Longstanding bug with video preview thumbnail generation - easily reproducible

Plex Media Server
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There is a very old bug in Plex which me and a few others reported about a year ago, regarding the generation of video preview thumbnails. This happens with certain titles that have an unusual encoding format.
It’s very easy to reproduce.

  1. Take any Blu-ray or 4K disc published by the Arrow label. This is not confined to Arrow, but I know for a fact they all use the same GOP encoding.
  2. Rip the title to MKV using any method you prefer.
  3. Add it to a Plex library with video preview thumbnail generation turned on. Watch the logs, and eventually it will fail with an error message like this:

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The preview is not produced.
4. The only way to get around this is to change the option for “GenerateBIFKeyframesOnly” from 1 to 0, meaning that every frame in in the file gets indexed, rather than just keyframes. If you do this it results in much, much slower thumbnail generation, as well as considerably increased size for the video preview thumbnail file stored in the database - especially for 4K video.
SOLUTION: The video preview thumbnail scanner needs to be updated to recognise unusual but entirely valid H264 encoding GOPs which it currently cannot parse.
NOTE: This does not apply to chapter thumbnail generation. That works fine with any Arrow title.

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Is that true? Extracted keyframes for the preview thumbnails should still be generated on a consistent time interval. The skip_frame optimization should mostly improve extraction/generation speed, not size.

I definitely can imagine a change in size in some cases. If a video had keyframes(+GOP) spaced further apart than the GenerateBIFFrameInterval, then some of the time intervals would be missed.

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