public double Profile(double x)
{
#if DEBUG
if (x < 0)
throw new Exception();
#endif
// Equation 9 in (Comaniciu & Meer)
// https://courses.csail.mit.edu/6.869/handouts/PAMIMeanshift.pdf
// and http://www.serc.iisc.ernet.in/~venky/SE263/slides/Mean-Shift-Theory.pdf
return Math.Exp(-0.5 * x);
}