[Misc] Improve gif output compression rate

[neozhaoliang]

Issue: #6183

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[yuanming-hu]

Thanks, will take a look next week!

[yuanming-hu]

Many thanks for the PR - could you help benchmark the output size of the old ffmpeg approach against the new moviepy approach?

[neozhaoliang]

Many thanks for the PR - could you help benchmark the output size of the old ffmpeg approach against the new moviepy approach?

The improvement in compression size is quite good. I used a 1.8MB size mp4 video to test the two methods:

1. The original way results in a 13.3MB gif.
2. moviepy outputs a 4.3MB gif.

@houkensjtu also has some insights on this comparison.

[houkensjtu]

FYI, I tested this PR using the taichi_dem repo. Previously, with the default frame rate and resolution, the gif generated by the first 100 frames output is around 45 MB. With this PR, the size is reduced to 32 MB.

I guess the reason why the compression rate is not as good as @neozhaoliang 's result is that the frames from DEM are all black & white.

The commands I used to generate the gif are as follows:

ti video
ti gif -i video.mp4

Overall, I think it's a good improvement over the ffmpeg method. We may include a comment in the doc to warn the users about the size of the generated gif, and point them to third-party compression tools (such as ezgif) if they wish so.

[yuanming-hu]

LGTM, thank you both! :-)