The function calculates 2-D Gaussian function:
f(x, y) = exp( x * x + y * y / ( -2 * s * s ) ) / ( s * s * 2 * PI )
public static double Gaussian2D(double sigmaSquared, double x, double y) { return Math.Exp((x * x + y * y) / (-2 * sigmaSquared)) / (2 * Math.PI * sigmaSquared); }