// calc tangent at x=a
public final void compute() {
if (!(f.isDefined() && P.isDefined() && deriv.isDefined())) {
tangent.setUndefined();
return;
}
// calc the tangent;
double a = P.inhomX;
double fa = f.evaluate(a);
double slope = deriv.evaluate(a);
tangent.setCoords(-slope, 1.0, a * slope - fa);
if (!pointOnFunction) T.setCoords(a, fa, 1.0);
}
// calc axes
@Override
public final void compute() {
// only parabola has directrix
if (c.type == GeoConicNDConstants.CONIC_PARABOLA) {
// directrix has direction of second eigenvector
// through point (b - p/2* eigenvec1)
directrix.x = -eigenvec[1].getY();
directrix.y = eigenvec[1].getX();
double px = b.getX() - c.p / 2.0 * eigenvec[0].getX();
double py = b.getY() - c.p / 2.0 * eigenvec[0].getY();
directrix.z = -(directrix.x * px + directrix.y * py);
P.setCoords(px, py, 1.0);
} else directrix.setUndefined();
}