class_docstr: Mask to keep track of discrete regions crossed by streamlines.
  
  The resolution of this grid determines the approximate spacing between
  trajectories. Streamlines are only allowed to pass through zeroed cells:
  When a streamline enters a cell, that cell is set to 1, and no new
  streamlines are allowed to enter.
class_docstr_tok: ["Mask","to","keep","track","of","discrete","regions","crossed","by","streamlines",".","The","resolution","of","this","grid","determines","the","approximate","spacing","between","trajectories",".","Streamlines","are","only","allowed","to","pass","through","zeroed","cells",":","When","a","streamline","enters","a","cell",",","that","cell","is","set","to","1",",","and","no","new","streamlines","are","allowed","to","enter","."]
docstr: Start recording streamline trajectory
docstr_tok: ["Start","recording","streamline","trajectory"]
code: def _start_trajectory(self, xm, ym):
      """Start recording streamline trajectory"""
      self._traj = []
      self._update_trajectory(xm, ym)
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import six
  from six.moves import xrange
  import numpy
  import matplotlib
  import matplotlib.cm
  import matplotlib.colors
  import matplotlib.collections
  import matplotlib.lines
  import matplotlib.patches
label_desc: Use image node_id 3 for calling the StreamMask obj's underlying member method code with example usage: obj._start_trajectory(xm, ym) without return types