Orion3D physic engine

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Orion3D physic engine

In this page, we will give an in-depth explanation of how the design of the Orion3D physic engine. We will not review the maths and physic derivations of how all this stuff works. This is up to you to read the inline documentation (formula are always expanded).

A brief overview

What is a physic engine ?

The main feature of this physic engine is it's modularity. It is fully object oriented, and it's easy to derive new class and implement new forces, constraints, etc. Althought it is not really optimised yet, it use powerfull maths algorithms (sparse systems resolution, etc), and should be easy to tuned for real time applications.

Here are the main layers of the physic engine :

build on top of Orion3D :
- rigid bodies can be linked to Orion3D objects.
- particules systems can be overloaded to produce special effects (cloth, fountains ...).
classical forward dynamics resolutions :
- Forces system : support for various kind of forces.
- Different integrators for the equations of movement (Euler, RK4, Verlet).
colliding contact resolution :
- allow multiple kind of primitive collision (rigid body, particules system, gizmo).
- use of OPCODE library for rigid body versus rigid body collision check.
constraint resolution between rigid body :
- use Lagrange multiplier velocity based model
- different joint primitive (spherical, hinge, etc) for simulating articulated rigid body structures.
particule system management :
- resolution of equation of movement for the particules.
- allow for various use of the system (cloth simulation, flow rendering, etc).

The rigid body

What is a rigid body ?

How to link a rigid body with \b Orion3D object ?

Particules system

What is a particules system ?

A particule system is a list of poncutal particules (class OR_Particule), that move according to ponctual mecanics.

How to use a particule system ?

Collision detection

The different kind of colliders

To track the collision between two objects (called the collidees), Orion3D uses colliders. According to the kind of objects pairs, there are for the moment two main different colliders :

Rigid body colliders : track collisions between too rigid objects, of class OR_RigidBody. The class of this collider is OR_RigidBodyCollider.
Deflector collider : track collisions between a deflector (ie. a class deriving from the abstract class OR_Deflector_ABC) and a particule system (class OR_ParticuleManager or a derivated class such as OR_ParticuleSystem).

All collider inherit from base class OR_Collider_ABC and have to overload mainly one method : \begincode virtual void PerformCollisionDetection( OR_CollisionManager& CollisionManager )=0; This is the method that is called by the physic engine (in fact by the manager of colliders) to resolve the collision between the two collidee.

The main advantage of using collider is to provide a high level control over the way collision is performed. For example, collision between to object can easyly be turn on/off simply by disabling the collider (a complete list of such methods are provided by OR_Collider_ABC).

The rigid body colliders

The rigid body collider use OPCODE collision detection library to check the collisions between its two rigid body collidee. To perform this task, it keeps in its internal two OPCODE objects that tracks the relative positions of the two rigid body :

AABBTreeCollider : this keeps track of the collision states between the two objects. To access the data of the two rigid body (mainly the vertex of the mesh that supports the rigid body), we assign to the TreeCollider a callback method.
BVTCache : this cache is used by OPCODE to access it's information about the two objects. This information is packed in an opcode model (class OPCODE_Model).

The rigid body collider, at its creation, is responsible for insitializing these both object (linking the cache with the OPCODE model, set up the tree collider options, ...).

Linkage between Orion3D and OPCODE

There are two kind of datas that OPCODE used :

the datas that are common to a pair of collidde. This is the tree collider (AABBTreeCollider) and the collision cache (BVTCache). These datas are kept inside the rigid body colliders.
the datas that are proper to each object. These data are stored by OPCODE in an OPCODE_Model, and are encapsulated by Orion3D in a class named OR_PhysicModel.

The class OR_PhysicModel links the OPCODE model with Orion3D mesh. An OR_PhysicModel must be specified for each rigid body (class OR_RigidBody) that is used in a rigid body collider.