Discussion thread in the OgreOde forum
Introduction
After implementing the first application that uses OgreOde, you should already have some experience on how you set things like bouncyness, fraction, etc. But how to handle every object individually with different behavior for every type of object and different values for every single object itself? In this tutorial I will show you an object orientated approach that is capable of doing all this. It has the price of some overhead though and an iterative browsing through all contacts might be faster, but probably not as easy to handle. You have to decide yourself if it is worth the cost, I think it will not be visible unless you have really lots of simulated objects in your app.
After implementing this system, you will be able to easily add new classes to your project that do anything you want on a collision, e.g water, teleporters, damage models and/or hit zones, trigger zones, etc.
This tutorial does not cover basic rigid body physics usage. For this refer to the simple scenes demo.
Concept
Now how does it work?
For every object that you want to specify collision behavior for, you will have to identify this object in the geometry that belongs to it, just by setting it as a UserObject. To ensure that you can interact with that object afterwards, you will have to derive it from a base class that has a function such as Collide(). So in your CollisionListener collision() function you can now extract the user object of the geometry and call that function.
The base class
class CollisionTestedObject
{
public:
CollisionTestedObject(void);
virtual ~CollisionTestedObject(void);
/*!
\brief
This function will be called, if a geometry collides that has a user object that points to this object.
*/
bool virtual Collide(bool MineIsFirst, OgreOde::Contact *Contact) = 0;
};
As you know, the two intersecting geometries are stored in Contact. The parameter MineIsFirst says, whether the first or second geometry in contact belongs to the object. This will be important if you want to to do something more than just setting some new contact parameters.
Note: If Collide() returns true, OgreOde will handle them like rigid bodies. If it returns false, the objects will not interact at all, so you can implement new behavior, imagine a trigger zone or something like that.
The CollisionListener
Modify your collision() function according to this:
bool MyClass::collision(OgreOde::Contact *Contact)
{
// Check for collisions between things that are connected and ignore them
OgreOde::Geometry * const g1 = Contact->getFirstGeometry();
OgreOde::Geometry * const g2 = Contact->getSecondGeometry();
if (g1 && g2)
{
const OgreOde::Body * const b1 = g2->getBody();
const OgreOde::Body * const b2 = g1->getBody();
if (b1 && b2 && OgreOde::Joint::areConnected(b1, b2))
return false;
}
//set contact parameters:
Contact->setBouncyness(1.0);
Contact->setCoulombFriction(OgreOde::Utility::Infinity);
Contact->setForceDependentSlip(1.0);
Contact->setAdditionalFDS(1.0);
/*we have 2 collidable objects from our object system, if one of the Collide function returns false, we return false in this method, too,
else we return true, so ode computes a normal collision.
true means ode will treat this like a normal collison => rigid body behavior
false means ode will not treat this collision at all => objects ignore each other*/
bool Return = true;
if (g1->getUserAny().isEmpty() == false))
if (!any_cast<CollisionTestedObject*>(g1->getUserAny())->Collide(true, Contact))
Return = false;
if (g2->getUserAny().isEmpty() == false)
if (!any_cast<CollisionTestedObject*>(g2->getUserAny())->Collide(false, Contact))
Return = false;
return Return;
}
As you see, from now on the CollisionListener takes care of everything, you don't need to worry about anything yourself. Be aware that you are forced to derive every object from our CollisionTestedObject class, if any of its geometries store it as a user object. If you don't, calling Collide() will end up in an exception. If you wonder why we set absolute contact parameters, take a look at the following class:
The CollidingObject class
We will now create another class that handles the adjustment of the contact parameters. Wonder why we need an extra class for it and not just use CollisionTestedObject? It's a concept thing. Imagine you implement a water class that does not really want ode to react on a collision, so calling collide() will return false. Now there is memory used for those adjustment variables we do not really need. If you think this is nonsense, feel free to implement it in one class.
class CollidingObject : public virtual CollisionTestedObject
{
protected:
float Friction;
float Bouncyness;
float BounceVelocity;
float ForceDependentSlip;
public:
CollidingObject(void);
virtual ~CollidingObject(void);
bool virtual WriteToIni(IniFile& Ini) const;
bool virtual LoadFromIni(const std::basic_string<wchar_t>& ObjectID, IniFile& Ini);
float GetFriction(void) const {return Friction;}
float GetBouncyness(void) const {return Bouncyness;}
float GetBounceVelocity(void) const {return BounceVelocity;}
float GetFDS(void) const {return ForceDependentSlip;}
bool virtual Collide(bool MineIsFirst, OgreOde::Contact *Contact);
private:
static const wchar_t* KEY_FRICTION;
static const float DEF_FRICTION;
static const wchar_t* KEY_BOUNCYNESS;
static const float DEF_BOUNCYNESS;
static const wchar_t* KEY_FDS;
static const float DEF_FDS;
static const wchar_t* KEY_BOUNCE_VELO;
static const float DEF_BOUNCE_VELO;
};
If you look at the member variables and functions, you have some idea of how the class works, so I will only show you the Collide() function:
bool CollidingObject::Collide(bool MineIsFirst, OgreOde::Contact *Contact)
{
Contact->setForceDependentSlip(ForceDependentSlip);
Contact->setAdditionalFDS(ForceDependentSlip);
Contact->setCoulombFriction(Friction);
Contact->setBouncyness( Bouncyness,BounceVelocity);
return true;
}
See it is really easy now to change the values for every contact. The variables should be between 0 and 1 so the values will still be in the targeted range.
You can now derive your object from CollidingObject instead of CollisionTestedObject and it will be able to adjust the contact parameters automatically. If you want you can write new classes that derive from CollisionTestedObject, to implement different behavior, think of a class that simulates water, a teleporter or whatever.
Create a Geometry
The object you want to control normally has one or more geometries and maybe some bodies for these geometries. You now need to derive the class of this object from CollidingObject or any other class you wrote and everytime you create a geometry in it, add the following line to the end of that creation:
Geometry->setUserAny(Any(static_cast<CollisionTestedObject*>(this)));
The geometry's userAny will point to the class that is derived from the CollidingObject Class.
The object is hereby identified in every geometry of that object and can easily be backtracked. Note that you cast it to CollisionTestedObject and not to CollidingObject or your new class.
Sample Code
Header
Here's some sample usage of the code:
//Crate.h
class Crate :public CollidingObject
{
public:
Crate();
Crate(const String& name,Vector3 pos,Vector3 size);
virtual ~Crate();
void CreateCrate(const String& name,Vector3 pos = Vector3::ZERO,Vector3 size = Vector3(5,5,5));
protected:
Entity* crate;
SceneManager *mSceneManager;
SceneNode* BoxNode;
};
Source
Then the definitions :
//Crate.cpp
Crate::Crate(SceneManager* mgr,const String& name,Vector3 pos,Vector3 size)
: mSceneManager(mgr)
{
CreateCrate(name,pos,size);
Bouncyness = 1.0;
ForceDependentSlip = 1.0;
Friction = OgreOde::Utility::Infinity;
BounceVelocity = -1.0;
}
Crate::Crate()
{
}
Crate::~Crate()
{
}
void Crate::CreateCrate(const String& name,Ogre::Vector3 pos,Ogre::Vector3 size)
{
crate = mSceneManager->createEntity(name,"crate.mesh");
crate->setCastShadows(true);
BoxNode = mSceneManager->getRootSceneNode()->createChildSceneNode(name);
BoxNode->attachObject(crate);
BoxNode->setScale(size);
// Set the position
BoxNode->setPosition(pos);
OgreOde::BoxMass mMass(0.5,size);
// Create a box for ODE and attach it to the Ogre version
OgreOde::Body* body = new OgreOde::Body(mWorld);
BoxNode->attachObject(body);
body->setMass(mMass);
OgreOde::Geometry* geom = (OgreOde::Geometry*) new OgreOde::BoxGeometry(size, mWorld, mSpace);
geom->setBody(body);
// Link to CollisionTestedObject Class
geom->setUserAny(Any(static_cast<CollisionTestedObject*>(this)));
}
Collision CallBack Class
Now you need to implement a class that receives the Collision callback from ODE:
class CollisionCallBack : public OgreOde::CollisionListener
{
public:
CollisionCallBack(OgreOde::World * wld)
: mWorld(wld)
{
mWorld->setCollisionListener(this);
new Crate(mWorld->getSceneManager(),"Crate1",Ogre::Vector3::ZERO,Ogre::Vector3(10,10,10));
}
virtual bool collision(OgreOde::Contact* Contact)
{
// Check for collisions between things that are connected and ignore them
OgreOde::Geometry * const g1 = Contact->getFirstGeometry();
OgreOde::Geometry * const g2 = Contact->getSecondGeometry();
if (g1 && g2)
{
const OgreOde::Body * const b1 = g2->getBody();
const OgreOde::Body * const b2 = g1->getBody();
if (b1 && b2 && OgreOde::Joint::areConnected(b1, b2))
return false;
}
//set contact parameters:
Contact->setBouncyness(1.0);
Contact->setCoulombFriction(OgreOde::Utility::Infinity);
Contact->setForceDependentSlip(1.0);
Contact->setAdditionalFDS(1.0);
/*we have 2 collidable objects from our object system, if one of the Collide function returns false, we return false in this method, too,
else we return true, so ode computes a normal collision.
true means ode will treat this like a normal collison => rigid body behavior
false means ode will not treat this collision at all => objects ignore each other*/
bool Return = true;
if (g1->getUserAny().isEmpty() == false))
if (!any_cast<CollisionTestedObject*>(g1->getUserAny())->Collide(true, Contact))
Return = false;
if (g2->getUserAny().isEmpty() == false)
if (!any_cast<CollisionTestedObject*>(g2->getUserAny())->Collide(false, Contact))
Return = false;
return Return;
}
protected:
OgreOde::World *mWorld;
};
Now in your setupScene() funtion (or where ever you first defined your World) , add this line.
new CollisionCallBack(mWorld);
At this point you must make sure you are stepping your world and there should be a Large crate on the ground. If you are unsure how to step the world, you should read http://www.ogre3d.org/wiki/index.php/First_steps_with_OgreODE First Steps With OgreODE first and then return here.
Contact Parameters
ForceDependentSlip
Says how slippy the object is. If your player can not push the crates smoothly over the ground, you might want to increase this one. The Difference between Slip and Slip2 is just the axis that it counts for. I think its not necessary to make a difference, so I just use the same value for both.
Friction
How fast does the object lose speed when hitting another object.
Bouncyness
The higher these values are, the harder the object jumps back from a contact. If you experience that your objects move into each other from time to time, the reason could be that your bouncyness values are too low.
Other
There are some more values that can be changed, but I have not tried them out all yet, feel free to add them here with a short description.
Note: In the current version of OgreOde, the getters for the contact class are not implemented yet, but I am going to create a patch, so they will be added in cvs soon. If they are not included by now, it should be no problem for you to add them yourself.
Closing words
Thats it, feedback, questions and error/mistake reports are welcome and go to this thread or ruben-gerlachREMOVE at webTHIS.de.
written by rewb0rn, November 7, 2007
In this thread is a code for rebuilding a hightmap terrain and generating a collision object. (if I understood right)
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