python - 高效的 Matplotlib 重绘

Question

我使用 Matplotlib 允许用户通过鼠标点击选择感兴趣的数据点，使用与 this answer. 非常相似的方法

实际上，散点图显示在热图图像上，鼠标点击可以添加或删除散点。

我的数据是使用 pcolormesh() 在后台绘制的，所以当我使用 axis.figure.canvas.draw() 更新 Canvas 时，散点和重新绘制背景热图。考虑到热图的大小，这对于一个可用的界面来说太慢了。

有没有一种方法可以有选择地重绘散点而不重绘背景？

示例代码:

points = []  # Holds a list of (x,y) scatter points 

def onclick(event):
    # Click event handler to add points
    points.append( (event.x, event.y) )
    ax.figure.canvas.draw()

fig = plt.figure()
ax = plt.figure()
# Plot the background
ax.pcolormesh(heatmap_data)

# Add the listener to handle clicks
cid = fig.canvas.mpl_connect("button_press_event", onclick)
plt.show()

Answer 1

当然!你想要的是blitting。如果您不是在编写图形用户界面，则可以使用 matplotlib.animation 来简化其中的一些操作，但如果您希望内容具有交互性，则需要直接处理它。

在 matplotlib 术语中，您想要 fig.canvas.copy_from_bbox 的组合，然后交替调用 fig.canvas.restore_region(background)，ax。 draw_artist(what_you_want_to_draw) 和 fig.canvas.blit:

background = fig.canvas.copy_from_bbox(ax.bbox)

for x, y in user_interactions:
    fig.canvas.restore_region(background)
    points.append([x, y])
    scatter.set_offsets(points)
    ax.draw_artist(scatter)
    fig.canvas.blit(ax.bbox)

---

简单的 Blitting 示例:添加点

在你的情况下，如果你只是添加点，你实际上可以跳过保存和恢复背景。但是，如果你走那条路，由于抗锯齿点被反复重新绘制在彼此之上，你最终会在情节上做出一些细微的变化。

无论如何，这是您想要的事物类型的最简单示例。这只处理添加点，并跳过我上面提到的保存和恢复背景:

import matplotlib.pyplot as plt
import numpy as np

def main():
    fig, ax = plt.subplots()
    ax.pcolormesh(np.random.random((100, 100)), cmap='gray')

    ClickToDrawPoints(ax).show()

class ClickToDrawPoints(object):
    def __init__(self, ax):
        self.ax = ax
        self.fig = ax.figure
        self.xy = []
        self.points = ax.scatter([], [], s=200, color='red', picker=20)
        self.fig.canvas.mpl_connect('button_press_event', self.on_click)

    def on_click(self, event):
        if event.inaxes is None:
            return
        self.xy.append([event.xdata, event.ydata])
        self.points.set_offsets(self.xy)
        self.ax.draw_artist(self.points)
        self.fig.canvas.blit(self.ax.bbox)

    def show(self):
        plt.show()

main()

---

有时候简单就是太简单

但是，假设我们想让右键单击删除一个点。

在那种情况下，我们需要能够在不重绘的情况下恢复背景。

好的，一切都很好。我们将使用类似于我在答案顶部提到的伪代码片段的东西。

但是，有一个警告:如果调整图形大小，我们需要更新背景。同样，如果坐标轴是交互缩放/平移的，我们需要更新背景。基本上，您需要在绘制绘图时随时更新背景。

很快您就需要变得相当复杂。

---

更复杂:添加/拖动/删除点

以下是您最终搭建的那种“脚手架”的一般示例。

这有点低效，因为绘图被绘制了两次。 (例如，平移将慢)。可以解决这个问题，但我会把这些例子留到下次再说。

这个实现了加点、拖点、删点。要在交互式缩放/平移后添加/拖动点，请再次单击工具栏上的缩放/平移工具以禁用它们。

这是一个相当复杂的示例，但希望它能让您了解通常构建的框架类型，以交互方式绘制/拖动/编辑/删除 matplotlib 艺术家，而无需重新绘制整个绘图。

import numpy as np
import matplotlib.pyplot as plt

class DrawDragPoints(object):
    """
    Demonstrates a basic example of the "scaffolding" you need to efficiently
    blit drawable/draggable/deleteable artists on top of a background.
    """
    def __init__(self):
        self.fig, self.ax = self.setup_axes()
        self.xy = []
        self.tolerance = 10
        self._num_clicks = 0

        # The artist we'll be modifying...
        self.points = self.ax.scatter([], [], s=200, color='red',
                                      picker=self.tolerance, animated=True)

        connect = self.fig.canvas.mpl_connect
        connect('button_press_event', self.on_click)
        self.draw_cid = connect('draw_event', self.grab_background)

    def setup_axes(self):
        """Setup the figure/axes and plot any background artists."""
        fig, ax = plt.subplots()

        # imshow would be _much_ faster in this case, but let's deliberately
        # use something slow...
        ax.pcolormesh(np.random.random((1000, 1000)), cmap='gray')

        ax.set_title('Left click to add/drag a point\nRight-click to delete')
        return fig, ax

    def on_click(self, event):
        """Decide whether to add, delete, or drag a point."""
        # If we're using a tool on the toolbar, don't add/draw a point...
        if self.fig.canvas.toolbar._active is not None:
            return

        contains, info = self.points.contains(event)
        if contains:
            i = info['ind'][0]
            if event.button == 1:
                self.start_drag(i)
            elif event.button == 3:
                self.delete_point(i)
        else:
            self.add_point(event)

    def update(self):
        """Update the artist for any changes to self.xy."""
        self.points.set_offsets(self.xy)
        self.blit()

    def add_point(self, event):
        self.xy.append([event.xdata, event.ydata])
        self.update()

    def delete_point(self, i):
        self.xy.pop(i)
        self.update()

    def start_drag(self, i):
        """Bind mouse motion to updating a particular point."""
        self.drag_i = i
        connect = self.fig.canvas.mpl_connect
        cid1 = connect('motion_notify_event', self.drag_update)
        cid2 = connect('button_release_event', self.end_drag)
        self.drag_cids = [cid1, cid2]

    def drag_update(self, event):
        """Update a point that's being moved interactively."""
        self.xy[self.drag_i] = [event.xdata, event.ydata]
        self.update()

    def end_drag(self, event):
        """End the binding of mouse motion to a particular point."""
        for cid in self.drag_cids:
            self.fig.canvas.mpl_disconnect(cid)

    def safe_draw(self):
        """Temporarily disconnect the draw_event callback to avoid recursion"""
        canvas = self.fig.canvas
        canvas.mpl_disconnect(self.draw_cid)
        canvas.draw()
        self.draw_cid = canvas.mpl_connect('draw_event', self.grab_background)

    def grab_background(self, event=None):
        """
        When the figure is resized, hide the points, draw everything,
        and update the background.
        """
        self.points.set_visible(False)
        self.safe_draw()

        # With most backends (e.g. TkAgg), we could grab (and refresh, in
        # self.blit) self.ax.bbox instead of self.fig.bbox, but Qt4Agg, and
        # some others, requires us to update the _full_ canvas, instead.
        self.background = self.fig.canvas.copy_from_bbox(self.fig.bbox)

        self.points.set_visible(True)
        self.blit()

    def blit(self):
        """
        Efficiently update the figure, without needing to redraw the
        "background" artists.
        """
        self.fig.canvas.restore_region(self.background)
        self.ax.draw_artist(self.points)
        self.fig.canvas.blit(self.fig.bbox)

    def show(self):
        plt.show()

DrawDragPoints().show()