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Lecture 4

Introducing Physics to OgreReferences:[1] Gregory Junker, Pro OGRE 3D Programming, Apress, 2006 [2] Ogre Tutorials – Ogre Wiki

http://www.ogre3d.org/wiki/index.php/Ogre_Tutorials[3] Microsoft MSDN, C++ reference[4] Newton Game Dynamics http://newtondynamics.com[5] OgreNewt http://walaber.com/index.php?action=showitem&id=9[6] OgreNewt Source

https://svn.ogre3d.org/svnroot/ogreaddons/branches/ogrenewt/newton20/src/

EIE360 Integrated Project

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Architecture of the Interactive Virtual Aquarium System

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USB port

3D GraphicsSystem

3D Graphics

Your program

Your program

Network

Computer A

Computer B

Kinect Sensor Device

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Game Physics

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Computer animation physics or game physics involves the introduction of the laws of physics into a simulation or game engine

With game physics, we will see game objects react to stimulation similar to real objects in real world

Become more and more important in computer game development since it provides the realism to the game

To facilitate the implementation of game physics, physics engines have been developed Professional ones: Havok – very expensive Free engines: ODE, Tokamak, PhysX, and Newton Game

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Newton Game Dynamics Newton Game Dynamics is a physics engine for realistically

simulating rigid bodies in games In contrast to most other real-time physics engines it goes for

accuracy over speed Its solver is deterministic and not based on traditional iterative

methods Advantages: it can handle higher mass ratios (up to 400:1) and the simulation

is very robust and easy to tune Disadvantage: a bit slower than engines with an iterative solver

However the new Newton 2.0 has greatly improved the simulation speed

To use Newton, a SDK should be downloaded from [4] and installed in the computer

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OgreNewt Library OgreNewt is a library that wraps the Newton Game Dynamics

physics SDK into an object-oriented set of classes that facilitate integration with the OGRE 3D engine

OgreNewt library is pretty much a 1:1 conversion of the Newton functions into a class-based environment

OgreNewt is available as a part of "ogreaddons" in the Ogre CVS. It is also available from [5]

Different from Ogre, there is not a well-documented API reference for OgreNewt Can check the usage of APIs from their source [6]

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Newton Basic Elements

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Basic Elements

World Collisions JointsRigid Bodies Materials Others

Primities

Convex Hulls

TreeCollisions MaterialID

MaterialPair

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Newton Basic Elements: World WORLD (OgreNewt::World) 

This is the "space" in which all objects exist In most applications, we only need 1 World object,

inside which all other objects are placed However the system allows for multiple worlds to co-

exist

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OgreNewt::World *mWorld = new OgreNewt::World();mWorld->setWorldSize( … , …); //specify a box with 2 3-dimensional points

//as the min and max

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Newton Basic Elements: World – framelistener

The Newton world has its own framelistener to update all the objects in each time step. Need to be created and add to Ogre A framelistener receives notification before and after a

frame is rendered to the screen

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Ogre::FrameListener *mNewtonListener = new OgreNewt::BasicFrameListener(mWindow,

mWorld);mRoot->addFrameListener(mNewtonListener);

//One more framelistener is added

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Newton Basic Elements: Rigid Body

RIGID BODY (OgreNewt::Body)  This is the basic object in the physics world It represents a solid (rigid) body, which can interact

with other bodies in the scene Bodies can have mass, size, and shape. Basically

everything we want to be affected by physics calculations needs a Body

However, to create a Body, we need to first define its collision primitive

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Newton Basic Elements: Rigid Body – Example

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Body for the barrel

Body for the fish

Body for the floor

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Newton Basic Elements:Collision COLLISION (OgreNewt::Collision) 

Visible shape of an object is defined by its mesh model Rigid Bodies require a Collision object to define their

shape for physics calculation – E.g. to calculate the reaction after colliding with another object Not necessarily equal to the visible shape, actually often much

simple One of the purposes is to reduce the computation complexity

since visible shape can be very complex

Newton provides several different kinds of collision objects. They include Primitive shapes, Convex Hulls and Tree Collisions

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Newton Basic Elements:Collision - TreeCollisions TREE COLLISIONS

TreeCollision objects are just polygon collision objects For any object made up by polygons, we can create a

TreeCollision object from it However, TreeCollision objects CANNOT be used for

active rigid bodies (e.g. movable bodies) All Bodies created from a TreeCollision will

automatically have infinite mass, and therefore be completely immobile

Best used for "background" objects that will not move For moving objects, use convex hulls or primitives.

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Newton Basic Elements:Collision - TreeCollisions

Assume the entity of the ground groundEnt has been created

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OgreNewt::CollisionPtr col( new OgreNewt::CollisionPrimitives::TreeCollision( mWorld, groundNode, false, 1 ));// Create the TreeCollision object

OgreNewt::Body *ground_body = new OgreNewt::Body( mWorld, col );// Create the rigid body

• In the example, we would like to create a TreeCollision object for the ground, since it will not move

mean we don’t want newton to try to optimize the object for us

An id for the object

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Newton Basic Elements:Collision – Convex Hulls Convex Hulls

Convex hulls are a more general primitive type They take a series of points in space, and create the

smallest possible convex shape based on all of those points

In most cases, we would use the vertices that makeup a model for the points

This results in a primitive shape that looks something like our 3D model wrapped up in wrapping paper

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Newton Basic Elements:Collision – Convex Hulls

In the example, the collision objects for the barrel and the fish are of the primitive type “convex hulls”

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Newton Basic Elements:Collision – Convex Hulls

Assume the entity for the Barrel has been created and pointed by barrelEnt

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OgreNewt::ConvexCollisionPtr col_convex( newOgreNewt::CollisionPrimitives::ConvexHull(mWorld, barrelEnt, 2));// Create the convex hull object

OgreNewt::Body *barrel_body = new OgreNewt::Body( mWorld, col_convex );// Create the Body

An id for the object

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How the Body is connected to the 3D World?

Newton will automatically update the position and rotation of object bodies based on physics laws

Newton has a callback system built in, which automatically calls only the callback for bodies that are moving, bodies that have not moved since the last update are properly ignored

To enable the above, we only need to attach the body to the scene node and set the initial position and orientation

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ground_body->attachToNode( groundNode );ground_body->setPositionOrientation( … );

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Mass, MOI, and Force If we want objects to move following physics laws, a few

parameters of the objects should be defined Mass – the mass of the object MOI – moment of inertia Force – the force applied to the object

Mass is simple, we can use any units that see fit, although usually Kilograms is used

Inertia might not be so intuitive, as it's a value that represents an objects resistance to rotation around a specific axis, many factors can affect this value

An object having mass and inertia defined still cannot move since it needs a force to trigger the motion

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Mass and MOI

OgreNewt provides standard functions to define the mass and inertia of objects

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Ogre::Real mass = 1.0; Ogre::Vector3 inertia = OgreNewt:: MomentOfInertia::CalcSphereSolid(mass, 10);

//Compute the MOI of a sphere of size 10//Check OgreNewt reference for other shapes//such as cylinder, box, etc.

bod->setMassMatrix( mass, inertia ); //Assume bod is the Body we want to define

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Mass and MOI (cont) For convex hulls, OgreNewt also provides a more

general function calculateInertiaMatrix() It calculates the MOI for a collision primitive, as well as

the computed center of mass Two Vector3 objects should be passed into the function

and they will be populated with the info It is only for boxes, ellipsoids and convex hulls but NOT

work on TreeCollisions.

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Ogre::Vector3 inertia, centre_of_mass;col_convex->calculateInertialMatrix(inertia,

centre_of_mass);//Assume the collision object is col_convex

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Force

In Newton world, object cannot be translated by moving its scene node, since it does not follow physics

Need to apply a force to the object to make it move The most common type of force is the gravity OgreNewt provides a standard function setStandardCallback() to apply a constant gravitational (-Y) force of 9.8units/sec^2 to bodies

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barrel_body-> setStandardForceCallback();//So the barrel will fall //onto the floor

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Force (cont) For custom force, one needs to add a special callback

function to apply to a body

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EIE360Project::createScene(…) {mFish_body->setCustomForceAndTorqueCallback

<EIE360ProjectApp>(&EIE360ProjectApp:: customSwimCallback, this);// register the callback function

}EIE360Project::customSwimCallback(OgreNewt::Body* body, float timestep, int threadIndex) {//This callback function will be called when newton// wants to apply force to the body

body->addForce(…); body->setForce(…); }

Time since last call Id of the thread calling this function

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Example – Fish Swimming

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EIE360Project:: customSwimCallback(OgreNewt::Body* body, float timestep, int threadIndex) { Ogre::Real mass; Ogre::Vector3 inertia; body->getMassMatrix(mass, inertia); Ogre::Vector3 velocity =

(mFishLastPosition - pos)/ timeStep; Ogre::Vector3 accel = (velocity –

body->getVelocity()) / timeStep; Ogre::Vector3 force = mass * accel; body->addForce(force);}

Current pos

Pos it wants to go

F = M*A!!!Velocity needed to go the target pos

Current velocity

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Example – Changing Orientation

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Ogre::Vector3 direction = mFishLastPosition - newPos;direction.normalise();Ogre::Vector3 pos = mFish_body->getPosition();

mFish_body->setPositionOrientation(pos, Ogre::Vector3::UNIT_X.getRotationTo(direction)); 

The default orientation of the fish model

Return the orientation of the fish that heads to the required direction

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Collision Detection

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Collision detection is one of the most important operations to enable interactions between objects in the game world

By collision detection, it refers to the automatic detection and subsequent actions when two game objects are collided

An automatic approach is essential since there can be numerous objects collide with each other at a particular time. It will be extremely time consuming if programmers need to manually do this in their programs

A standard procedure for collision detection can be found in Ogre

Newton (and OgreNewt) provides an even simpler approach by using materialID and materialPair

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Newton Basic Elements:Material

MATERIAL (OgreNewt::MaterialID && OgreNewt::MaterialPair)  Materials are how Newton lets one adjust the interaction

between bodies when they collide This can be as simple as adjusting the friction, or much more

complex The material system is pretty simple

First create "MaterialID" objects to represent each material that might be required in the system

Then build what is called a "MaterialPair". A material pair is a description of what happens when 2 materials collide with each other

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OgreNewt’s Collision Detection System When the screen is updated with a new frame,

OgreNewt will also update the position and orientation of each body by calling OgreNewt::World::update()

At the same time, OgreNewt also checks if two bodies are collided

The collision detection procedure starts with the overlapping of the AABB of two bodies

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OgreNewt’s Collision Detection System (cont)

AABB overlap?

Update()

End

Move all bodies

onAABBOverlap()

Bodies actually contact? contactsProcess()

Y

YN

N

A simplified flow diagram(assume bodies only contact once and all functions return 1)

A simplified flow diagram(assume bodies only contact once and all functions return 1)

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OgreNewt’s Collision Detection System (cont) onAABBOverlap()

Called if the AABB of the 2 bodies in question overlap If it returns 0, OgreNewt will not perform further action even if it

is really a collision. Return 1 otherwise. contactsProcess()

Called if the 2 bodies in question really collide Inside this function you can get lots of information about the

contact (contact normal, speed of collision along the normal, tangent vectors, etc), or even modify some collision parameters

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A Simple Example

Assume we want to detect if a fish collides with the barrel in the game world. If yes, some bubbles are generated around the barrel

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Step 1

Create a materialID for the fish and the barrel. Then group them into a materialPair and connect them to their body. In createScene(), add the following codes

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OgreNewt::MaterialID* fish_material_id = new OgreNewt::MaterialID(mWorld); OgreNewt::MaterialID* barrel_material_id = new OgreNewt::MaterialID(mWorld);

//Group them into a pairOgreNewt::MaterialPair* pair = new OgreNewt::

MaterialPair(mWorld, fish_material_id, barrel_material_id );

mFish_body->setMaterialGroupID(fish_material_id);barrel_body->setMaterialGroupID(barrel_material_id);

Relationship between classes

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EIE360Project

FishBarrelContactCallback

mProject = this

contactsProcess(){ mProject->

startBubbles();}

new FishBarrelContactCallback(this);

pair-> setContactCallback(fishBarrelCallback);

:startBubbles(){ ...}

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Call if there is contact between fish and barrel

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Step 2

Tell OgreNewt where the callback functions of this materialPair can be found

In this example, the callback functions are implemented in a class called FishBarrelContactCallback

In createScene(), add the following statements:

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FishBarrelContactCallback* fishBarrelCallback = new FishBarrelContactCallback(this);

//Instantiate an object of that classpair->setContactCallback(fishBarrelCallback);//Tell the system the callback functions of //the materialPair pair can be found in that//object

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Step 3 Design the class FishBarrelContactCallback FishBarrelContactCallback::FishBarrelContactCallback

(EIE360Project *project){ mProject = project;

//mProject is a member variable of the class}FishBarrelContactCallback::

~FishBarrelContactCallback(void) { }void FishBarrelContactCallback::contactsProcess( OgreNewt::ContactJoint &contactJoint, Ogre::Real timeStep, int threadIndex ){ //Codes for collision detection

// you can use the ContactJoint to iterate through//contact-points

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Step 4 Implement contactsProcess()

void FishBarrelContactCallback::constactsProcess( OgreNewt::ContactJoint &contactJoint, Ogre::Real timeStep,int threadIndex)

{mProject->startBubbles();

}

The function startBubbles() should have been implemented In EIE360Project() that will generate bubbles around the barrel

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