code: public double Function2D(double x, double y) {
          return Math.exp(-(x * x + y * y) / (2 * sqrSigma)) / (2 * Math.PI * sqrSigma);
      }
docs: 2-D Gaussian function.
  
  @param x value.
  @param y value.
  @return Function's value at point (x,y).