import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D

x, y = np.mgrid[-1:1:31j, -1:1:31j]
xs, ys = np.mgrid[-1:1:9j, -1:1:9j]

z = x*np.exp(-x**2 - y**2)
zs = xs*np.exp(-xs**2 - ys**2)
u = -(1 - 2*xs**2)*np.exp(-xs**2 - ys**2)
v = 2*ys*zs
w = np.ones_like(xs)

fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
ax.view_init(azim=145, elev=45)
ax.plot_surface(x, y, z, rstride=1, cstride=1, cmap="summer", lw=0.3,
                edgecolors="k")
ax.quiver(xs, ys, zs, u, v, w, colors='k', pivot='tail', length=0.3)
plt.axis("off")
plt.savefig("Surface normals.svg", bbox_inches="tight", transparent=True)
plt.show()