PR Summary

As of #17401, we enabled an error-free path to save animated GIFs with ffmpeg, which is the default writer. This means with matplotlib 3.3.0 the follow code works without error:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.animation as animation

fig, ax = plt.subplots(figsize=(2, 2))
x = np.arange(0, 2*np.pi, 0.01)
line, = ax.plot(x, np.sin(x))

def animate(i):
    line.set_ydata(np.sin(x + i / 50))  # update the data.
    return line,

anim = animation.FuncAnimation(fig, animate, interval=20, blit=True, save_count=50)
anim.save('test.gif')

Unfortunately, it also produces this by default (notice the weird coloring):

This is a consequence of GIF being a palette-d image format and the default palette used by ffmpeg being...suboptimal for matplotlib (I'm pretty sure anti-aliasing plays a role, otherwise there are only 3 colors in the original image). Luckily this blog from GIPHY, which helped me understand this, also provides some magic incantations for ffmpeg to get it to determine an optimal palette for us. The result is:

Best of all, if I run that same animation through ffmpeg, imagemagick, and pillow we see:

 128K  ffmpeg.gif
 661K  pillow.gif
 231K  imagemagick.gif

So our default writer now produces our smallest animated GIFs.

I also fixed a bug where parameters to save() like codec were not being passed and set in the base class--thus the rcParam defaults were always used. (Introduced in #15961.) I tried to add a test for this, but couldn't figure out how since you can't pass them along with a class instance, which was the straightforward path to check they're set properly.

I'll leave it to @QuLogic to decide whether this belongs in 3.3.1.

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/next_api_changes/* if API changed in a backward-incompatible way