal;      // normal vector
    float depth;        // penetration depth
    iCollisionGeometry *g1, *g2; // colliding geoms
};

Vector pos: The world space location of the point of collision.
Vector normal: A world space unit vector perpendicular to the collision surface. This always points at g2.
float depth: The depth of penetration between the geometries.
iCollisionGeometry* g1: The first geometry involved in this collision.
iCollisionGeometry* g2: The second geometry involved in this collision.

With this information, the computer can be programmed to respond appropriately to any number of collisions :)

Listening For Collisions

While RigidBodies are not directly responsible for holding collision information or for keeping track of collisions, a decision was made early on to let RigidBody hold the CollisionListener list. The logic behind this is foul and mysterious; in the future, the CollisionListener list will most likely be held by instances of iCollisionGeometry.

A CollisionListener is an object that performs some action in response to a collision event between two objects. This is done by implementing the iCollisionListener interface in a class and then adding an instance of the listener class to the CollisionListener list of a given RigidBody. From that point on, that RigidBody will forward all collision events that it is involved in to the added CollisionListener as well as any other CollisionListeners that may have been added to the RigidBody previously. CollisionListeners can also be removed from a given RigidBody.

The CollisionListener list of a given RigidBody may be edited by calling the following functions:

    virtual void attachListener(iCollisionListener* l)      = 0;
    virtual void detachListener(const iCollisionListener* l)= 0;
    virtual std::vector<iCollisionListener*>& getListeners()= 0;

I assume that the purpose of these functions is mostly self explanatory :)

CollisionListener

#include "iCollisionListener.h"

Let's look at the CollisionListener interface:

/* Collision Listener Interface - Physics Scene Component - Model Branch
 *
 * iCollisionListener.h
 * December 8 2010
 */

//--------------------------- iCollisionListener ------------------------------
//
// A listener for collision events. Can be attached to a rigid body.
//

#include "MathDeclarations.h"

class iRigidBody;

class iCollisionListener {
public:
    virtual void HitWall(
        iRigidBody* other,
        Vector hitNormal,
        Vector hitLocation)                                             = 0;
};

The interface requires a single function to be implemented:

virtual void HitWall(iRigidBody* other, Vector hitNormal, Vector hitLocation)

This function will be called by PhysicsScene every frame that two objects are colliding.

iRigidBody* other: The RigidBody that the RigidBody that this CollisionListener is attached to has collided with.
Vector hitNormal: A unit vector in world space perpendicular to the surface of collision, points in the direction of other.
Vector hitLocation: The location in world space of the point of collision.

Integrating Framework Into Existing Projects

There are three ways to integrate the framework into your game:

As a physics simulator and collision detector combination
As a physics simulator only
As a collision detector only

Please skip to the section relevant to your interests :)

Combined Physics Simulator/Collision Detector

This is the framework in its default state.

RBDynamics Properties Reference

Vector position: Current position of COM
Vector velocity: Current velocity of COM
Vector lastVelocity: Used to help detect velocity spikes
Vector acceleration: Additive acceleration of COM
Vector force: Constant force applied to COM
Vector temporalForce: Force that will be applied at the nexttick and then reset to 0. Total linear acceleration is equal to: <math>acceleration + ((force + temporalForce)/mass)</math>
float mass: Mass, defaults to 1, the more mass present the more force required to move the object. Set this to INFINITE_MASS (#include "ModelSettings.h") to prevent the object from moving.
float dragCoefficient: The drag coefficient, 0 implies no drag, 0.25~0.45 is the drag of a car, etc...
float restitution: How much energy does the object keep after a collision? 1 - Superball, the object loses none, 0 - Clay, the object loses all
float friction: Tangential impulse applied during a collision that hinders movement along the plane of the collision normal. Coefficient of friction of a given collision is calculated by adding together the friction values of the two objects colliding.
Vector com: Center of Mass offset.
PhysicsType physicsType: Type of physics applied to body, defaults to Falling (BUGGY, use PHYS_Falling and PHYS_Floating for now)
Matrix orientation: Matrix representing rotation in XYZ
Vector angularVelocity: Angular velocity in radians/sec in XYZ
Vector angularMomentum: Angular momentum on the COM
Vector torque: Torque force on the COM
Vector temporalTorque: Torque force that will be applied at the next tick and then reset to 0; Total angular acceleration is equal to: angularAcceleration + ((torque + temporalTorque)/inertiaMoment)

RBDynamics Functions Reference

Vector getWorldCOM() const: Returns the world position of the COM with respect to this body.
Vector getVelocityAtWorldPoint(const Vector& p) const: Given a point in worldspace, returns its speed taking into account velocity and angular velocity.
Void setInverseInertiaTensor(const Matrix& i): Sets the inverse inertia tensor of this body. Automatically sets the inertia tensor as well by inversing the incoming matrix. DOES NOT NEED TO BE CALLED AFTER CALLING setInertiaTensor()
Void setInertiaTensor(const Matrix& i): Opposite of setInverseInertiaTensor. DOES NOT NEED TO BE CALLED AFTER CALLING setInverseInertiaTensor()
Matrix& getInertiaTensor()
Matrix& getInverseInertiaTensor(): Returns the inertia tensor or inverse inertia tensor contained by this object.
Void ApplyImpulse(const Vector& impulse, const Vector& pointOfApplication): Applies a given impulse instantaneously onto this object at a given world point of application. Can be used to shoot or prod objects.

Team Guardian Physics

Contents

Physics System Overview

Major Classes

How to Use

Framework Breakdown

Physics

PhysicsScene

RigidBody

Collision

CollisionSpace

SimpleCollisionSpace

CollisionContact

Listening For Collisions

CollisionListener

Integrating Framework Into Existing Projects

Combined Physics Simulator/Collision Detector

RBDynamics Properties Reference

RBDynamics Functions Reference

Other Topics Reference

Inertia Tensor

Impulse

Collision Geometry

Collision Space

Matrix Inverse

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