matplotlib 中的交互线

Question

I'm trying to make an interactive plot using matplotlib that creates a line segment with two handles at the endpoints. You can click and drag the handles and the line will refresh to match the positions specified in this way, in a similar fashion to this matplotlib example poly_editor : (if you see the example, imagine that I want the same thing but with just one edge of the polygon).

I have tried altering the poly_editor code to work with just the Line2D element, and my program runs without any errors, except that it doesn't draw anything on the axis at all. I think it might be an error in the scope of the variables or something to do with the draw calls from matplotlib. Any guidance as to what the errors are would be greatly appreciated.

Edit: I advanced some more, simplified the code and now I can get it to draw the line and print the index of the nearest vertex within epsilon distance, but the line stays stationary and does not animate. The updated code is bellow

import matplotlib.pyplot as plt
import numpy as np
from matplotlib.lines import Line2D

class LineBuilder(object):

    epsilon = 0.5

    def __init__(self, line):
        canvas = line.figure.canvas
        self.canvas = canvas
        self.line = line
        self.axes = line.axes
        self.xs = list(line.get_xdata())
        self.ys = list(line.get_ydata())
    
        self.ind = None
    
        canvas.mpl_connect('button_press_event', self.button_press_callback)
        canvas.mpl_connect('button_release_event', self.button_release_callback)
        canvas.mpl_connect('motion_notify_event', self.motion_notify_callback)

    def get_ind(self, event):
        x = np.array(self.line.get_xdata())
        y = np.array(self.line.get_ydata())
        d = np.sqrt((x-event.xdata)**2 + (y - event.ydata)**2)
        if min(d) > self.epsilon:
            return None
        if d[0] < d[1]:
            return 0
        else:
            return 1

    def button_press_callback(self, event):
        if event.button != 1:
            return
        self.ind = self.get_ind(event)
        print(self.ind)
    
        self.line.set_animated(True)
        self.canvas.draw()
        self.background = self.canvas.copy_from_bbox(self.line.axes.bbox)
    
        self.axes.draw_artist(self.line)
        self.canvas.blit(self.axes.bbox)

    def button_release_callback(self, event):
        if event.button != 1:
            return
        self.ind = None
        self.line.set_animated(False)
        self.background = None
        self.line.figure.canvas.draw()

    def motion_notify_callback(self, event):
        if event.inaxes != self.line.axes:
            return
        if event.button != 1:
            return
        if self.ind is None:
            return
        self.xs[self.ind] = event.xdata
        self.ys[self.ind] = event.ydata
        self.line.set_data(self.xs, self.ys)
    
        self.canvas.restore_region(self.background)
        self.axes.draw_artist(self.line)
        self.canvas.blit(self.axes.bbox)


if __name__ == '__main__':
    fig, ax = plt.subplots()
    line = Line2D([0,1], [0,1], marker='o', markerfacecolor='red')
    ax.add_line(line)

    linebuilder = LineBuilder(line)

    ax.set_title('click to create lines')
    ax.set_xlim(-2,2)
    ax.set_ylim(-2,2)
    plt.show()

Thanks in advance, Kevin.

Answer 1

Okay, I solved the problem. The new code (above) actually works, there was a mistake in it. The mpl_connect call for the motion notify event had the wrong event type, now it is working as intended.

Answer 2

I am new here so hope I don't make to many mistakes by replying to this self.replied question. :)

First thanks for posting this, it helped me a lot by saving some time, I wanted almost exactly this code. I did some updates that I propose here, so that it's possible to manipulate more than two points, and use the key handling events to create or delete points in the line, as the PolygonInteractor does.

from matplotlib.lines import Line2D
import matplotlib.pyplot as plt
import numpy as np

def dist(x, y):
    """
    Return the distance between two points.
    """
    d = x - y
    return np.sqrt(np.dot(d, d))


def dist_point_to_segment(p, s0, s1):
    """
    Get the distance of a point to a segment.
      *p*, *s0*, *s1* are *xy* sequences
    This algorithm from
    http://geomalgorithms.com/a02-_lines.html
    """
    v = s1 - s0
    w = p - s0
    c1 = np.dot(w, v)
    if c1 <= 0:
        return dist(p, s0)
    c2 = np.dot(v, v)
    if c2 <= c1:
        return dist(p, s1)
    b = c1 / c2
    pb = s0 + b * v
    return dist(p, pb)

class LineBuilder(object):

    epsilon = 30 #in pixels

    def __init__(self, line):
        canvas = line.figure.canvas
        self.canvas = canvas
        self.line = line
        self.axes = line.axes
        self.xs = list(line.get_xdata())
        self.ys = list(line.get_ydata())

        self.ind = None

        canvas.mpl_connect('button_press_event', self.button_press_callback)
        canvas.mpl_connect('button_release_event', self.button_release_callback)
        canvas.mpl_connect('key_press_event', self.key_press_callback)
        canvas.mpl_connect('motion_notify_event', self.motion_notify_callback)

    def get_ind(self, event):
        xy = np.asarray(self.line._xy)
        xyt = self.line.get_transform().transform(xy)
        x, y = xyt[:, 0], xyt[:, 1]
        d = np.sqrt((x-event.x)**2 + (y - event.y)**2)
        indseq, = np.nonzero(d == d.min())
        ind = indseq[0]

        if d[ind] >= self.epsilon:
            ind = None

        return ind

    def button_press_callback(self, event):
        if event.button != 1:
            return
        if event.inaxes is None:
            return
        self.ind = self.get_ind(event)
        print(self.ind)

        self.line.set_animated(True)
        self.canvas.draw()
        self.background = self.canvas.copy_from_bbox(self.line.axes.bbox)

        self.axes.draw_artist(self.line)
        self.canvas.blit(self.axes.bbox)

    def button_release_callback(self, event):
        if event.button != 1:
            return
        self.ind = None
        self.line.set_animated(False)
        self.background = None
        self.line.figure.canvas.draw()

    def motion_notify_callback(self, event):
        if event.inaxes != self.line.axes:
            return
        if event.button != 1:
            return
        if self.ind is None:
            return
        self.xs[self.ind] = event.xdata
        self.ys[self.ind] = event.ydata
        self.line.set_data(self.xs, self.ys)

        self.canvas.restore_region(self.background)
        self.axes.draw_artist(self.line)
        self.canvas.blit(self.axes.bbox)

    def key_press_callback(self, event):
        """Callback for key presses."""

        if not event.inaxes:
            return
        elif event.key == 'd':
            ind = self.get_ind(event)
            if ind is not None and len(self.xs) > 2:
                self.xs = np.delete(self.xs, ind)
                self.ys = np.delete(self.ys, ind)
                self.line.set_data(self.xs, self.ys)
                self.axes.draw_artist(self.line)
                self.canvas.draw_idle()
        elif event.key == 'i':
            p = np.array([event.x, event.y])  # display coords
            xy = np.asarray(self.line._xy)
            xyt = self.line.get_transform().transform(xy)
            for i in range(len(xyt) - 1):
                s0 = xyt[i]
                s1 = xyt[i+1]
                d = dist_point_to_segment(p, s0, s1)
                if d <= self.epsilon:
                    self.xs = np.insert(self.xs, i+1, event.xdata)
                    self.ys = np.insert(self.ys, i+1, event.ydata)
                    self.line.set_data(self.xs, self.ys)
                    self.axes.draw_artist(self.line)
                    self.canvas.draw_idle()
                    break

if __name__ == '__main__':

    fig, ax = plt.subplots()
    line = Line2D([0,0.5,1], [0,0.5,1], marker = 'o', markerfacecolor = 'red')
    ax.add_line(line)

    linebuilder = LineBuilder(line)

    ax.set_title('click to create lines')
    ax.set_xlim(-2,2)
    ax.set_ylim(-2,2)
    plt.show()

This actually leads to a question/problem i have but it will be in another message.

Kristen