Team Guardian Physics

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Revision as of 00:32, 6 April 2011 by NorthWind87 (talk | contribs) (Framework Breakdown)
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Physics System Overview

By 'northWind'

Major Classes

  • 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.
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.


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 {
    // 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 update

Integrating Framework Into Existing Projects

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

  1. As a physics simulator and collision detector combination
  2. As a physics simulator only
  3. 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.

Other Topics Reference

Inertia Tensor

The inertia tensor is a Matrix representation of the distribution of mass within an object. This property is important in that it determines how objects rotate in reaction to given impulses. Provided are two methods to create them:

  1. Use the quick inertia tensor calculators (getBoxInertiaTensor, getSphereInertiaTensor, #include “MathDefinitions.h”)
  2. Create a sequence of PointMass objects and calculate the inertia tensor on them by using getInertiaTensor (const PointMass* points, size_t n). If possible, please use method 1 as method 2 has been returning somewhat exaggerated results.


A force that is applied to an object over a very small time period. Practically, this force is applied instantaneously in the case of this engine.

Collision Geometry

The collision representation of an object. Typically in the form of boxes or spheres but may be other shapes. Currently implemented shapes are spheres and boxes, future additions include planes (possibly although use appears limited since no graphical representation of infinite plane) and compound objects.

Collision Space

A geometric space that contains a set of collision geometries. It is responsible for weeding out impossible collisions and generally controls the number of maximum collision calculations in a frame. Currently, the “simple” collision space is implemented ((N^2)/2 collisions) but the octree collision space is being currently implemented and should alleviate the number of objects bottleneck currently plaguing the system.

Matrix Inverse

Has a similar effect to the traditional division operation when multiplied by a float or vector (ie it acts as the divisor). This explanation is not rigorous in any way shape or form :)