使用 blitting 更快地渲染

Blitting是光栅图形中的一种标准技术，在 Matplotlib 的上下文中，可用于（大幅）提高交互式图形的性能。例如， animationandwidgets模块在内部使用 blitting。在这里，我们演示了如何在这些类之外实现您自己的 blitting。

Blitting 通过将所有不变的图形元素渲染到背景图像中来加速重复绘制。然后，对于每次绘制，只需要在这个背景上绘制变化的元素。例如，如果 Axes 的限制没有改变，我们可以将包含所有刻度和标签的空 Axes 渲染一次，然后再绘制变化的数据。

策略是

• 准备恒定的背景：

  • 绘制图形，但通过将其标记为动画来排除您想要动画的所有艺术家（请参阅Artist.set_animated）。

  • 保存 RBGA 缓冲区的副本。

• 渲染单个图像：

  • 恢复 RGBA 缓冲区的副本。

  • Axes.draw_artist使用/ 重绘动画艺术家Figure.draw_artist。

  • 在屏幕上显示生成的图像。

此过程的一个结果是您的动画艺术家总是被绘制在静态艺术家之上。

并非所有后端都支持 blitting。您可以通过该FigureCanvasBase.supports_blit属性检查给定的画布是否存在。

警告

此代码不适用于 OSX 后端（但适用于 mac 上的其他 GUI 后端）。

最小的例子

我们可以使用这些FigureCanvasAgg方法 copy_from_bbox并 restore_region结合 animated=True我们的艺术家的设置来实现一个使用 blitting 加速渲染的最小示例

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0, 2 * np.pi, 100)

fig, ax = plt.subplots()

# animated=True tells matplotlib to only draw the artist when we
# explicitly request it
(ln,) = ax.plot(x, np.sin(x), animated=True)

# make sure the window is raised, but the script keeps going
plt.show(block=False)

# stop to admire our empty window axes and ensure it is rendered at
# least once.
#
# We need to fully draw the figure at its final size on the screen
# before we continue on so that :
#  a) we have the correctly sized and drawn background to grab
#  b) we have a cached renderer so that ``ax.draw_artist`` works
# so we spin the event loop to let the backend process any pending operations
plt.pause(0.1)

# get copy of entire figure (everything inside fig.bbox) sans animated artist
bg = fig.canvas.copy_from_bbox(fig.bbox)
# draw the animated artist, this uses a cached renderer
ax.draw_artist(ln)
# show the result to the screen, this pushes the updated RGBA buffer from the
# renderer to the GUI framework so you can see it
fig.canvas.blit(fig.bbox)

for j in range(100):
    # reset the background back in the canvas state, screen unchanged
    fig.canvas.restore_region(bg)
    # update the artist, neither the canvas state nor the screen have changed
    ln.set_ydata(np.sin(x + (j / 100) * np.pi))
    # re-render the artist, updating the canvas state, but not the screen
    ax.draw_artist(ln)
    # copy the image to the GUI state, but screen might not be changed yet
    fig.canvas.blit(fig.bbox)
    # flush any pending GUI events, re-painting the screen if needed
    fig.canvas.flush_events()
    # you can put a pause in if you want to slow things down
    # plt.pause(.1)

此示例有效并显示了一个简单的动画，但是因为我们只抓取了一次背景，如果图形的像素大小发生变化（由于图形的大小或 dpi 变化），背景将无效并导致不正确（但有时看起来很酷！）图像。还有一个全局变量和相当数量的样板，这表明我们应该将它包装在一个类中。

基于类的示例

我们可以使用一个类来封装样板逻辑和恢复背景、绘制艺术家、然后将结果blitting 到屏幕的状态。此外，我们可以使用'draw_event' 回调来捕获新背景，只要完全重新绘制发生正确处理调整大小。

class BlitManager:
    def __init__(self, canvas, animated_artists=()):
        """
        Parameters
        ----------
        canvas : FigureCanvasAgg
            The canvas to work with, this only works for sub-classes of the Agg
            canvas which have the `~FigureCanvasAgg.copy_from_bbox` and
            `~FigureCanvasAgg.restore_region` methods.

        animated_artists : Iterable[Artist]
            List of the artists to manage
        """
        self.canvas = canvas
        self._bg = None
        self._artists = []

        for a in animated_artists:
            self.add_artist(a)
        # grab the background on every draw
        self.cid = canvas.mpl_connect("draw_event", self.on_draw)

    def on_draw(self, event):
        """Callback to register with 'draw_event'."""
        cv = self.canvas
        if event is not None:
            if event.canvas != cv:
                raise RuntimeError
        self._bg = cv.copy_from_bbox(cv.figure.bbox)
        self._draw_animated()

    def add_artist(self, art):
        """
        Add an artist to be managed.

        Parameters
        ----------
        art : Artist

            The artist to be added.  Will be set to 'animated' (just
            to be safe).  *art* must be in the figure associated with
            the canvas this class is managing.

        """
        if art.figure != self.canvas.figure:
            raise RuntimeError
        art.set_animated(True)
        self._artists.append(art)

    def _draw_animated(self):
        """Draw all of the animated artists."""
        fig = self.canvas.figure
        for a in self._artists:
            fig.draw_artist(a)

    def update(self):
        """Update the screen with animated artists."""
        cv = self.canvas
        fig = cv.figure
        # paranoia in case we missed the draw event,
        if self._bg is None:
            self.on_draw(None)
        else:
            # restore the background
            cv.restore_region(self._bg)
            # draw all of the animated artists
            self._draw_animated()
            # update the GUI state
            cv.blit(fig.bbox)
        # let the GUI event loop process anything it has to do
        cv.flush_events()

下面是我们将如何使用我们的类。这是一个比第一种情况稍微复杂的例子，因为我们还添加了一个文本帧计数器。

# make a new figure
fig, ax = plt.subplots()
# add a line
(ln,) = ax.plot(x, np.sin(x), animated=True)
# add a frame number
fr_number = ax.annotate(
    "0",
    (0, 1),
    xycoords="axes fraction",
    xytext=(10, -10),
    textcoords="offset points",
    ha="left",
    va="top",
    animated=True,
)
bm = BlitManager(fig.canvas, [ln, fr_number])
# make sure our window is on the screen and drawn
plt.show(block=False)
plt.pause(.1)

for j in range(100):
    # update the artists
    ln.set_ydata(np.sin(x + (j / 100) * np.pi))
    fr_number.set_text("frame: {j}".format(j=j))
    # tell the blitting manager to do its thing
    bm.update()

该类不依赖于pyplot并适合嵌入到较大的 GUI 应用程序中。