gestion of collisions : discussion on some improvements...

[je.a.le]

hi,

don't know on what you're working on but for collision what I need is :
1) which part on the entity collide, (front, back (of a car) etc...)
2) where (and who!) on the other entity, (a bush, a truck (rip) )
3) which force/velocity (1Km/h 100km/h ??? not the same result)
4) must be done as fast as possible !!


the current implementation doesn't fit really well. You have to cross ref others
entity/shape - which is time consuming - even many informations are missing.

So here some changes I made (would make) and I would like to know what you all
think about ;

a) The ode callback in physics::Level will call :
physics::Shape::onCollide( Shape* remote, vector& velocity )

: added the velocity information and "remote" is the other entity's shape it
collided with.

b) The shape will call (from onCollide) :
physics::entity::onCollide( Shape* local, Shape* remote, vector& velocity );

: from the entity you directly know where, with who, how.

c) I change the entity collided array (GetCollidedShapes). It will store a
struct holding :
struct collision {
        Shape* local // shape on this entity
        Shape* remote // shape on the collided entity
        vector3 velocity // velocity of the collision
}

d) new xml optional token for shape object : "shapeid" (int)
By distributing the value between all shapes on your game - like and ip address
- it's allow you to identify the shape without having to look at the composite :
cheap, fast. (and allow switch/case algorithm structures)

here an example of implementation :

xml for a car :

<Composite type="standard" numBodies="5" numJoints="4" numMeshes="0">
      <RigidBody name="body" pos="0.000000 0.682949 0.000000" rot="0.0000 0.0000
0.0000 1.0000" model="body" numShapes="1">
        <BoxShape shapeid="100" pos="0.0  0.296 0.0" rot="0.0000 0.0000 0.0000
1.0000" size="2 0.692 1" mat="EmptyWood" />
      </RigidBody>

      <RigidBody name="wheel_front_l" pos="0.822664 0.682949 -0.596252"
rot="0.0000 0.0000 0.0000 1.0000" model="wheel_front_l" numShapes="1">
        <SphereShape shapeid="10" pos="0.000 0.000 0.000" radius="0.3" mat="Tire" />
      </RigidBody>
.............

xml for a pylon (static obj)

<Composite type="standard" numBodies="0" numJoints="0" numMeshes="1">
         <MeshShape name="pylone" shapeid="1000"
file="home:/export/meshes/testcar/pylone.n3d2" mat="Soil"/>
</Composite>


most of collision check must be done into an derived physics::entity but here
how to implement those into an property::OnMoveAfter

..............
// init some variables etc....
...............
i = phyentity->GetNumCollisions() -1;
while( i >= 0 ) {
struct Physics::Entity::collision data = phyentity->GetCollidedShapes().At( i );
               
switch( data.local->GetShapeId() ) {
// left front wheel
case 10 :
..........
// right front wheel
case 11 :
        switch( data.remote->GetShapeId() ) {
        // wheel collide with ground ; don't care
        case 0 :
                break;
       
        // it a "tiny" tree, no consequences, the tree entity
        // will care about destroying it
        case 500 :
                break;
       
        // hit a pylon, may "break the joint
        case 1000 :
                // check force...
                if( (data.velocity.x * data.velocity.x) +
                (data.velocity.y * data.velocity.y) +
                (data.velocity.z * data.velocity.z) > 0.1 )  {
               
                        // too fast, break the joint between wheel and body
                        dJointAttach( joint_g->GetJointId(), 0, 0 );
                        ....... etc
                }
                break;
        .................etc
        default :
                break;
        }
.............etc
}

In my point of view those changes make collision based decision fast and cheap,
I tried it and working with collision is now an easy part;

So WHAT do you THINK about IT ?????????

--
for my 3d work it's over there --> http://j.a.l.free.fr/3dsmax
update 26 Aug 2006


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