code: def on_mouse_move(event):
      if 3 in event.buttons:
          tr = canvas.scene.node_transform(vfield)
          vfield.shared_program["attractor"] = tr.map(event.pos)[:2]
  
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 2 for calling a global function with example usage: on_mouse_move(event) without return types