For anyone who’s not in the Synology ecosystem, this is what the release notes are:
Starting from this version, the processing of media files using HEVC (H.265), AVC (H.264), and VC-1 codecs will be transitioned from the server to end devices to reduce unnecessary resource usage on the system and enhance system efficiency. These codecs are widespread on end devices such as smartphones, tablets, computers, and smart TVs. If the end device does not support the required codecs, the use of media files may be limited.
This mostly affects things like streaming to a TV, streaming box or tablet with limited codec support.
When watching videos on Linux, the support on the NAS itself doesn’t matter, just the support only your PC. When opening videos over SMB in dolphin, the codec support on the NAS does not come into play. The thumbnails are generated by your PC.
Just install VLC on your PC and it will play whatever you throw at it, regardless of OS codecs. I would not re-encode anything.
edit: It looks like the biggest impact is using Synology Photos - it can’t generate thumbnails for HEIF photos/HEIC videos anymore
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