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Re: [media-and-entertainment] Frame accurate seeking of HTML5 MediaElement

From: Nigel Megitt via GitHub <sysbot+gh@w3.org>
Date: Fri, 15 Jun 2018 09:20:12 +0000
To: public-web-and-tv@w3.org
Message-ID: <issue_comment.created-397564406-1529054411-sysbot+gh@w3.org>
> the most important use case for frame accuracy right now would be to accurately snap subtitle cue changes to frame changes. A frame counter like described above would definitely work for this. 

@Daiz I agree the use case is important and would like to achieve the same result, but I disagree that a frame counter would work. In fact, a frame counter would absolutely not work!

The reason is that we typically distribute a single subtitle file but have multiple profiles of video encoding, where one approach to managing the bitrate adaptively is to vary the frame rate. I think our lowest frame rate profile is about 6.25 fps. In that situation, quantizing subtitle times to the frame rate is a very bad idea. For more on this, see [EBU-TT-D](https://tech.ebu.ch/publications/tech3380) annex E.

That's why we use media time expressions with decimal fractions of seconds, and arrange that those time expressions work against the media at some canonical frame rate, such as 25fps in the knowledge that it will also work at other frame rates.

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