/// <summary>
/// Returns the value of a window function.
/// </summary>
/// <param name="x">Argument</param>
/// <param name="frameSize">Window size</param>
/// <returns>float precision floating point number</returns>
public override float Function(float x, int frameSize)
{
// Kaiser window:
float u = 2 * x / (frameSize - 1);
float r = 1 - u * u;
float v = r >= 0 ? (float)Math.Sqrt(1 - u * u) : 0;
float z = Maths.Pi * this.a;
float q = Special.I(z * v, 0);
return(q / Special.I(z, 0));
}