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

Michael Bradshaw mjbshaw at google.com
Sat Oct 24 01:44:04 CEST 2015


On Mon, Oct 12, 2015 at 10:10 AM, Michael Bradshaw <mjbshaw at google.com>
wrote:
>
> On Sun, Oct 11, 2015 at 12:13 AM, Steve Lhomme <slhomme at matroska.org>
> wrote:
>
>> 2015-10-05 18:07 GMT+02:00 Michael Bradshaw <mjbshaw at google.com>:
>> > On Sun, Oct 4, 2015 at 6:43 AM, Steve Lhomme <slhomme at matroska.org>
>> wrote:
>> >>
>> >> On Sep 29, 2015 03:04, "Michael Bradshaw" <mjbshaw at google.com> wrote:
>> >>
>> >> > What’s the point of default values for non-mandatory elements in the
>> MKV
>> >> > spec? Why not make them mandatory if they have a default value?
>> >>
>> >> You could set the element in the file if you want, but if it's the
>> default
>> >> value (likely a very common value) you don't have to write it, the
>> semantic
>> >> reader will get it.
>> >
>> >
>> > But is there any real difference between a non-mandatory element with a
>> > default value, and a mandatory element with a default value? The two
>> seem
>> > effectively equivalent. If there's no meaningful difference, why not
>> just
>> > make any element with a default value a mandatory element?
>>
>> In the of mandatory, you don't even have to write it in the file at
>> all. Whenever you read the parent element, either it's there and you
>> read the value (if there's one) or you assume you have the default
>> value.
>>
>> If it's not mandatory and not written, then you do not have the
>> element and its value. So if you want the element you have to write at
>> least the ID and a size of 0. Each time you want it.
>
>
> Interesting. The current EBML spec says signed/unsigned integers, dates,
> and float elements have the value 0 if their octet length is zero[1]. To
> make sure I'm understanding you correctly, you're saying that optional
> elements can have a default value (specified by the document spec) that is
> used if their octet length is zero (even if their default value is not 0),
> correct? If that's the case, could this be clarified in the EBML/MKV specs?
> I don't see that behavior mentioned in the MKV spec, and the EBML spec
> makes it sound like these elements always get the value of 0 (with no
> indication of other default values being possible).
>
> [1]:
> https://github.com/Matroska-Org/ebml-specification/blob/master/specification.markdown#ebml-element-types
>

Any clarifications on this point?
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