[Matroska-devel] Several (minor) issues or underspecified areas in the MKV spec

Michael Bradshaw mjbshaw at google.com
Wed Oct 28 01:14:23 CET 2015


On Sun, Oct 25, 2015 at 1:10 AM, Steve Lhomme <slhomme at matroska.org> wrote:

> ...


Some follow up questions:


   1. If the DocType spec (e.g. the MKV spec or WebM spec) doesn't define a
   default value for an optional element, and that element is encoded with a
   zero length, does its default value fall back to the EBML spec (so it would
   be 0 for int/float types)?
   2. BlockDuration defaults to TrackDuration, but there is no element
   named TrackDuration. Is TrackDuration meant to refer to DefaultDuration for
   the corresponding track (as referenced by the block's header)?
   3. BlockDuration's description is kinda confusing. Specifically the
   following sentence: "When not written and with no DefaultDuration, the
   value is assumed to be the difference between the timestamp of this Block
   and the timestamp of the next Block in "display" order (not coding order)."
   If BlockDuration is not written, and there is no DefaultDuration (thus it's
   not mandatory), I'm not sure it makes sense to talk about its value and
   make assumptions about it. If it's not written it doesn't exist and can't
   have a value. I'm guessing the sentence is trying to talk about how users
   can compute the duration of the block, in which case I might suggest
   replacing "the value is assumed to be" with "the duration of the block is".
   I think this helps separate the distinction between the literal element
   itself and the semantic value it's attempting to represent (the
   BlockDuration element may not exist, but the semantic value it represents
   (the block's duration) still does (philosophically speaking) and can be
   computed).
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