"""Plot streamlines of the 2D field:
u(x,y) = -1 - x^2 + y
v(x,y) =  1 + x   - y^2
"""
from vtkplotter import *
import numpy as np

# a grid with a vector field (U,V):
X, Y = np.mgrid[-5:5 :15j, -4:4 :15j]
U = -1 - X**2 + Y
V =  1 + X    - Y**2

# optionally, pick some random points as seeds:
prob_pts = np.random.rand(200, 2)*8 - [4,4]

sp = streamplot(X,Y, U,V,
                lw=0.001,            # line width in abs. units
                direction='forward', # 'both' or 'backward'
                probes=prob_pts,     # try comment out this
               )

pts = Points(prob_pts, r=5, c='white')

show(sp, pts,
     Text2D(__doc__, c='w'),
     axes=1, bg='bb')
