content: """
  Support for plotting vector fields.
  
  Presently this contains Quiver and Barb. Quiver plots an arrow in the
  direction of the vector, with the size of the arrow related to the
  magnitude of the vector.
  
  Barbs are like quiver in that they point along a vector, but
  the magnitude of the vector is given schematically by the presence of barbs
  or flags on the barb.
  
  This will also become a home for things such as standard
  deviation ellipses, which can and will be derived very easily from
  the Quiver code.
  """
  
  
  import numpy as np
  from numpy import ma
  import matplotlib.collections as collections
  import matplotlib.transforms as transforms
  import matplotlib.text as mtext
  import matplotlib.artist as martist
  import matplotlib.font_manager as font_manager
  from matplotlib.cbook import delete_masked_points
  from matplotlib.patches import CirclePolygon
  import math
  
  
  _quiver_doc = """
  Plot a 2-D field of arrows.
  
  call signatures::
  
    quiver(U, V, **kw)
    quiver(U, V, C, **kw)
    quiver(X, Y, U, V, **kw)
    quiver(X, Y, U, V, C, **kw)
  
  Arguments:
  
    *X*, *Y*:
  
      The x and y coordinates of the arrow locations (default is tail of
      arrow; see *pivot* kwarg)
  
    *U*, *V*:
  
      give the *x* and *y* components of the arrow vectors
  
    *C*:
      an optional array used to map colors to the arrows
  
  All arguments may be 1-D or 2-D arrays or sequences. If *X* and *Y*
  are absent, they will be generated as a uniform grid.  If *U* and *V*
  are 2-D arrays but *X* and *Y* are 1-D, and if len(*X*) and len(*Y*)
  match the column and row dimensions of *U*, then *X* and *Y* will be
  expanded with :func:`numpy.meshgrid`.
  
  *U*, *V*, *C* may be masked arrays, but masked *X*, *Y* are not
  supported at present.
  
  Keyword arguments:
  
    *units*: ['width' | 'height' | 'dots' | 'inches' | 'x' | 'y' ]
      arrow units; the arrow dimensions *except for length* are in
      multiples of this unit.
  
      * 'width' or 'height': the width or height of the axes
  
      * 'dots' or 'inches': pixels or inches, based on the figure dpi
  
      * 'x' or 'y': *X* or *Y* data units
  
      The arrows scale differently depending on the units.  For
      'x' or 'y', the arrows get larger as one zooms in; for other
      units, the arrow size is independent of the zoom state.  For
      'width or 'height', the arrow size increases with the width and
      height of the axes, respectively, when the the window is resized;
      for 'dots' or 'inches', resizing does not change the arrows.
  
     *angles*: ['uv' | 'xy' | array]
      With the default 'uv', the arrow aspect ratio is 1, so that
      if *U*==*V* the angle of the arrow on the plot is 45 degrees
      CCW from the *x*-axis.
      With 'xy', the arrow points from (x,y) to (x+u, y+v).
      Alternatively, arbitrary angles may be specified as an array
      of values in degrees, CCW from the *x*-axis.
  
    *scale*: [ None | float ]
      data units per arrow unit, e.g. m/s per plot width; a smaller
      scale parameter makes the arrow longer.  If *None*, a simple
      autoscaling algorithm is used, based on the average vector length
      and the number of vectors.
  
    *width*:
      shaft width in arrow units; default depends on choice of units,
      above, and number of vectors; a typical starting value is about
      0.005 times the width of the plot.
  
    *headwidth*: scalar
      head width as multiple of shaft width, default is 3
  
    *headlength*: scalar
      head length as multiple of shaft width, default is 5
  
    *headaxislength*: scalar
      head length at shaft intersection, default is 4.5
  
    *minshaft*: scalar
      length below which arrow scales, in units of head length. Do not
      set this to less than 1, or small arrows will look terrible!
      Default is 1
  
    *minlength*: scalar
      minimum length as a multiple of shaft width; if an arrow length
      is less than this, plot a dot (hexagon) of this diameter instead.
      Default is 1.
  
    *pivot*: [ 'tail' | 'middle' | 'tip' ]
      The part of the arrow that is at the grid point; the arrow rotates
      about this point, hence the name *pivot*.
  
    *color*: [ color | color sequence ]
      This is a synonym for the
      :class:`~matplotlib.collections.PolyCollection` facecolor kwarg.
      If *C* has been set, *color* has no effect.
  
  The defaults give a slightly swept-back arrow; to make the head a
  triangle, make *headaxislength* the same as *headlength*. To make the
  arrow more pointed, reduce *headwidth* or increase *headlength* and
  *headaxislength*. To make the head smaller relative to the shaft,
  scale down all the head parameters. You will probably do best to leave
  minshaft alone.
  
  linewidths and edgecolors can be used to customize the arrow
  outlines. Additional :class:`~matplotlib.collections.PolyCollection`
  keyword arguments:
  
  %(PolyCollection)s
  """ % martist.kwdocd
  
  _quiverkey_doc = """
  Add a key to a quiver plot.
  
  call signature::
  
    quiverkey(Q, X, Y, U, label, **kw)
  
  Arguments:
  
    *Q*:
      The Quiver instance returned by a call to quiver.
  
    *X*, *Y*:
      The location of the key; additional explanation follows.
  
    *U*:
      The length of the key
  
    *label*:
      a string with the length and units of the key
  
  Keyword arguments:
  
    *coordinates* = [ 'axes' | 'figure' | 'data' | 'inches' ]
      Coordinate system and units for *X*, *Y*: 'axes' and 'figure' are
      normalized coordinate systems with 0,0 in the lower left and 1,1
      in the upper right; 'data' are the axes data coordinates (used for
      the locations of the vectors in the quiver plot itself); 'inches'
      is position in the figure in inches, with 0,0 at the lower left
      corner.
  
    *color*:
      overrides face and edge colors from *Q*.
  
    *labelpos* = [ 'N' | 'S' | 'E' | 'W' ]
      Position the label above, below, to the right, to the left of the
      arrow, respectively.
  
    *labelsep*:
      Distance in inches between the arrow and the label.  Default is
      0.1
  
    *labelcolor*:
      defaults to default :class:`~matplotlib.text.Text` color.
  
    *fontproperties*:
      A dictionary with keyword arguments accepted by the
      :class:`~matplotlib.font_manager.FontProperties` initializer:
      *family*, *style*, *variant*, *size*, *weight*
  
  Any additional keyword arguments are used to override vector
  properties taken from *Q*.
  
  The positioning of the key depends on *X*, *Y*, *coordinates*, and
  *labelpos*.  If *labelpos* is 'N' or 'S', *X*, *Y* give the position
  of the middle of the key arrow.  If *labelpos* is 'E', *X*, *Y*
  positions the head, and if *labelpos* is 'W', *X*, *Y* positions the
  tail; in either of these two cases, *X*, *Y* is somewhere in the
  middle of the arrow+label key object.
  """
  
  
  class QuiverKey(martist.Artist):
      """ Labelled arrow for use as a quiver plot scale key.
      """
      halign = {'N': 'center', 'S': 'center', 'E': 'left',   'W': 'right'}
      valign = {'N': 'bottom', 'S': 'top',    'E': 'center', 'W': 'center'}
      pivot  = {'N': 'mid',    'S': 'mid',    'E': 'tip',    'W': 'tail'}
  
      def __init__(self, Q, X, Y, U, label, **kw):
          martist.Artist.__init__(self)
          self.Q = Q
          self.X = X
          self.Y = Y
          self.U = U
          self.coord = kw.pop('coordinates', 'axes')
          self.color = kw.pop('color', None)
          self.label = label
          self._labelsep_inches = kw.pop('labelsep', 0.1)
          self.labelsep = (self._labelsep_inches * Q.ax.figure.dpi)
  
          def on_dpi_change(fig):
              self.labelsep = (self._labelsep_inches * fig.dpi)
              self._initialized = False # simple brute force update
                                        # works because _init is called
                                        # at the start of draw.
  
          Q.ax.figure.callbacks.connect('dpi_changed', on_dpi_change)
  
          self.labelpos = kw.pop('labelpos', 'N')
          self.labelcolor = kw.pop('labelcolor', None)
          self.fontproperties = kw.pop('fontproperties', dict())
          self.kw = kw
          _fp = self.fontproperties
          #boxprops = dict(facecolor='red')
          self.text = mtext.Text(text=label,  #      bbox=boxprops,
                         horizontalalignment=self.halign[self.labelpos],
                         verticalalignment=self.valign[self.labelpos],
                         fontproperties=font_manager.FontProperties(**_fp))
          if self.labelcolor is not None:
              self.text.set_color(self.labelcolor)
          self._initialized = False
          self.zorder = Q.zorder + 0.1
  
  
      __init__.__doc__ = _quiverkey_doc
  
      def _init(self):
          if True: ##not self._initialized:
              self._set_transform()
              _pivot = self.Q.pivot
              self.Q.pivot = self.pivot[self.labelpos]
              self.verts = self.Q._make_verts(np.array([self.U]),
                                                          np.zeros((1,)))
              self.Q.pivot = _pivot
              kw = self.Q.polykw
              kw.update(self.kw)
              self.vector = collections.PolyCollection(self.verts,
                                           offsets=[(self.X,self.Y)],
                                           transOffset=self.get_transform(),
                                           **kw)
              if self.color is not None:
                  self.vector.set_color(self.color)
              self.vector.set_transform(self.Q.get_transform())
              self._initialized = True
  
      def _text_x(self, x):
          if self.labelpos == 'E':
              return x + self.labelsep
          elif self.labelpos == 'W':
              return x - self.labelsep
          else:
              return x
  
      def _text_y(self, y):
          if self.labelpos == 'N':
              return y + self.labelsep
          elif self.labelpos == 'S':
              return y - self.labelsep
          else:
              return y
  
      def draw(self, renderer):
          self._init()
          self.vector.draw(renderer)
          x, y = self.get_transform().transform_point((self.X, self.Y))
          self.text.set_x(self._text_x(x))
          self.text.set_y(self._text_y(y))
          self.text.draw(renderer)
  
  
      def _set_transform(self):
          if self.coord == 'data':
              self.set_transform(self.Q.ax.transData)
          elif self.coord == 'axes':
              self.set_transform(self.Q.ax.transAxes)
          elif self.coord == 'figure':
              self.set_transform(self.Q.ax.figure.transFigure)
          elif self.coord == 'inches':
              self.set_transform(self.Q.ax.figure.dpi_scale_trans)
          else:
              raise ValueError('unrecognized coordinates')
  
      def set_figure(self, fig):
          martist.Artist.set_figure(self, fig)
          self.text.set_figure(fig)
  
      def contains(self, mouseevent):
          # Maybe the dictionary should allow one to
          # distinguish between a text hit and a vector hit.
          if (self.text.contains(mouseevent)[0]
                  or self.vector.contains(mouseevent)[0]):
              return True, {}
          return False, {}
  
      quiverkey_doc = _quiverkey_doc
  
  
  class Quiver(collections.PolyCollection):
      """
      Specialized PolyCollection for arrows.
  
      The only API method is set_UVC(), which can be used
      to change the size, orientation, and color of the
      arrows; their locations are fixed when the class is
      instantiated.  Possibly this method will be useful
      in animations.
  
      Much of the work in this class is done in the draw()
      method so that as much information as possible is available
      about the plot.  In subsequent draw() calls, recalculation
      is limited to things that might have changed, so there
      should be no performance penalty from putting the calculations
      in the draw() method.
      """
      def __init__(self, ax, *args, **kw):
          self.ax = ax
          X, Y, U, V, C = self._parse_args(*args)
          self.X = X
          self.Y = Y
          self.XY = np.hstack((X[:,np.newaxis], Y[:,np.newaxis]))
          self.N = len(X)
          self.scale = kw.pop('scale', None)
          self.headwidth = kw.pop('headwidth', 3)
          self.headlength = float(kw.pop('headlength', 5))
          self.headaxislength = kw.pop('headaxislength', 4.5)
          self.minshaft = kw.pop('minshaft', 1)
          self.minlength = kw.pop('minlength', 1)
          self.units = kw.pop('units', 'width')
          self.angles = kw.pop('angles', 'uv')
          self.width = kw.pop('width', None)
          self.color = kw.pop('color', 'k')
          self.pivot = kw.pop('pivot', 'tail')
          kw.setdefault('facecolors', self.color)
          kw.setdefault('linewidths', (0,))
          collections.PolyCollection.__init__(self, [], offsets=self.XY,
                                              transOffset=ax.transData,
                                              closed=False,
                                              **kw)
          self.polykw = kw
          self.set_UVC(U, V, C)
          self._initialized = False
  
          self.keyvec = None
          self.keytext = None
  
          def on_dpi_change(fig):
              self._new_UV = True # vertices depend on width, span
                                  # which in turn depend on dpi
              self._initialized = False # simple brute force update
                                        # works because _init is called
                                        # at the start of draw.
  
          self.ax.figure.callbacks.connect('dpi_changed', on_dpi_change)
  
  
      __init__.__doc__ = """
          The constructor takes one required argument, an Axes
          instance, followed by the args and kwargs described
          by the following pylab interface documentation:
          %s""" % _quiver_doc
  
      def _parse_args(self, *args):
          X, Y, U, V, C = [None]*5
          args = list(args)
          if len(args) == 3 or len(args) == 5:
              C = ma.asarray(args.pop(-1)).ravel()
          V = ma.asarray(args.pop(-1))
          U = ma.asarray(args.pop(-1))
          nn = np.shape(U)
          nc = nn[0]
          nr = 1
          if len(nn) > 1:
              nr = nn[1]
          if len(args) == 2: # remaining after removing U,V,C
              X, Y = [np.array(a).ravel() for a in args]
              if len(X) == nc and len(Y) == nr:
                  X, Y = [a.ravel() for a in np.meshgrid(X, Y)]
          else:
              indexgrid = np.meshgrid(np.arange(nc), np.arange(nr))
              X, Y = [np.ravel(a) for a in indexgrid]
          return X, Y, U, V, C
  
      def _init(self):
          """initialization delayed until first draw;
          allow time for axes setup.
          """
          # It seems that there are not enough event notifications
          # available to have this work on an as-needed basis at present.
          if True: ##not self._initialized:
              trans = self._set_transform()
              ax = self.ax
              sx, sy = trans.inverted().transform_point(
                                              (ax.bbox.width, ax.bbox.height))
              self.span = sx
              sn = max(8, min(25, math.sqrt(self.N)))
              if self.width is None:
                  self.width = 0.06 * self.span / sn
  
      def draw(self, renderer):
          self._init()
          if self._new_UV or self.angles == 'xy':
              verts = self._make_verts(self.U, self.V)
              self.set_verts(verts, closed=False)
              self._new_UV = False
          collections.PolyCollection.draw(self, renderer)
  
      def set_UVC(self, U, V, C=None):
          self.U = U.ravel()
          self.V = V.ravel()
          if C is not None:
              self.set_array(C.ravel())
          self._new_UV = True
  
      def _set_transform(self):
          ax = self.ax
          if self.units in ('x', 'y'):
              if self.units == 'x':
                  dx0 = ax.viewLim.width
                  dx1 = ax.bbox.width
              else:
                  dx0 = ax.viewLim.height
                  dx1 = ax.bbox.height
              dx = dx1/dx0
          else:
              if self.units == 'width':
                  dx = ax.bbox.width
              elif self.units == 'height':
                  dx = ax.bbox.height
              elif self.units == 'dots':
                  dx = 1.0
              elif self.units == 'inches':
                  dx = ax.figure.dpi
              else:
                  raise ValueError('unrecognized units')
          trans = transforms.Affine2D().scale(dx)
          self.set_transform(trans)
          return trans
  
      def _angles(self, U, V, eps=0.001):
          xy = self.ax.transData.transform(self.XY)
          uv = ma.hstack((U[:,np.newaxis], V[:,np.newaxis])).filled(0)
          xyp = self.ax.transData.transform(self.XY + eps * uv)
          dxy = xyp - xy
          ang = ma.arctan2(dxy[:,1], dxy[:,0])
          return ang
  
      def _make_verts(self, U, V):
          uv = ma.asarray(U+V*1j)
          a = ma.absolute(uv)
          if self.scale is None:
              sn = max(10, math.sqrt(self.N))
              scale = 1.8 * a.mean() * sn / self.span # crude auto-scaling
              self.scale = scale
          length = a/(self.scale*self.width)
          X, Y = self._h_arrows(length)
          if self.angles == 'xy':
              theta = self._angles(U, V).filled(0)[:,np.newaxis]
          elif self.angles == 'uv':
              theta = np.angle(ma.asarray(uv[..., np.newaxis]).filled(0))
          else:
              theta = ma.asarray(self.angles*np.pi/180.0).filled(0)
          xy = (X+Y*1j) * np.exp(1j*theta)*self.width
          xy = xy[:,:,np.newaxis]
          XY = ma.concatenate((xy.real, xy.imag), axis=2)
          return XY
  
  
      def _h_arrows(self, length):
          """ length is in arrow width units """
          # It might be possible to streamline the code
          # and speed it up a bit by using complex (x,y)
          # instead of separate arrays; but any gain would be slight.
          minsh = self.minshaft * self.headlength
          N = len(length)
          length = length.reshape(N, 1)
          # x, y: normal horizontal arrow
          x = np.array([0, -self.headaxislength,
                          -self.headlength, 0], np.float64)
          x = x + np.array([0,1,1,1]) * length
          y = 0.5 * np.array([1, 1, self.headwidth, 0], np.float64)
          y = np.repeat(y[np.newaxis,:], N, axis=0)
          # x0, y0: arrow without shaft, for short vectors
          x0 = np.array([0, minsh-self.headaxislength,
                          minsh-self.headlength, minsh], np.float64)
          y0 = 0.5 * np.array([1, 1, self.headwidth, 0], np.float64)
          ii = [0,1,2,3,2,1,0]
          X = x.take(ii, 1)
          Y = y.take(ii, 1)
          Y[:, 3:] *= -1
          X0 = x0.take(ii)
          Y0 = y0.take(ii)
          Y0[3:] *= -1
          shrink = length/minsh
          X0 = shrink * X0[np.newaxis,:]
          Y0 = shrink * Y0[np.newaxis,:]
          short = np.repeat(length < minsh, 7, axis=1)
          #print 'short', length < minsh
          # Now select X0, Y0 if short, otherwise X, Y
          X = ma.where(short, X0, X)
          Y = ma.where(short, Y0, Y)
          if self.pivot[:3] == 'mid':
              X -= 0.5 * X[:,3, np.newaxis]
          elif self.pivot[:3] == 'tip':
              X = X - X[:,3, np.newaxis]   #numpy bug? using -= does not
                                           # work here unless we multiply
                                           # by a float first, as with 'mid'.
          tooshort = length < self.minlength
          if tooshort.any():
              # Use a heptagonal dot:
              th = np.arange(0,7,1, np.float64) * (np.pi/3.0)
              x1 = np.cos(th) * self.minlength * 0.5
              y1 = np.sin(th) * self.minlength * 0.5
              X1 = np.repeat(x1[np.newaxis, :], N, axis=0)
              Y1 = np.repeat(y1[np.newaxis, :], N, axis=0)
              tooshort = ma.repeat(tooshort, 7, 1)
              X = ma.where(tooshort, X1, X)
              Y = ma.where(tooshort, Y1, Y)
          return X, Y
  
      quiver_doc = _quiver_doc
  
  _barbs_doc = """
  Plot a 2-D field of barbs.
  
  call signatures::
  
    barb(U, V, **kw)
    barb(U, V, C, **kw)
    barb(X, Y, U, V, **kw)
    barb(X, Y, U, V, C, **kw)
  
  Arguments:
  
    *X*, *Y*:
      The x and y coordinates of the barb locations
      (default is head of barb; see *pivot* kwarg)
  
    *U*, *V*:
      give the *x* and *y* components of the barb shaft
  
    *C*:
      an optional array used to map colors to the barbs
  
  All arguments may be 1-D or 2-D arrays or sequences. If *X* and *Y*
  are absent, they will be generated as a uniform grid.  If *U* and *V*
  are 2-D arrays but *X* and *Y* are 1-D, and if len(*X*) and len(*Y*)
  match the column and row dimensions of *U*, then *X* and *Y* will be
  expanded with :func:`numpy.meshgrid`.
  
  *U*, *V*, *C* may be masked arrays, but masked *X*, *Y* are not
  supported at present.
  
  Keyword arguments:
  
    *length*:
      Length of the barb in points; the other parts of the barb
      are scaled against this.
      Default is 9
  
    *pivot*: [ 'tip' | 'middle' ]
      The part of the arrow that is at the grid point; the arrow rotates
      about this point, hence the name *pivot*.  Default is 'tip'
  
    *barbcolor*: [ color | color sequence ]
      Specifies the color all parts of the barb except any flags.  This
      parameter is analagous to the *edgecolor* parameter for polygons,
      which can be used instead. However this parameter will override
      facecolor.
  
    *flagcolor*: [ color | color sequence ]
      Specifies the color of any flags on the barb.  This parameter is
      analagous to the *facecolor* parameter for polygons, which can be
      used instead. However this parameter will override facecolor.  If
      this is not set (and *C* has not either) then *flagcolor* will be
      set to match *barbcolor* so that the barb has a uniform color. If
      *C* has been set, *flagcolor* has no effect.
  
    *sizes*:
      A dictionary of coefficients specifying the ratio of a given
      feature to the length of the barb. Only those values one wishes to
      override need to be included.  These features include:
  
          - 'spacing' - space between features (flags, full/half barbs)
  
          - 'height' - height (distance from shaft to top) of a flag or
            full barb
  
          - 'width' - width of a flag, twice the width of a full barb
  
          - 'emptybarb' - radius of the circle used for low magnitudes
  
    *fill_empty*:
      A flag on whether the empty barbs (circles) that are drawn should
      be filled with the flag color.  If they are not filled, they will
      be drawn such that no color is applied to the center.  Default is
      False
  
    *rounding*:
      A flag to indicate whether the vector magnitude should be rounded
      when allocating barb components.  If True, the magnitude is
      rounded to the nearest multiple of the half-barb increment.  If
      False, the magnitude is simply truncated to the next lowest
      multiple.  Default is True
  
    *barb_increments*:
      A dictionary of increments specifying values to associate with
      different parts of the barb. Only those values one wishes to
      override need to be included.
  
          - 'half' - half barbs (Default is 5)
  
          - 'full' - full barbs (Default is 10)
  
          - 'flag' - flags (default is 50)
  
    *flip_barb*:
      Either a single boolean flag or an array of booleans.  Single
      boolean indicates whether the lines and flags should point
      opposite to normal for all barbs.  An array (which should be the
      same size as the other data arrays) indicates whether to flip for
      each individual barb.  Normal behavior is for the barbs and lines
      to point right (comes from wind barbs having these features point
      towards low pressure in the Northern Hemisphere.)  Default is
      False
  
  Barbs are traditionally used in meteorology as a way to plot the speed
  and direction of wind observations, but can technically be used to
  plot any two dimensional vector quantity.  As opposed to arrows, which
  give vector magnitude by the length of the arrow, the barbs give more
  quantitative information about the vector magnitude by putting slanted
  lines or a triangle for various increments in magnitude, as show
  schematically below::
  
   :     /\    \\
   :    /  \    \\
   :   /    \    \    \\
   :  /      \    \    \\
   : ------------------------------
  
  .. note the double \\ at the end of each line to make the figure
  .. render correctly
  
  The largest increment is given by a triangle (or "flag"). After those
  come full lines (barbs). The smallest increment is a half line.  There
  is only, of course, ever at most 1 half line.  If the magnitude is
  small and only needs a single half-line and no full lines or
  triangles, the half-line is offset from the end of the barb so that it
  can be easily distinguished from barbs with a single full line.  The
  magnitude for the barb shown above would nominally be 65, using the
  standard increments of 50, 10, and 5.
  
  linewidths and edgecolors can be used to customize the barb.
  Additional :class:`~matplotlib.collections.PolyCollection` keyword
  arguments:
  
  %(PolyCollection)s
  """ % martist.kwdocd
  
  class Barbs(collections.PolyCollection):
      '''
      Specialized PolyCollection for barbs.
  
      The only API method is :meth:`set_UVC`, which can be used to
      change the size, orientation, and color of the arrows.  Locations
      are changed using the :meth:`set_offsets` collection method.
      Possibly this method will be useful in animations.
  
      There is one internal function :meth:`_find_tails` which finds
      exactly what should be put on the barb given the vector magnitude.
      From there :meth:`_make_barbs` is used to find the vertices of the
      polygon to represent the barb based on this information.
      '''
      #This may be an abuse of polygons here to render what is essentially maybe
      #1 triangle and a series of lines.  It works fine as far as I can tell
      #however.
      def __init__(self, ax, *args, **kw):
          self._pivot = kw.pop('pivot', 'tip')
          self._length = kw.pop('length', 7)
          barbcolor = kw.pop('barbcolor', None)
          flagcolor = kw.pop('flagcolor', None)
          self.sizes = kw.pop('sizes', dict())
          self.fill_empty = kw.pop('fill_empty', False)
          self.barb_increments = kw.pop('barb_increments', dict())
          self.rounding = kw.pop('rounding', True)
          self.flip = kw.pop('flip_barb', False)
  
          #Flagcolor and and barbcolor provide convenience parameters for setting
          #the facecolor and edgecolor, respectively, of the barb polygon.  We
          #also work here to make the flag the same color as the rest of the barb
          #by default
          if None in (barbcolor, flagcolor):
              kw['edgecolors'] = 'face'
              if flagcolor:
                  kw['facecolors'] = flagcolor
              elif barbcolor:
                  kw['facecolors'] = barbcolor
              else:
                  #Set to facecolor passed in or default to black
                  kw.setdefault('facecolors', 'k')
          else:
              kw['edgecolors'] = barbcolor
              kw['facecolors'] = flagcolor
  
          #Parse out the data arrays from the various configurations supported
          x, y, u, v, c = self._parse_args(*args)
          self.x = x
          self.y = y
          xy = np.hstack((x[:,np.newaxis], y[:,np.newaxis]))
  
          #Make a collection
          barb_size = self._length**2 / 4 #Empirically determined
          collections.PolyCollection.__init__(self, [], (barb_size,), offsets=xy,
              transOffset=ax.transData, **kw)
          self.set_transform(transforms.IdentityTransform())
  
          self.set_UVC(u, v, c)
  
      __init__.__doc__ = """
          The constructor takes one required argument, an Axes
          instance, followed by the args and kwargs described
          by the following pylab interface documentation:
          %s""" % _barbs_doc
  
      def _find_tails(self, mag, rounding=True, half=5, full=10, flag=50):
          '''
          Find how many of each of the tail pieces is necessary.  Flag
          specifies the increment for a flag, barb for a full barb, and half for
          half a barb. Mag should be the magnitude of a vector (ie. >= 0).
  
          This returns a tuple of:
  
              (*number of flags*, *number of barbs*, *half_flag*, *empty_flag*)
  
          *half_flag* is a boolean whether half of a barb is needed,
          since there should only ever be one half on a given
          barb. *empty_flag* flag is an array of flags to easily tell if
          a barb is empty (too low to plot any barbs/flags.
          '''
  
          #If rounding, round to the nearest multiple of half, the smallest
          #increment
          if rounding:
              mag = half * (mag / half + 0.5).astype(np.int)
  
          num_flags = np.floor(mag / flag).astype(np.int)
          mag = np.mod(mag, flag)
  
          num_barb = np.floor(mag / full).astype(np.int)
          mag = np.mod(mag, full)
  
          half_flag = mag >= half
          empty_flag = ~(half_flag | (num_flags > 0) | (num_barb > 0))
  
          return num_flags, num_barb, half_flag, empty_flag
  
      def _make_barbs(self, u, v, nflags, nbarbs, half_barb, empty_flag, length,
          pivot, sizes, fill_empty, flip):
          '''
          This function actually creates the wind barbs.  *u* and *v*
          are components of the vector in the *x* and *y* directions,
          respectively.
  
          *nflags*, *nbarbs*, and *half_barb*, empty_flag* are,
          *respectively, the number of flags, number of barbs, flag for
          *half a barb, and flag for empty barb, ostensibly obtained
          *from :meth:`_find_tails`.
  
          *length* is the length of the barb staff in points.
  
          *pivot* specifies the point on the barb around which the
          entire barb should be rotated.  Right now, valid options are
          'head' and 'middle'.
  
          *sizes* is a dictionary of coefficients specifying the ratio
          of a given feature to the length of the barb. These features
          include:
  
              - *spacing*: space between features (flags, full/half
                 barbs)
  
              - *height*: distance from shaft of top of a flag or full
                 barb
  
              - *width* - width of a flag, twice the width of a full barb
  
              - *emptybarb* - radius of the circle used for low
                 magnitudes
  
          *fill_empty* specifies whether the circle representing an
          empty barb should be filled or not (this changes the drawing
          of the polygon).
  
          *flip* is a flag indicating whether the features should be flipped to
          the other side of the barb (useful for winds in the southern
          hemisphere.
  
          This function returns list of arrays of vertices, defining a polygon for
          each of the wind barbs.  These polygons have been rotated to properly
          align with the vector direction.
          '''
  
          #These control the spacing and size of barb elements relative to the
          #length of the shaft
          spacing = length * sizes.get('spacing', 0.125)
          full_height = length * sizes.get('height', 0.4)
          full_width = length * sizes.get('width', 0.25)
          empty_rad = length * sizes.get('emptybarb', 0.15)
  
          #Controls y point where to pivot the barb.
          pivot_points = dict(tip=0.0, middle=-length/2.)
  
          #Check for flip
          if flip: full_height = -full_height
  
          endx = 0.0
          endy = pivot_points[pivot.lower()]
  
          #Get the appropriate angle for the vector components.  The offset is due
          #to the way the barb is initially drawn, going down the y-axis.  This
          #makes sense in a meteorological mode of thinking since there 0 degrees
          #corresponds to north (the y-axis traditionally)
          angles = -(ma.arctan2(v, u) + np.pi/2)
  
          #Used for low magnitude.  We just get the vertices, so if we make it
          #out here, it can be reused.  The center set here should put the
          #center of the circle at the location(offset), rather than at the
          #same point as the barb pivot; this seems more sensible.
          circ = CirclePolygon((0,0), radius=empty_rad).get_verts()
          if fill_empty:
              empty_barb = circ
          else:
              #If we don't want the empty one filled, we make a degenerate polygon
              #that wraps back over itself
              empty_barb = np.concatenate((circ, circ[::-1]))
  
          barb_list = []
          for index, angle in np.ndenumerate(angles):
              #If the vector magnitude is too weak to draw anything, plot an
              #empty circle instead
              if empty_flag[index]:
                  #We can skip the transform since the circle has no preferred
                  #orientation
                  barb_list.append(empty_barb)
                  continue
  
              poly_verts = [(endx, endy)]
              offset = length
  
              #Add vertices for each flag
              for i in range(nflags[index]):
                  #The spacing that works for the barbs is a little to much for
                  #the flags, but this only occurs when we have more than 1 flag.
                  if offset != length: offset += spacing / 2.
                  poly_verts.extend([[endx, endy + offset],
                      [endx + full_height, endy - full_width/2 + offset],
                      [endx, endy - full_width + offset]])
  
                  offset -= full_width + spacing
  
              #Add vertices for each barb.  These really are lines, but works
              #great adding 3 vertices that basically pull the polygon out and
              #back down the line
              for i in range(nbarbs[index]):
                  poly_verts.extend([(endx, endy + offset),
                      (endx + full_height, endy + offset + full_width/2),
                      (endx, endy + offset)])
  
                  offset -= spacing
  
              #Add the vertices for half a barb, if needed
              if half_barb[index]:
                  #If the half barb is the first on the staff, traditionally it is
                  #offset from the end to make it easy to distinguish from a barb
                  #with a full one
                  if offset == length:
                      poly_verts.append((endx, endy + offset))
                      offset -= 1.5 * spacing
                  poly_verts.extend([(endx, endy + offset),
                      (endx + full_height/2, endy + offset + full_width/4),
                      (endx, endy + offset)])
  
              #Rotate the barb according the angle. Making the barb first and then
              #rotating it made the math for drawing the barb really easy.  Also,
              #the transform framework makes doing the rotation simple.
              poly_verts = transforms.Affine2D().rotate(-angle).transform(
                  poly_verts)
              barb_list.append(poly_verts)
  
          return barb_list
  
      #Taken shamelessly from Quiver
      def _parse_args(self, *args):
          X, Y, U, V, C = [None]*5
          args = list(args)
          if len(args) == 3 or len(args) == 5:
              C = ma.asarray(args.pop(-1)).ravel()
          V = ma.asarray(args.pop(-1))
          U = ma.asarray(args.pop(-1))
          nn = np.shape(U)
          nc = nn[0]
          nr = 1
          if len(nn) > 1:
              nr = nn[1]
          if len(args) == 2: # remaining after removing U,V,C
              X, Y = [np.array(a).ravel() for a in args]
              if len(X) == nc and len(Y) == nr:
                  X, Y = [a.ravel() for a in np.meshgrid(X, Y)]
          else:
              indexgrid = np.meshgrid(np.arange(nc), np.arange(nr))
              X, Y = [np.ravel(a) for a in indexgrid]
          return X, Y, U, V, C
  
      def set_UVC(self, U, V, C=None):
          self.u = ma.asarray(U).ravel()
          self.v = ma.asarray(V).ravel()
          if C is not None:
              c = ma.asarray(C).ravel()
              x,y,u,v,c = delete_masked_points(self.x.ravel(), self.y.ravel(),
                  self.u, self.v, c)
          else:
              x,y,u,v = delete_masked_points(self.x.ravel(), self.y.ravel(),
                  self.u, self.v)
  
          magnitude = np.sqrt(u*u + v*v)
          flags, barbs, halves, empty = self._find_tails(magnitude,
              self.rounding, **self.barb_increments)
  
          #Get the vertices for each of the barbs
  
          plot_barbs = self._make_barbs(u, v, flags, barbs, halves, empty,
              self._length, self._pivot, self.sizes, self.fill_empty, self.flip)
          self.set_verts(plot_barbs)
  
          #Set the color array
          if C is not None:
              self.set_array(c)
  
          #Update the offsets in case the masked data changed
          xy = np.hstack((x[:,np.newaxis], y[:,np.newaxis]))
          self._offsets = xy
  
      def set_offsets(self, xy):
          '''
          Set the offsets for the barb polygons.  This saves the offets passed in
          and actually sets version masked as appropriate for the existing U/V
          data. *offsets* should be a sequence.
  
          ACCEPTS: sequence of pairs of floats
          '''
          self.x = xy[:,0]
          self.y = xy[:,1]
          x,y,u,v = delete_masked_points(self.x.ravel(), self.y.ravel(), self.u,
              self.v)
          xy = np.hstack((x[:,np.newaxis], y[:,np.newaxis]))
          collections.PolyCollection.set_offsets(self, xy)
      set_offsets.__doc__ = collections.PolyCollection.set_offsets.__doc__
  
      barbs_doc = _barbs_doc
  