float Classifier::classify2D_checkcondnum(const Point2D& P, const Point2D& R, float& condnumber) const
{
condnumber = 0;
if (path.size() < 2)
{
assert(false);
return 0;
}
// consider each path segment as a mini-classifier
// the segment PR[Pt-->Refpt] and each path's segment cross
// iff each one classifies the end point of the other in different classes
Point2D PR = R-P;
Point2D u = PR; u.normalize();
unsigned numcross = 0;
//we'll also look for the distance between P and the nearest segment
float closestSquareDist = -1.0f;
size_t segCount = path.size() - 1;
for (size_t i=0; i<segCount; ++i)
{
//current path segment (or half-line!)
Point2D AP = P - path[i];
Point2D AB = path[i+1] - path[i];
Point2D v = AB; v.normalize();
condnumber = std::max<float>(condnumber, fabs(v.dot(u)));
// Compute whether PR[Pt-->Refpt] and that segment cross
float denom = (u.x*v.y-v.x*u.y);
if (denom != 0)
{
// 1. check whether the given pt and the refpt are on different sides of the classifier line
//we search for alpha and beta so that
// P + alpha * PR = A + beta * AB
float alpha = (AP.y * v.x - AP.x * v.y)/denom;
bool pathIntersects = (alpha >= 0 && alpha*alpha <= PR.norm2());
if (pathIntersects)
{
float beta = (AP.y * u.x - AP.x * u.y)/denom;
// first and last lines are projected to infinity
bool refSegIntersects = ((i == 0 || beta >= 0) && (i+1 == segCount || beta*beta < AB.norm2())); //not "beta*beta <= AB.norm2()" because the equality case will be managed by the next segment!
// crossing iif each segment/line separates the other
if (refSegIntersects)
numcross++;
}
}
// closest distance from the point to that segment
// 1. projection of the point of the line
float squareDistToSeg = 0;
float distAH = v.dot(AP);
if ((i == 0 || distAH >= 0.0) && (i+1 == segCount || distAH <= AB.norm()))
{
// 2. Is the projection within the segment limit? yes => closest
Point2D PH = (path[i] + v * distAH) - P;
squareDistToSeg = PH.norm2();
}
else
{
// 3. otherwise closest is the minimum of the distance to the segment ends
Point2D BP = P - path[i+1];
squareDistToSeg = std::min( AP.norm2(), BP.norm2() );
}
if (closestSquareDist < 0 || squareDistToSeg < closestSquareDist)
{
closestSquareDist = squareDistToSeg;
}
}
assert(closestSquareDist >= 0);
float deltaNorm = sqrt(closestSquareDist);
return ((numcross & 1) == 0 ? deltaNorm : -deltaNorm);
}
