Mastodon have implemented mandatory H264+AAC transcoding for any .mp4 that isn't H264+AAC. Apart from being stupid because AV1 is now being used and Opus has been used for half a decade now, it also opens an easy DDOS attack possibility.
Just encode low bitrate JPEG2000 video in .mp4 (yes, its in spec). The ffmpeg j2k decoder is RIDICULOUSLY slow. Decoding even a few j2k frames will take an eternity.
Of course, I'll permit all codecs in my instance. Something doesn't decode? Not my problem.

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A Mastodon instance for people interested in multimedia, codecs, assembly, SIMD, and the occasional weeb stuff.