Method reduceContactPoints

src/systems/CollisionDetectionSystem.cpp:1483–1662

Reduce the number of contact points of a potential contact manifold This is based on the technique described by Dirk Gregorius in his "Contacts Creation" GDC presentation. This method will reduce the number of contact points to a maximum of 4 points (but it can be less).

// "Contacts Creation" GDC presentation. This method will reduce the number of
// contact points to a maximum of 4 points (but it can be less).
void CollisionDetectionSystem::reduceContactPoints(ContactManifoldInfo& manifold, const Transform& shape1ToWorldTransform,
                                             const Array<ContactPointInfo>& potentialContactPoints) const {

    assert(manifold.nbPotentialContactPoints > MAX_CONTACT_POINTS_IN_MANIFOLD);

    // The following algorithm only works to reduce to a maximum of 4 contact points
    assert(MAX_CONTACT_POINTS_IN_MANIFOLD == 4);

    // Array of the candidate contact points indices in the manifold. Every time that we have found a
    // point we want to keep, we will remove it from this array
    uint candidatePointsIndices[NB_MAX_CONTACT_POINTS_IN_POTENTIAL_MANIFOLD];
    uint8 nbCandidatePoints = manifold.nbPotentialContactPoints;
    for (uint8 i=0 ; i < manifold.nbPotentialContactPoints; i++) {
        candidatePointsIndices[i] = manifold.potentialContactPointsIndices[i];
    }

    int8 nbReducedPoints = 0;

    uint32 pointsToKeepIndices[MAX_CONTACT_POINTS_IN_MANIFOLD];
    for (int8 i=0; i<MAX_CONTACT_POINTS_IN_MANIFOLD; i++) {
        pointsToKeepIndices[i] = 0;
    }

    //  Compute the initial contact point we need to keep.
    // The first point we keep is always the point in a given
    // constant direction (in order to always have same contact points
    // between frames for better stability)

    const Transform worldToShape1Transform = shape1ToWorldTransform.getInverse();

    // Compute the contact normal of the manifold (we use the first contact point)
    // in the local-space of the first collision shape
    const Vector3 contactNormalShape1Space = worldToShape1Transform.getOrientation() * potentialContactPoints[candidatePointsIndices[0]].normal;

    // Compute a search direction
    const Vector3 searchDirection(1, 1, 1);
    decimal maxDotProduct = DECIMAL_SMALLEST;
    uint32 elementIndexToKeep = 0;
    for (uint32 i=0; i < nbCandidatePoints; i++) {

        const ContactPointInfo& element = potentialContactPoints[candidatePointsIndices[i]];
        decimal dotProduct = searchDirection.dot(element.localPoint1);
        if (dotProduct > maxDotProduct) {
            maxDotProduct = dotProduct;
            elementIndexToKeep = i;
            nbReducedPoints = 1;
        }
    }
    pointsToKeepIndices[0] = candidatePointsIndices[elementIndexToKeep];
    removeItemAtInArray(candidatePointsIndices, elementIndexToKeep, nbCandidatePoints);
    //candidatePointsIndices.removeAt(elementIndexToKeep);
    assert(nbReducedPoints == 1);

    // Compute the second contact point we need to keep.
    // The second point we keep is the one farthest away from the first point.

    decimal maxDistance = decimal(0.0);
    elementIndexToKeep = 0;