Re: [matplotlib-devel] Re: collection initializer color handling vs quiver

[Jordan D. <jdawe@u.washington.edu>]

> from collections import LineCollection
> class Arrow(LineCollection):
>     """
>     An arrow
>     """
>     def __init__( self, x, y, dx, dy, width=1.0, arrowstyle='solid', 
> **kwargs ):
>         """Draws an arrow, starting at (x,y), direction and length
>         given by (dx,dy) the width of the arrow is scaled by width.
>         arrowstyle may be 'solid' or 'barbed'
>         """
>         L = math.hypot(dx,dy) or 1 # account for div by zero
>         S = 0.1
>         arrow = {'barbed': array([[0.,0.], [L,0.], [L-S,S/3],
>                                   [L,0.], [L,-S/3], [L,0.]]),
>                  'solid': array([[0.,0.], [L-S,0.], [L-S,S/3],
>                                   [L,0.], [L-S,-S/3], [L-S,0.]])
>                 }[arrowstyle]
>
>         cx = float(dx)/L
>         sx = float(dy)/L
>         M = array([[cx, sx], [-sx, cx]])
>         verts = matrixmultiply(arrow, M) + [x,y]
>         LineCollection.__init__(self, [tuple(t) for t in verts], 
> **kwargs)
I've found one problem with your Arrow LineCollection; it's not actually 
a line collection.  It's one line, so some of the LineCollection 
functions fail on it.  You need to break up the arrow into segments, 
like this:

'barbed': array([ [ [0.,0.], [L,0.] ],
                          [ [L,0.], [L-S,S/3] ],
                          [ [L,0.], [L-S,-S/3] ] ]

Except just doing this will break the matrixmultiply.  Just a heads-up.

Jordan