Two-dimensional oriented exponential decay pattern.

def exponential(x, y, xscale, yscale):
    """
    Two-dimensional oriented exponential decay pattern.
    """
    if xscale==0.0 or yscale==0.0:
        return x*0.0

    with float_error_ignore():
        x_w = np.divide(x,xscale)
        y_h = np.divide(y,yscale)
        return np.exp(-np.sqrt(x_w*x_w+y_h*y_h))