闪电教程

“BLIT”是一个 standard technique 在栅格图形中，在Matplotlib的上下文中，可以用来（极大地）提高交互式图形的性能。例如 animation 和 widgets 模块内部使用blitting。在这里，我们将演示如何在这些类之外实现您自己的blitting。

绩效提升的根源就是不必重新做我们不需要做的工作。如果轴的限制没有改变，那么就不需要重新绘制所有的记号和记号标签（特别是因为文本是渲染成本较高的对象之一）。

保存我们工作的程序大致如下：

• 绘制图形，但不包括任何标记为“动画”的艺术家
• 保存RBGA缓冲区的副本

在未来，为了更新“动画”艺术家，我们

• 还原RGBA缓冲区的副本
• 只重绘动画艺术家
• 在屏幕上显示结果图像

从而使我们不必重新绘制所有 _not_ 有生气的。此过程的一个结果是，动画艺术家总是以比静态艺术家更高的z顺序绘制。

不是所有的后端都支持blitting。您可以通过 FigureCanvasBase.supports_blit 财产。

警告

此代码不适用于OSX后端（但适用于mac上的其他GUI后端）。

最小示例

我们可以使用 FigureCanvasAgg 方法 copy_from_bbox 和 restore_region 结合设置 animated=True 在我们的艺术家实现一个最小的例子，使用光点加速渲染

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 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)

这个例子工作并显示了一个简单的动画，但是，因为我们只抓取一次背景，如果图形的大小（以像素为单位）发生变化（由于图形的大小或dpi发生变化），背景将无效并导致不正确（但有时看起来很酷！）图像。在这个类中，我们建议有一个变量。

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的状态，绘制艺术家，然后将结果显示在屏幕上。另外，我们可以使用 'draw_event' 回调以在完全重新绘制时捕获新背景，以正确处理大小调整。

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()

下面是我们如何使用我们的课堂。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框计数器。

# 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 it's thing
    bm.update()

该类不依赖于 pyplot 适合嵌入到更大的GUI应用程序中。