Differences in Animation Creation Between FFmpeg and Pillow Writer

[munechika-koyo]

Summary

When saving the animation using FuncAnimation's save method with writer="ffmpeg", it produces a strange layout, and layout shifts occur around the first frames compared to webp or gif created with writer="pillow".

Reproducer

import matplotlib
import numpy as np
import ultraplot as uplt
from matplotlib.animation import FuncAnimation

matplotlib.use("Agg")
state = np.random.RandomState(51423)

fig, axs = uplt.subplots(
    nrows=1,
    ncols=2,
    width="14cm",
)

mappables = []
for ax in axs:
    data = state.rand(10, 10)
    m = ax.heatmap(data, cmap="dusk")
    ax.colorbar(m, loc="t", tickdir="out", label="Axes Colorbars")
    mappables.append(m)

axs.format(
    abc="(a)",
    abcloc="ul",
    xlabel="xlabel",
    ylabel="ylabel",
    toplabels=("Left Axes", "Right Axes"),
    urtitle="1",
    suptitle="Test Animation",
)


def update(frame):
    global axs, mappables

    for m in mappables:
        data = state.rand(10, 10)
        m.set_array(data)

    axs.format(
        urtitle=f"{frame + 1}",
    )
    return mappables


ani = FuncAnimation(
    fig,
    update,
    frames=10,
    interval=150,
)

ani.save("test_animation.mp4", writer="ffmpeg")
# ani.save("test_animation.webp", writer="pillow")  # <--- change here

This is the weird video in the MP4 format.

test_animation.mp4

This is the WEBP-formatted one that I probably prefer.

Environment

• ultraplot v2.2.0
• ffmpeg v8.1.1
• python v3.14.4
• matplotlib v3.10.9

Miscellaneous

This is inherited from the discussion at #488

[munechika-koyo]

Because the MP4 video doesn't display correctly somehow, I reference the link to the almost identical one: #488 (comment)

[munechika-koyo]

I found a workaround for changing the tight parameter of a figure to resolve the shifting issue.
First, set tight=False when creating the figure, like:

fig, axs = uplt.subplots(
    nrows=1,
    ncols=2,
    width="14cm",
    tight=False,
)

Then, tighten the layout at the first frame, like:

def update(frame):

    if frame == 0:
        fig.auto_layout(tight=True)
    else:
        fig.auto_layout(tight=False)

test_animation.mp4