matplotlib
diff --git a/‎04-spline_demo.py‎
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+import scipy.interpolate as si
+import numpy as np
+from functools import reduce
+
+# uncomment this to set the backend
+# import matplotlib
+# matplotlib.use('Qt4Agg')
+import matplotlib.pyplot as plt
+
+
+class TooFewPointsException(Exception):
+    ...
+
+
+class SplineFitter:
+    def __init__(self, ax, pix_err=1):
+        self.canvas = ax.get_figure().canvas
+        self.cid = None
+        self.pt_lst = []
+        self.pt_plot = ax.plot([], [], marker='o',
+                               linestyle='none', zorder=5)[0]
+        self.sp_plot = ax.plot([], [], lw=3, color='r')[0]
+        self.pix_err = pix_err
+        self.connect_sf()
+
+    def clear(self):
+        '''Clears the points'''
+        self.pt_lst = []
+        self.redraw()
+
+    def connect_sf(self):
+        if self.cid is None:
+            self.cid = self.canvas.mpl_connect('button_press_event',
+                                               self.click_event)
+
+    def disconnect_sf(self):
+        if self.cid is not None:
+            self.canvas.mpl_disconnect(self.cid)
+            self.cid = None
+
+    def click_event(self, event):
+        ''' Extracts locations from the user'''
+        if event.key == 'shift':
+            self.clear()
+            return
+        if event.xdata is None or event.ydata is None:
+            return
+        if event.button == 1:
+            self.pt_lst.append((event.xdata, event.ydata))
+        elif event.button == 3:
+            self.remove_pt((event.xdata, event.ydata))
+        self.ev = event
+        self.redraw()
+
+    def remove_pt(self, loc):
+        if len(self.pt_lst) > 0:
+            self.pt_lst.pop(np.argmin(list(map(lambda x:
+                                               np.sqrt((x[0] - loc[0]) ** 2 +
+                                                       (x[1] - loc[1]) ** 2),
+                                               self.pt_lst))))
+
+    def redraw(self):
+        if len(self.pt_lst) > 5:
+            SC = SplineCurve.from_pts(self.pt_lst, pix_err=self.pix_err)
+            new_pts = SC.q_phi_to_xy(0, np.linspace(0, 2 * np.pi, 1000))
+            center = SC.cntr
+            self.sp_plot.set_xdata(new_pts[0])
+            self.sp_plot.set_ydata(new_pts[1])
+            self.pt_lst.sort(key=lambda x:
+                             np.arctan2(x[1] - center[1], x[0] - center[0]))
+        else:
+            self.sp_plot.set_xdata([])
+            self.sp_plot.set_ydata([])
+        if len(self.pt_lst) > 0:
+            x, y = zip(*self.pt_lst)
+        else:
+            x, y = [], []
+        self.pt_plot.set_xdata(x)
+        self.pt_plot.set_ydata(y)
+
+        self.canvas.draw_idle()
+
+    @property
+    def points(self):
+        '''Returns the clicked points in the format the rest of the
+        code expects'''
+        return np.vstack(self.pt_lst).T
+
+    @property
+    def SplineCurve(self):
+        curve = SplineCurve.from_pts(self.pt_lst, pix_err=self.pix_err)
+        return curve
+
+
+class SplineCurve:
+    '''
+    A class that wraps the scipy.interpolation objects
+    '''
+    @classmethod
+    def _get_spline(cls, points, pix_err=2, need_sort=True, **kwargs):
+        '''
+        Returns a closed spline for the points handed in.
+        Input is assumed to be a (2xN) array
+
+        =====
+        input
+        =====
+
+        :param points: the points to fit the spline to
+        :type points: a 2xN ndarray or a list of len =2 tuples
+
+        :param pix_err: the error is finding the spline in pixels
+        :param need_sort: if the points need to be sorted
+            or should be processed as-is
+
+        =====
+        output
+        =====
+        tck
+           The return data from the spline fitting
+        '''
+        if type(points) is np.ndarray:
+            # make into a list
+            pt_lst = zip(*points)
+            # get center
+            center = np.mean(points, axis=1).reshape(2, 1)
+        else:
+            # make a copy of the list
+            pt_lst = list(points)
+
+            # compute center
+            def tmp_fun(x, y): (x[0] + y[0], x[1] + y[1])
+
+            center = np.array(reduce(tmp_fun, pt_lst)).reshape(2, 1)
+            center /= len(pt_lst)
+        if len(pt_lst) < 5:
+            raise TooFewPointsException("not enough points")
+
+        if need_sort:
+            # sort the list by angle around center
+            pt_lst.sort(key=lambda x: np.arctan2(x[1] - center[1],
+                                                 x[0] - center[0]))
+        # add first point to end because it is periodic (makes the
+        # interpolation code happy)
+        pt_lst.append(pt_lst[0])
+        # make array for handing in to spline fitting
+        pt_array = np.vstack(pt_lst).T
+        # do spline fitting
+
+        tck, u = si.splprep(pt_array, s=len(pt_lst) * (pix_err ** 2), per=True)
+        return tck
+
+    @classmethod
+    def from_pts(cls, new_pts, **kwargs):
+        tck = cls._get_spline(new_pts, **kwargs)
+        this = cls(tck)
+        this.raw_pts = new_pts
+        return this
+
+    def __init__(self, tck):
+        '''Use `from_pts` class method to construct instance
+        '''
+        self.tck = tck
+        self._cntr = None
+        self._circ = None
+        self._th_offset = None
+
+    def write_to_hdf(self, parent_group, name=None):
+        '''
+        Writes out the essential data (spline of central curve) to hdf file.
+        '''
+        if name is not None:
+            curve_group = parent_group.create_group(name)
+        else:
+            curve_group = parent_group
+        curve_group.attrs['tck0'] = self.tck[0]
+        curve_group.attrs['tck1'] = np.vstack(self.tck[1])
+        curve_group.attrs['tck2'] = self.tck[2]
+
+    @property
+    def circ(self):
+        '''returns a rough estimate of the circumference'''
+        if self._circ is None:
+            new_pts = si.splev(np.linspace(0, 1, 1000), self.tck, ext=2)
+            self._circ = np.sum(np.sqrt(np.sum(np.diff(new_pts, axis=1) ** 2,
+                                               axis=0)))
+        return self._circ
+
+    @property
+    def cntr(self):
+        '''returns a rough estimate of the circumference'''
+        if self._cntr is None:
+            new_pts = si.splev(np.linspace(0, 1, 1000), self.tck, ext=2)
+            self._cntr = np.mean(new_pts, 1)
+        return self._cntr
+
+    @property
+    def th_offset(self):
+        """
+        The angle from the y-axis for (x, y) at `phi=0`
+        """
+        if self._th_offset is None:
+            x, y = self.q_phi_to_xy(0, 0) - self.cntr.reshape(2, 1)
+            self._th_offset = np.arctan2(y, x)
+        return self._th_offset
+
+    @property
+    def tck0(self):
+        return self.tck[0]
+
+    @property
+    def tck1(self):
+        return self.tck[1]
+
+    @property
+    def tck2(self):
+        return self.tck[2]
+
+    def q_phi_to_xy(self, q, phi, cross=None):
+        '''Converts q, phi pairs -> x, y pairs.  All other code that
+        does this should move to using this so that there is minimal
+        breakage when we change over to using additive q instead of
+        multiplicative'''
+        # make sure data is arrays
+        q = np.asarray(q)
+        # convert real units -> interpolation units
+        phi = np.mod(np.asarray(phi), 2 * np.pi) / (2 * np.pi)
+        # get the shapes
+        q_shape, phi_shape = [_.shape if (_.shape != () and
+                                          len(_) > 1) else None for
+                              _ in (q, phi)]
+
+        # flatten everything
+        q = q.ravel()
+        phi = phi.ravel()
+        # sanity checks on shapes
+        if cross is False:
+            if phi_shape != q_shape:
+                raise ValueError("q and phi must have same" +
+                                 " dimensions to broadcast")
+        if cross is None:
+            if ((phi_shape is not None) and (q_shape is not None)
+                    and (phi_shape == q_shape)):
+                cross = False
+            elif q_shape is None:
+                cross = False
+                q = q[0]
+            else:
+                cross = True
+
+        x, y = si.splev(phi, self.tck, ext=2)
+        dx, dy = si.splev(phi, self.tck, der=1, ext=2)
+        norm = np.sqrt(dx ** 2 + dy ** 2)
+        nx, ny = dy / norm, -dx / norm
+
+        # if cross, then
+        if cross:
+            data_out = zip(
+                *map(lambda q_: ((x + q_ * nx).reshape(phi_shape),
+                                 (y + q_ * ny).reshape(phi_shape)),
+                     q)
+            )
+        else:
+
+            data_out = np.vstack([(x + q * nx).reshape(phi_shape),
+                                  (y + q * ny).reshape(phi_shape)])
+
+        return data_out
+
+
+fig, ax = plt.subplots()
+sp = SplineFitter(ax, .001)
+plt.ion()
+plt.show()