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Re: Timecodes that are not zero-based

From: Conrad Parker <conrad@metadecks.org>
Date: Sat, 28 Mar 2009 08:55:27 +0900
Message-ID: <dba6c0830903271655q4fcfc4d5o8449e3e320762064@mail.gmail.com>
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Cc: Media Fragment <public-media-fragment@w3.org>
2009/3/28 Silvia Pfeiffer <silviapfeiffer1@gmail.com>:
>
> In Annodex/Ogg Skeleton we deal with this situation by having it
> included in the header of the delivered file. It will say that it
> starts for offset 12s, but that the display time should be 15s. This
> will make the video decoder skip the beginning and display 15s as the
> first timestamp.
>
> Other formats probably do not support this approach yet, and then your
> user agent will indeed need to do this manually as described.

Aye, we designed ogg/skeleton specifically to support this kind of
usage. It would be interesting to see what other formats that info
could be applied to.

Perhaps another way to retro-fit it would be to put the
Presentation-TIme in an HTTP response header. That would be less
accurate but could be a workaround for other formats.

> What we are doing in Annodex is: we convert a smpte timecode
> specification to a npt time and then do all the offset calculations in
> npt/seconds. You can see my time handling for CMML in
> http://svn.annodex.net/libcmml/trunk/src/cmml_time.c .

not quite :-) that's the default behaviour for the CMML encapsulation,
but the offsets encoded in Skeleton in general can be in any rational
base. The data start times for each track can be in the units for that
track, eg. video frames, audio samples etc.; and the overall
presentation time for the segment can be specified in an arbitrary
unit.

Most applications so far (like oggz-chop) just choose milliseconds for
the presentation time, but that is up to the implementation.

cheers,

Conrad.
Received on Friday, 27 March 2009 23:56:02 GMT

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