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[Bug 26066] New: Enhance language around edit list in BMFF bytestream spec

From: <bugzilla@jessica.w3.org>
Date: Thu, 12 Jun 2014 06:46:20 +0000
To: public-html-media@w3.org
Message-ID: <bug-26066-5436@http.www.w3.org/Bugs/Public/>

            Bug ID: 26066
           Summary: Enhance language around edit list in BMFF bytestream
           Product: HTML WG
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Media Source Extensions
          Assignee: adrianba@microsoft.com
          Reporter: strobe@google.com
        QA Contact: public-html-bugzilla@w3.org
                CC: mike@w3.org, public-html-media@w3.org

In private correspondence, it was suggested that the language around edit lists
in the BMFF spec for MSE is underspecified.

The full extent of edit lists are a pain to get right and difficult to test. I
suggest that we narrow the language to support for the one case that's actually
required for proper operation of most existing MSE-compatible media corpora.

>From ISO 14496-12:2012:

A non-empty edit may insert a portion of the media timeline that is not present
in the initial movie, and is present only in subsequent movie fragments.
Particularly in an empty initial movie of a fragmented movie file (when there
are no media samples yet present), the segment_duration of this edit may be
zero, whereupon the edit provides the offset from media composition time to
movie presentation time, for the movie and subsequent movie fragments. It is
recommended that such an edit be used to establish a presentation time of 0 for
the first presented sample, when composition offsets are used. 

>From the current MSE BMFF spec:

The user agent must handle Edit Boxes (edts), in initialization segment, which
provide a mapping of composition times for each track to the global
presentation time.

Proposed revised text:

The user agent must support setting the offset from media composition time to
movie presentation time by handling an Edit Box (edts) with an Edit List (elst)
element that contains an non-empty edit list with a duration of 0 (indicating
that it spans all subsequent media).

This does not preclude a UA from implementing more complete support, but
clarifying the (pretty simple) minimum requirement makes it more likely for
that minimum baseline to be implemented IMO.

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Received on Thursday, 12 June 2014 06:46:21 UTC

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