Image

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

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)

<HR>
Creative Commons Copyright -- Some rights reserved.


THE WORK (AS DEFINED BELOW) IS PROVIDED UNDER THE TERMS OF THIS CREATIVE COMMONS PUBLIC LICENSE ("CCPL" OR "LICENSE"). THE WORK IS PROTECTED BY COPYRIGHT AND/OR OTHER APPLICABLE LAW. ANY USE OF THE WORK OTHER THAN AS AUTHORIZED UNDER THIS LICENSE OR COPYRIGHT LAW IS PROHIBITED.

BY EXERCISING ANY RIGHTS TO THE WORK PROVIDED HERE, YOU ACCEPT AND AGREE TO BE BOUND BY THE TERMS OF THIS LICENSE. THE LICENSOR GRANTS YOU THE RIGHTS CONTAINED HERE IN CONSIDERATION OF YOUR ACCEPTANCE OF SUCH TERMS AND CONDITIONS.

1. Definitions

  • "Collective Work" means a work, such as a periodical issue, anthology or encyclopedia, in which the Work in its entirety in unmodified form, along with a number of other contributions, constituting separate and independent works in themselves, are assembled into a collective whole. A work that constitutes a Collective Work will not be considered a Derivative Work (as defined below) for the purposes of this License.
  • "Derivative Work" means a work based upon the Work or upon the Work and other pre-existing works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which the Work may be recast, transformed, or adapted, except that a work that constitutes a Collective Work will not be considered a Derivative Work for the purpose of this License. For the avoidance of doubt, where the Work is a musical composition or sound recording, the synchronization of the Work in timed-relation with a moving image ("synching") will be considered a Derivative Work for the purpose of this License.
  • "Licensor" means the individual or entity that offers the Work under the terms of this License.
  • "Original Author" means the individual or entity who created the Work.
  • "Work" means the copyrightable work of authorship offered under the terms of this License.
  • "You" means an individual or entity exercising rights under this License who has not previously violated the terms of this License with respect to the Work, or who has received express permission from the Licensor to exercise rights under this License despite a previous violation.
  • "License Elements" means the following high-level license attributes as selected by Licensor and indicated in the title of this License: Attribution, ShareAlike.

2. Fair Use Rights

Nothing in this license is intended to reduce, limit, or restrict any rights arising from fair use, first sale or other limitations on the exclusive rights of the copyright owner under copyright law or other applicable laws.

3. License Grant

Subject to the terms and conditions of this License, Licensor hereby grants You a worldwide, royalty-free, non-exclusive, perpetual (for the duration of the applicable copyright) license to exercise the rights in the Work as stated below:

  • to reproduce the Work, to incorporate the Work into one or more Collective Works, and to reproduce the Work as incorporated in the Collective Works;
  • to create and reproduce Derivative Works;
  • to distribute copies or phonorecords of, display publicly, perform publicly, and perform publicly by means of a digital audio transmission the Work including as incorporated in Collective Works;
  • to distribute copies or phonorecords of, display publicly, perform publicly, and perform publicly by means of a digital audio transmission Derivative Works.
  • For the avoidance of doubt, where the work is a musical composition:
    • Performance Royalties Under Blanket Licenses. Licensor waives the exclusive right to collect, whether individually or via a performance rights society (e.g. ASCAP, BMI, SESAC), royalties for the public performance or public digital performance (e.g. webcast) of the Work.
    • Mechanical Rights and Statutory Royalties. Licensor waives the exclusive right to collect, whether individually or via a music rights society or designated agent (e.g. Harry Fox Agency), royalties for any phonorecord You create from the Work ("cover version") and distribute, subject to the compulsory license created by 17 USC Section 115 of the US Copyright Act (or the equivalent in other jurisdictions).
    • Webcasting Rights and Statutory Royalties. For the avoidance of doubt, where the Work is a sound recording, Licensor waives the exclusive right to collect, whether individually or via a performance-rights society (e.g. SoundExchange), royalties for the public digital performance (e.g. webcast) of the Work, subject to the compulsory license created by 17 USC Section 114 of the US Copyright Act (or the equivalent in other jurisdictions).


The above rights may be exercised in all media and formats whether now known or hereafter devised. The above rights include the right to make such modifications as are technically necessary to exercise the rights in other media and formats. All rights not expressly granted by Licensor are hereby reserved.

4. Restrictions

The license granted in Section 3 above is expressly made subject to and limited by the following restrictions:

  • You may distribute, publicly display, publicly perform, or publicly digitally perform the Work only under the terms of this License, and You must include a copy of, or the Uniform Resource Identifier for, this License with every copy or phonorecord of the Work You distribute, publicly display, publicly perform, or publicly digitally perform. You may not offer or impose any terms on the Work that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder. You may not sublicense the Work. You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute, publicly display, publicly perform, or publicly digitally perform the Work with any technological measures that control access or use of the Work in a manner inconsistent with the terms of this License Agreement. The above applies to the Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Work itself to be made subject to the terms of this License. If You create a Collective Work, upon notice from any Licensor You must, to the extent practicable, remove from the Collective Work any credit as required by clause 4(c), as requested. If You create a Derivative Work, upon notice from any Licensor You must, to the extent practicable, remove from the Derivative Work any credit as required by clause 4(c), as requested.
  • You may distribute, publicly display, publicly perform, or publicly digitally perform a Derivative Work only under the terms of this License, a later version of this License with the same License Elements as this License, or a Creative Commons iCommons license that contains the same License Elements as this License (e.g. Attribution-ShareAlike 2.5 Japan). You must include a copy of, or the Uniform Resource Identifier for, this License or other license specified in the previous sentence with every copy or phonorecord of each Derivative Work You distribute, publicly display, publicly perform, or publicly digitally perform. You may not offer or impose any terms on the Derivative Works that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder, and You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute, publicly display, publicly perform, or publicly digitally perform the Derivative Work with any technological measures that control access or use of the Work in a manner inconsistent with the terms of this License Agreement. The above applies to the Derivative Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Derivative Work itself to be made subject to the terms of this License.
  • If you distribute, publicly display, publicly perform, or publicly digitally perform the Work or any Derivative Works or Collective Works, You must keep intact all copyright notices for the Work and provide, reasonable to the medium or means You are utilizing: (i) the name of the Original Author (or pseudonym, if applicable) if supplied, and/or (ii) if the Original Author and/or Licensor designate another party or parties (e.g. a sponsor institute, publishing entity, journal) for attribution in Licensor's copyright notice, terms of service or by other reasonable means, the name of such party or parties; the title of the Work if supplied; to the extent reasonably practicable, the Uniform Resource Identifier, if any, that Licensor specifies to be associated with the Work, unless such URI does not refer to the copyright notice or licensing information for the Work; and in the case of a Derivative Work, a credit identifying the use of the Work in the Derivative Work (e.g., "French translation of the Work by Original Author," or "Screenplay based on original Work by Original Author"). Such credit may be implemented in any reasonable manner; provided, however, that in the case of a Derivative Work or Collective Work, at a minimum such credit will appear where any other comparable authorship credit appears and in a manner at least as prominent as such other comparable authorship credit.

5. Representations, Warranties and Disclaimer

UNLESS OTHERWISE AGREED TO BY THE PARTIES IN WRITING, LICENSOR OFFERS THE WORK AS-IS AND MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND CONCERNING THE MATERIALS, EXPRESS, IMPLIED, STATUTORY OR OTHERWISE, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF TITLE, MERCHANTIBILITY, FITNESS FOR A PARTICULAR PURPOSE, NONINFRINGEMENT, OR THE ABSENCE OF LATENT OR OTHER DEFECTS, ACCURACY, OR THE PRESENCE OF ABSENCE OF ERRORS, WHETHER OR NOT DISCOVERABLE. SOME JURISDICTIONS DO NOT ALLOW THE EXCLUSION OF IMPLIED WARRANTIES, SO SUCH EXCLUSION MAY NOT APPLY TO YOU.

6. Limitation on Liability.

EXCEPT TO THE EXTENT REQUIRED BY APPLICABLE LAW, IN NO EVENT WILL LICENSOR BE LIABLE TO YOU ON ANY LEGAL THEORY FOR ANY SPECIAL, INCIDENTAL, CONSEQUENTIAL, PUNITIVE OR EXEMPLARY DAMAGES ARISING OUT OF THIS LICENSE OR THE USE OF THE WORK, EVEN IF LICENSOR HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

7. Termination

  • This License and the rights granted hereunder will terminate automatically upon any breach by You of the terms of this License. Individuals or entities who have received Derivative Works or Collective Works from You under this License, however, will not have their licenses terminated provided such individuals or entities remain in full compliance with those licenses. Sections 1, 2, 5, 6, 7, and 8 will survive any termination of this License.
  • Subject to the above terms and conditions, the license granted here is perpetual (for the duration of the applicable copyright in the Work). Notwithstanding the above, Licensor reserves the right to release the Work under different license terms or to stop distributing the Work at any time; provided, however that any such election will not serve to withdraw this License (or any other license that has been, or is required to be, granted under the terms of this License), and this License will continue in full force and effect unless terminated as stated above.

8. Miscellaneous

  • Each time You distribute or publicly digitally perform the Work or a Collective Work, the Licensor offers to the recipient a license to the Work on the same terms and conditions as the license granted to You under this License.
  • Each time You distribute or publicly digitally perform a Derivative Work, Licensor offers to the recipient a license to the original Work on the same terms and conditions as the license granted to You under this License.
  • If any provision of this License is invalid or unenforceable under applicable law, it shall not affect the validity or enforceability of the remainder of the terms of this License, and without further action by the parties to this agreement, such provision shall be reformed to the minimum extent necessary to make such provision valid and enforceable.
  • No term or provision of this License shall be deemed waived and no breach consented to unless such waiver or consent shall be in writing and signed by the party to be charged with such waiver or consent.
  • This License constitutes the entire agreement between the parties with respect to the Work licensed here. There are no understandings, agreements or representations with respect to the Work not specified here. Licensor shall not be bound by any additional provisions that may appear in any communication from You. This License may not be modified without the mutual written agreement of the Licensor and You.