使用 Blitting 加快渲染速度

Blitting 是光柵圖形中的一種標準技術，在 Matplotlib 的環境中，可用於（大幅）提高互動式圖表的效能。例如，animation 和 widgets 模組在內部使用 blitting。在此，我們示範如何在這些類別之外實作您自己的 blitting。

Blitting 透過將所有不變的圖形元素一次渲染到背景圖像中來加速重複繪製。然後，每次繪製時，只需要將變化的元素繪製到此背景上。例如，如果軸的限制沒有改變，我們可以一次渲染空的軸，包括所有刻度和標籤，然後僅繪製稍後變化的資料。

策略是

• 準備恆定的背景

  • 繪製圖表，但排除所有您想要製作動畫的藝術家，方法是將它們標記為動畫 (請參閱Artist.set_animated)。

  • 儲存 RBGA 緩衝區的副本。

• 渲染個別圖像

  • 還原 RGBA 緩衝區的副本。

  • 使用 Axes.draw_artist / Figure.draw_artist重新繪製動畫藝術家。

  • 在螢幕上顯示產生的圖像。

此程序的結果之一是您的動畫藝術家總是繪製在靜態藝術家的頂部。

並非所有後端都支援 blitting。您可以透過 FigureCanvasBase.supports_blit 屬性檢查給定的畫布是否支援。

警告

此程式碼不適用於 macosx 後端（但適用於 Mac 上的其他 GUI 後端）。

最小範例

我們可以將 FigureCanvasAgg 方法 copy_from_bbox 和 restore_region 與在我們的藝術家上設定 animated=True 結合使用，來實作使用 blitting 加速渲染的最小範例

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)

此範例可以運作並顯示簡單的動畫，但因為我們只抓取一次背景，如果圖表的像素大小發生變化（因為圖表的大小或 dpi 發生變化），背景將會無效，並導致不正確（但有時看起來很酷！）的圖像。還有一個全域變數和相當多的樣板程式碼，這表示我們應該將其包裝在一個類別中。

基於類別的範例

我們可以利用類別來封裝樣板邏輯和還原背景、繪製藝術家，然後將結果 blit 到螢幕的狀態。此外，我們可以使用 'draw_event' 回呼，以便在完整重新繪製發生時擷取新的背景，以正確處理調整大小。

class BlitManager:
    def __init__(self, canvas, animated_artists=()):
        """
        Parameters
        ----------
        canvas : FigureCanvasAgg
            The canvas to work with, this only works for subclasses 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()

以下是我們如何使用我們的類別。這是一個比第一種情況稍微複雜的範例，因為我們還新增了一個文字影格計數器。

# 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(f"frame: {j}")
    # tell the blitting manager to do its thing
    bm.update()

此類別不依賴 pyplot，且適合嵌入到較大的 GUI 應用程式中。