Matplotlib drag overlapping points interactively

Question

In my case, I only want to drag one point each time. However, since the two points are heavily overlapping, dragging one point would cause another point to be dragged. How can I only drag the point that is on the above? Thank you!

from pylab import *
from scipy import *
import matplotlib.pyplot as plt
import matplotlib.patches as patches


class DraggablePoint:
    def __init__(self, p):
        self.point = p
        self.press = None

    def connect(self):
        self.cidpress = self.point.figure.canvas.mpl_connect('button_press_event', self.button_press_event)
        self.cidrelease = self.point.figure.canvas.mpl_connect('button_release_event', self.button_release_event)
        self.cidmotion = self.point.figure.canvas.mpl_connect('motion_notify_event', self.motion_notify_event)

    def disconnect(self):
    'disconnect all the stored connection ids'
        self.point.figure.canvas.mpl_disconnect(self.cidpress)
        self.point.figure.canvas.mpl_disconnect(self.cidrelease)
        self.point.figure.canvas.mpl_disconnect(self.cidmotion)


    def button_press_event(self,event):
        if event.inaxes != self.point.axes:
            return
        contains = self.point.contains(event)[0]
        if not contains: return
        self.press = self.point.center, event.xdata, event.ydata


    def button_release_event(self,event):
        self.press = None
        self.point.figure.canvas.draw()

    def motion_notify_event(self, event):
        if self.press is None: return
        if event.inaxes != self.point.axes: return
        self.point.center, xpress, ypress = self.press
        dx = event.xdata - xpress
        dy = event.ydata - ypress 
        self.point.center = (self.point.center[0]+dx, self.point.center[1]+dy)
        print self.point.center
        self.point.figure.canvas.draw()

if __name__ == '__main__':
    fig = plt.figure()
    ax = fig.add_subplot(111)
    ax.set_xlim(-1,2)
    ax.set_ylim(-1,2)
    circles = []
    circle1 = patches.Circle((0.32,0.3), 0.2, fc='r', alpha=0.5, picker=True)
    circle = patches.Circle((0.3,0.3), 0.2, fc='b', alpha=0.5, picker=True)
    circles.append(ax.add_patch(circle1))
    circles.append(ax.add_patch(circle))
    drs = []
    for c in circles:
        #print c.center[0]    
        dr = DraggablePoint(c)
        dr.connect()   
        drs.append(dr) 
    plt.show()

Answer 1

Joe's method works fine, but it makes a set of draggablepoints as a class instead of a single draggablepoint class. I just came across an alternative method to solve the above problem using animation blit techniques. It not only makes the dragging faster and smoother, but also only one point can be dragged. See the following code.

import matplotlib.pyplot as plt
import matplotlib.patches as patches
class DraggablePoint:
    lock = None #only one can be animated at a time
    def __init__(self, point):
        self.point = point
        self.press = None
        self.background = None

    def connect(self):
        'connect to all the events we need'
        self.cidpress = self.point.figure.canvas.mpl_connect('button_press_event', self.on_press)
        self.cidrelease = self.point.figure.canvas.mpl_connect('button_release_event', self.on_release)
        self.cidmotion = self.point.figure.canvas.mpl_connect('motion_notify_event', self.on_motion)

    def on_press(self, event):
        if event.inaxes != self.point.axes: return
        if DraggablePoint.lock is not None: return
        contains, attrd = self.point.contains(event)
        if not contains: return
        self.press = (self.point.center), event.xdata, event.ydata
        DraggablePoint.lock = self

        # draw everything but the selected rectangle and store the pixel buffer
        canvas = self.point.figure.canvas
        axes = self.point.axes
        self.point.set_animated(True)
        canvas.draw()
        self.background = canvas.copy_from_bbox(self.point.axes.bbox)

        # now redraw just the rectangle
        axes.draw_artist(self.point)

        # and blit just the redrawn area
        canvas.blit(axes.bbox)

    def on_motion(self, event):
        if DraggablePoint.lock is not self:
            return
        if event.inaxes != self.point.axes: return
        self.point.center, xpress, ypress = self.press
        dx = event.xdata - xpress
        dy = event.ydata - ypress
        self.point.center = (self.point.center[0]+dx, self.point.center[1]+dy)

        canvas = self.point.figure.canvas
        axes = self.point.axes
        # restore the background region
        canvas.restore_region(self.background)

        # redraw just the current rectangle
        axes.draw_artist(self.point)

        # blit just the redrawn area
        canvas.blit(axes.bbox)

    def on_release(self, event):
        'on release we reset the press data'
        if DraggablePoint.lock is not self:
            return

        self.press = None
        DraggablePoint.lock = None

        # turn off the rect animation property and reset the background
        self.point.set_animated(False)
        self.background = None

        # redraw the full figure
        self.point.figure.canvas.draw()

    def disconnect(self):
        'disconnect all the stored connection ids'
        self.point.figure.canvas.mpl_disconnect(self.cidpress)
        self.point.figure.canvas.mpl_disconnect(self.cidrelease)
        self.point.figure.canvas.mpl_disconnect(self.cidmotion)

fig = plt.figure()
ax = fig.add_subplot(111)
drs = []
circles = [patches.Circle((0.32, 0.3), 0.03, fc='r', alpha=0.5),
               patches.Circle((0.3,0.3), 0.03, fc='g', alpha=0.5)]

for circ in circles:
    ax.add_patch(circ)
    dr = DraggablePoint(circ)
    dr.connect()
    drs.append(dr)

plt.show()

Answer 2

I would synchronize which artist is currently active through a single class that works with multiple artists.

It's easiest to use the pick_event for this. It also makes it easier to generalize to other artists. As an example:

import matplotlib.pyplot as plt
import matplotlib.patches as patches

class DraggablePoints(object):
    def __init__(self, artists, tolerance=5):
        for artist in artists:
            artist.set_picker(tolerance)
        self.artists = artists
        self.currently_dragging = False
        self.current_artist = None
        self.offset = (0, 0)

        for canvas in set(artist.figure.canvas for artist in self.artists):
            canvas.mpl_connect('button_press_event', self.on_press)
            canvas.mpl_connect('button_release_event', self.on_release)
            canvas.mpl_connect('pick_event', self.on_pick)
            canvas.mpl_connect('motion_notify_event', self.on_motion)

    def on_press(self, event):
        self.currently_dragging = True

    def on_release(self, event):
        self.currently_dragging = False
        self.current_artist = None

    def on_pick(self, event):
        if self.current_artist is None:
            self.current_artist = event.artist
            x0, y0 = event.artist.center
            x1, y1 = event.mouseevent.xdata, event.mouseevent.ydata
            self.offset = (x0 - x1), (y0 - y1)

    def on_motion(self, event):
        if not self.currently_dragging:
            return
        if self.current_artist is None:
            return
        dx, dy = self.offset
        self.current_artist.center = event.xdata + dx, event.ydata + dy
        self.current_artist.figure.canvas.draw()

if __name__ == '__main__':
    fig, ax = plt.subplots()
    ax.set(xlim=[-1, 2], ylim=[-1, 2])

    circles = [patches.Circle((0.32, 0.3), 0.2, fc='r', alpha=0.5),
               patches.Circle((0.3, 0.3), 0.2, fc='b', alpha=0.5)]
    for circ in circles:
        ax.add_patch(circ)

    dr = DraggablePoints(circles)
    plt.show()