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Mpv – A free, open-source, and cross-platform media player

(mpv.io)
1113 points Bluestein | 1 comments | 17 Aug 24 18:56 UTC | HN request time: 0.212s | source
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lairv ◴[17 Aug 24 21:38 UTC] No.41278203[source]▶
I use it to inspect video frames by frames, particularly being able to go back one frame. VLC doesn't support it, this thread about the feature is hilarious https://forum.videolan.org/viewtopic.php?t=120627
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sergiotapia ◴[17 Aug 24 22:06 UTC] No.41278382[source]▶
i wonder why he's such an ass about it, and totally adamant that it's impossible when multiple players already do this fast. ego?
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variadix ◴[17 Aug 24 22:49 UTC] No.41278658[source]▶
I think technically he’s correct (I haven’t worked on media decoding code, but I understand how common video encoding formats work). If you have a long video with only a single key frame at the beginning then to step back you would need to, starting from the beginning of the video, decode every frame up to the previous frame you wanted to jump to in order to apply frame deltas, also assuming you have some sort of frame counter to determine when you’ve reached the target frame. In the worst case this does require a lot of compute, but this is an edge case if you primarily care about common video formats with normal encoding settings. I assume seeking backwards is also painfully slow on videos encoded in this manner, so why stepping back 1 frame is out of the question when compared to seeking backwards, I don’t fully understand, it must have something to do with precise frame counts being unavailable on some hardware decoders for some formats (and there being no good workaround) so you _may_ not actually go back 1 frame.

I don’t see any reason it couldn’t be supported for a set of formats with reasonable encoding/decoding settings, and provide some error message for other formats if a user attempts to step back, e.g. reverse frame stepping unavailable for current video due to format/encoding/decoding settings.

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1. oefrha ◴[18 Aug 24 11:57 UTC] No.41281805[source]▶
> I think technically he’s correct (I haven’t worked on media decoding code, but I understand how common video encoding formats work).

He’s technically simply wrong (I have worked on media decoding code, hell I’m working on a related project today). His player supports seeking back by 10 seconds or whatnot, but he insists that somehow to implement precise seeking to the previous frame, you need to seek from the very beginning of the video, no two ways about it:

> There is not a slight technical difficulty. On a logical level, this feature is algorithmically impossible, except for the extreme: You can decode all frames up to the previous one. But this would be far too slow for anything except really short videos.

It’s obvious bullshit, if you encounter one of those pathological videos (that don’t really exist except in his mind or in some test suites) you just give up after a reasonable amount of time, same as how you give up if you can’t seek back ~10s in a reasonable amount of time.

And players do give up seeking all the time already, not just on these hypothetical one-I-frame-per-hour videos, but real world videos with messed up pts/dts with no reasonable way to go back a short interval.