i got an article talking about collision detection with ellipsoid . this one General Collision Detection for Games Using Ellipsoids by Paul Nettle the only part that i don''t anderstand is when he come to detect collision . because he talk about sphere again. are we using an ellipsoid or a sphere ??????????? if some one can explain me this code how to use it . things to consider i anderstands the global algorithm and the respons too but the detection talk about a sphereintersectionpoint that we don''t have and i don''t anderstand why??????????????????? please help // The collision detection entry point collisionDetection(Point sourcePoint, Vector velocityVector, Vector gravityVector) { // We need to do any pre-collision detection work here. Such as adding // gravity to our veoclity vector. We want to do it in this // separate routine because the following routine is recursive, and we // don''t want to recursively add gravity. velocityVector += gravityVector; call collideWithWorld(sourcePoint, velocityVector); } // The collision detection recursive routine collideWithWorld(Point sourcePoint, Vector velocityVector) { // How far do we need to go? Double distanceToTravel = length of velocityVector; // Do we need to bother? if (distanceToTravel < EPSILON) return; // What''s our destination? Point destinationPoint = sourcePoint + velocityVector; // Whom might we collide with? List potentialColliders = determine list of potential colliders; // If there are none, we can safely move to the destination and bail if (potentialColliders is empty) { sourcePoint += velocityVector; return; } // Determine the nearest collider from the list potentialColliders bool firstTimeThrough = true; Double nearestDistance = -1.0; Poly nearestCollider = NULL; Point nearestIntersectionPoint = NULL; Point nearestPolygonIntersectionPoint = NULL; for (each polygon in potentialColliders) { // Plane origin/normal Point pOrigin = any vertex from the current polygon; Vector pNormal = surface normal from the current polygon; // Determine the distance from the plane to the source Double pDist = intersect(source, -pNormal, pOrigin, pNormal); Point sphereIntersectionPoint; Point planeIntersectionPoint; // The radius of the ellipsoid (in the direction of pNormal) Vector directionalRadius = -pNormal * radiusVector; Double radius = length of directionalRadius; // Is the plane embedded? if (fabs(pDist) <= radius) { // Calculate the plane intersection point Vector temp = -pNormal with length set to pDist; planeIntersectionPoint = source + temp; } else { // Calculate the ellipsoid intersection point Vector temp = -pNormal with length set to radius; ellipsoidIntersectionPoint = source + temp; // Calculate the plane intersection point Ray ray(sphereIntersectionPoint, Velocity); Double t = intersect(ellipsoidIntersectionPoint, Velocity, pOrigin, pNormal); // Calculate the plane intersection point Vector V = velocityVector with length set to t; planeIntersectionPoint = ellipsoidIntersectionPoint + V; } // Unless otherwise stated, our polygonIntersectionPoint is the // same point as planeIntersectionPoint Point polygonIntersectionPoint = planeIntersectionPoint; // So… are they the same? if (planeIntersectionPoint is not within the current polygon) { polygonIntersectionPoint = nearest point on polygon''s perimeter to planeIntersectionPoint; } // Invert the velocity vector Vector negativeVelocityVector = -velocityVector; // Using the polygonIntersectionPoint, we need to reverse-intersect // with the ellipsoid Double t = intersectEllipse(sourcePoint, radiusVector, polygonIntersectionPoint, negativeVelocityVector); // Was there an intersection with the ellipsoid? if (t >= 0.0 && t <= distanceToTravel) { Vector V = negativeVelocityVector with length set to t; // Where did we intersect the ellipsoid? Vector intersectionPoint = polygonIntersectionPoint + V; // Closest intersection thus far? if (firstTimeThrough || t < nearestDistance) { nearestDistance = t; nearestCollider = current collider; nearestIntersectionPoint = intersectionPoint; nearestPolygonIntersectionPoint = polygonIntersectionPoint; firstTimeThrough = false; } } } // If we never found a collision, we can safely move to the destination // and bail if (firstTimeThrough) { sourcePoint += velocityVector; return; } // Move to the nearest collision Vector V = velocityVector with length set to (nearestDistance - EPSILON); sourcePoint += V; // Determine the sliding plane (we do this now, because we''re about to // change sourcePoint) Point slidePlaneOrigin = nearestPolygonIntersectionPoint; Vector slidePlaneNormal = nearestPolygonIntersectionPoint - sourcePoint; // We now project the destination point onto the sliding plane Double time = intersect(destinationPoint, slidePlaneNormal, slidePlaneOrigin, slidePlaneNormal); Set length of slidePlaneNormal to time; Vector destinationProjectionNormal = slidePlaneNormal; Point newDestinationPoint = destination + destinationProjectionNormal; // Generate the slide vector, which will become our new velocity vector // for the next iteration Vector newVelocityVector = newDestinationPoint – nearestPolygonIntersectionPoint; // Recursively slide (without adding gravity) collideWithWorld(sourcePoint, newVelocityVector); } love and peace

The code is prototype rather than working code - some of it, such as

>Double distanceToTravel = length of velocityVector;

you can replace with vector functions, but a lot of it, such as

>List potentialColliders = determine list of potential colliders;
>polygonIntersectionPoint = nearest point on polygon''s perimeter to planeIntersectionPoint;
>Double t = intersect(...);
>Double t = intersectEllipse(...);

need to be replaced with a lot more specialised code. From its name I guess the last function does the actual work of dealing with the ellipse(oid), but without source code it''s impossible to say whether it''s correct or explain it.

i resolve my problem cause i ask to the autor.

but what i don''t anderstand is that he says that for an ellipse you have to define a radius vector

so can you explain me it
and show me how to implment it with my player class

like

player=class(tobject);
olpos:tvector;
.
.
.
.
.
.
raduis:glfloat;//for a simple sphere
but for an ellipse i don''t know how it goes

love and peace

Theres no difference for what you need to do between a sphere radius and an ellipsoid radius other than if you have an ellipsoid, all the radii may not be equal. So make a vector with x, y, z radius for it....

[edited by - rgirard413 on April 26, 2002 6:54:31 PM]

thank you i anderstand
thanks a lot man

love and peace