matplotlib.animation
#
Animation#
The easiest way to make a live animation in Matplotlib is to use one of the
Animation
classes.
See also
A base class for Animations. |
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In both cases it is critical to keep a reference to the instance
object. The animation is advanced by a timer (typically from the host
GUI framework) which the Animation
object holds the only reference
to. If you do not hold a reference to the Animation
object, it (and
hence the timers) will be garbage collected which will stop the
animation.
To save an animation use Animation.save
, Animation.to_html5_video
,
or Animation.to_jshtml
.
See Helper Classes below for details about what movie formats are supported.
FuncAnimation
#
The inner workings of FuncAnimation
is more-or-less:
for d in frames:
artists = func(d, *fargs)
fig.canvas.draw_idle()
fig.canvas.start_event_loop(interval)
with details to handle 'blitting' (to dramatically improve the live performance), to be non-blocking, not repeatedly start/stop the GUI event loop, handle repeats, multiple animated axes, and easily save the animation to a movie file.
'Blitting' is a standard technique in computer graphics. The
general gist is to take an existing bit map (in our case a mostly
rasterized figure) and then 'blit' one more artist on top. Thus, by
managing a saved 'clean' bitmap, we can only re-draw the few artists
that are changing at each frame and possibly save significant amounts of
time. When we use blitting (by passing blit=True
), the core loop of
FuncAnimation
gets a bit more complicated:
ax = fig.gca()
def update_blit(artists):
fig.canvas.restore_region(bg_cache)
for a in artists:
a.axes.draw_artist(a)
ax.figure.canvas.blit(ax.bbox)
artists = init_func()
for a in artists:
a.set_animated(True)
fig.canvas.draw()
bg_cache = fig.canvas.copy_from_bbox(ax.bbox)
for f in frames:
artists = func(f, *fargs)
update_blit(artists)
fig.canvas.start_event_loop(interval)
This is of course leaving out many details (such as updating the
background when the figure is resized or fully re-drawn). However,
this hopefully minimalist example gives a sense of how init_func
and func
are used inside of FuncAnimation
and the theory of how
'blitting' works.
Note
The zorder of artists is not taken into account when 'blitting' because the 'blitted' artists are always drawn on top.
The expected signature on func
and init_func
is very simple to
keep FuncAnimation
out of your book keeping and plotting logic, but
this means that the callable objects you pass in must know what
artists they should be working on. There are several approaches to
handling this, of varying complexity and encapsulation. The simplest
approach, which works quite well in the case of a script, is to define the
artist at a global scope and let Python sort things out. For example:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation
fig, ax = plt.subplots()
xdata, ydata = [], []
ln, = ax.plot([], [], 'ro')
def init():
ax.set_xlim(0, 2*np.pi)
ax.set_ylim(-1, 1)
return ln,
def update(frame):
xdata.append(frame)
ydata.append(np.sin(frame))
ln.set_data(xdata, ydata)
return ln,
ani = FuncAnimation(fig, update, frames=np.linspace(0, 2*np.pi, 128),
init_func=init, blit=True)
plt.show()
The second method is to use functools.partial
to pass arguments to the
function:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation
from functools import partial
fig, ax = plt.subplots()
line1, = ax.plot([], [], 'ro')
def init():
ax.set_xlim(0, 2*np.pi)
ax.set_ylim(-1, 1)
return line1,
def update(frame, ln, x, y):
x.append(frame)
y.append(np.sin(frame))
ln.set_data(x, y)
return ln,
ani = FuncAnimation(
fig, partial(update, ln=line1, x=[], y=[]),
frames=np.linspace(0, 2*np.pi, 128),
init_func=init, blit=True)
plt.show()
A third method is to use closures to build up the required artists and functions. A fourth method is to create a class.
Examples#
ArtistAnimation
#
Examples#
Writer Classes#
The provided writers fall into a few broad categories.
The Pillow writer relies on the Pillow library to write the animation, keeping all data in memory.
The HTML writer generates JavaScript-based animations.
Writer for JavaScript-based HTML movies. |
The pipe-based writers stream the captured frames over a pipe to an external process. The pipe-based variants tend to be more performant, but may not work on all systems.
Pipe-based ffmpeg writer. |
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Pipe-based animated gif writer. |
The file-based writers save temporary files for each frame which are stitched into a single file at the end. Although slower, these writers can be easier to debug.
File-based ffmpeg writer. |
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File-based animated gif writer. |
The writer classes provide a way to grab sequential frames from the same
underlying Figure
. They all provide three methods that
must be called in sequence:
setup
prepares the writer (e.g. opening a pipe). Pipe-based and file-based writers take different arguments tosetup()
.grab_frame
can then be called as often as needed to capture a single frame at a timefinish
finalizes the movie and writes the output file to disk.
Example:
moviewriter = MovieWriter(...)
moviewriter.setup(fig, 'my_movie.ext', dpi=100)
for j in range(n):
update_figure(j)
moviewriter.grab_frame()
moviewriter.finish()
If using the writer classes directly (not through Animation.save
), it is
strongly encouraged to use the saving
context manager:
with moviewriter.saving(fig, 'myfile.mp4', dpi=100):
for j in range(n):
update_figure(j)
moviewriter.grab_frame()
to ensure that setup and cleanup are performed as necessary.
Examples#
Helper Classes#
Animation Base Classes#
A base class for Animations. |
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Writer Registry#
A module-level registry is provided to map between the name of the
writer and the class to allow a string to be passed to
Animation.save
instead of a writer instance.
Registry of available writer classes by human readable name. |
Writer Base Classes#
To reduce code duplication base classes
Abstract base class for writing movies, providing a way to grab frames by calling |
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Base class for writing movies. |
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and mixins
Mixin class for FFMpeg output. |
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Mixin class for ImageMagick output. |
are provided.
See the source code for how to easily implement new MovieWriter
classes.