code: def test_offscreen_context():
      c1 = OffscreenContext()
      c2 = OffscreenContext.get_global_instance()
      c3 = OffscreenContext.get_global_instance()
      c4 = OffscreenContext()
  
      assert c1 is not c2
      assert c1 is not c4
      assert c2 is c3
  
      c1.make_current()
      c1.close()
  
file_path: vispy/vispy/app/backends/tests/test_offscreen_util.py
imports: from vispy.app.backends._offscreen_util import OffscreenContext, FrameBufferHelper
  from vispy.testing import run_tests_if_main, requires_application
  from vispy import gloo
  import numpy
label_desc: Use image node_id 1 for calling a global function with example usage: test_offscreen_context() without return types