code: def fn(y, x):
      dx = x - 50
      dy = y - 30
      hyp = (dx**2 + dy**2) ** 0.5 + 0.01
      return np.array([100 * dy / hyp**1.7, -100 * dx / hyp**1.8])
  
file_path: vispy/examples/demo/scene/flow_lines.py
imports: from __future__ import division
  from vispy import app, scene, visuals, gloo
  from vispy.util import ptime
  import numpy
label_desc: Use image node_id 1 for calling a global function with example usage: fn(y, x) and returns: np