static double filter_hamming(const double x)
{
	/* should be
	(0.54+0.46*cos(M_PI*(double) x));
	but this approximation is sufficient */
	if (x < -1.0f)
		return 0.0f;
	if (x < 0.0f)
		return 0.92f*(-2.0f*x-3.0f)*x*x+1.0f;
	if (x < 1.0f)
		return 0.92f*(2.0f*x-3.0f)*x*x+1.0f;
	return 0.0f;
}