I'm having some problems with my main entity movement algorithm.
What tutorials and docs usually describe is collision detection, but I've found documents about how to organize your movement around its results not to be as readily available. I got Paul Nettle's docs and demo for collision detection using ellipsoids, though, and learned quite a few things from it.
Anyway, the kind of response I'm aiming for is to slide against entities I collide with. The idea for the algorithm is to find the closest entity my moving object (a sphere, and all the other objects currently use bounding boxes) is going to collide with, to move the object as close to it as possible, and then to project what's remaining of my movement vector on it (the object we're collding with is only defined by its normal vector at the collision point and the distance we have to travel to hit it). This projection becomes my new movement vector, and I continue until the vector's depleted.
It's simple enough in theory, but obviously I'm making some error somewhere in the implementation. It works fine as long as I'm colliding with only one entity, but as soon as I'm colliding with several objects at once the moving object may be violently pushed away from its colliders.
For example in this situation the object, which should normally fall down and get stuck between the two boxes below, will be pushed upwards rather violently upon contact with both surfaces at the same time. The bounding volumes are displayed in white, by the way.
Here's a look at the movement code. Defined outside of this bit are m_position and m_speed, which are attributes of the moving object, fElapsedTime, which is provided by the application, and the code uses D3DX functions for all vector math, but I think the names are explanatory enough.
I'm pretty certain the problem doesn't come from the scene's collider detection method, since single-object collisions react exactly as I'd expect them to.
D3DXVECTOR3 movement=m_speed*fElapsedTime;
D3DXVECTOR3 originalPosition=m_position;
c_MoveInfo moveInfo;
bool collisionOccured=true;
unsigned int collisionLoops= MAXCOLLISIONLOOPS;
while (collisionLoops-- && collisionOccured
&& D3DXVec3LengthSq(&movement) > EPSILON)
{
//movement is decomposed into an unit vector for direction and a distance
moveInfo.moveDistance=D3DXVec3Length(&movement);
D3DXVec3Normalize(&moveInfo.moveDirection,&movement);
// the dynamic collision method returns the distance to the nearest collided entity
// and the normal to the collided surface if something is hit during movement.
collisionOccured=m_parentScene->dynamicCollision(this,&moveInfo);
if (collisionOccured){
//move entity right against the collider
m_position+=moveInfo.moveDirection*moveInfo.collisionDistance;
//substract the traveled bit from total intended movement
movement-=moveInfo.moveDirection*moveInfo.collisionDistance;
//project movement on the collided surface
movement=(movement- D3DXVec3Dot(&movement, &moveInfo.collisionNormal)*moveInfo.collisionNormal);
} else {
//no collision, simple movement
m_position+=movement;
}
}
m_speed=(m_position-originalPosition)/fElapsedTime;
If anybody's got some idea as to why it's incorrect, I'm open to suggestions. Possibly it's a rather silly error, but I've been staring at it for too long and can't seem to find it.
