[FFmpeg-devel] [PATCH 2/3] avformat/matroskaenc: write the FieldOrder element for non-interlaced video

[James Almer]

On 9/27/2016 6:36 PM, Hendrik Leppkes wrote:
> On Tue, Sep 27, 2016 at 11:14 PM, James Almer <jamrial at gmail.com> wrote:
>> On 9/27/2016 5:56 PM, Hendrik Leppkes wrote:
>>> On Tue, Sep 27, 2016 at 8:03 PM, James Almer <jamrial at gmail.com> wrote:
>>>> It's listed as mandatory in https://matroska.org/technical/specs/index.html
>>>>
>>>> Signed-off-by: James Almer <jamrial at gmail.com>
>>>> ---
>>>> The spec also mentions FieldOrder "MUST be ignored if FlagInterlaced is not
>>>> set to interlaced". Since it's a mandatory element, i interpreted that as a
>>>> demuxer guideline.
>>>>
>>>
>>> Its a mandatory field with a default value - that kind of definition
>>> is a bit weird, but the general consensus is that it does not have to
>>> be written if it would write the default anyway.
>>>
>>> - Hendrik
>>
>> I find it weird it was defined this way, considering the precedent set by
>> BlockDuration which is not tagged as mandatory but its description
>> explicitly says "This Element is mandatory when DefaultDuration is set for
>> the track".
>>
>> Alright then, patch dropped from the set.
>>
> 
> For the record, its written here:
> https://matroska.org/technical/specs/notes.html
> 
> 4. Mandatory
> - This element is mandatory in the file.
> - Mandatory elements with a default value may be left out of the file.
> In the absence of a mandatory element, the element's default value is
> used.
> - A mandatory element is not written if its parent is not in the file.

Ok, that clears things.

For that matter, since FlagInterlaced is also mandatory with a default
"Undetermined" value, i guess it would make sense to not write it in
such scenario.

> 
> The BlockDuration is a bit weird, but the same concept applies. If
> DefaultDuration is set, it becomes mandatory, but it still has a
> default value (ie. the DefaultDuration), so it doesn't have to be
> written if it matches this value.
> This whole concept of mandatory with default can be very confusing,
> but it is what it is.
> 
> And you conveniently left out the good part "This Element is mandatory
> when DefaultDuration is set for the track (but can be omitted as other
> default values).", specifically mentioning this case =)

Somehow missed that, heh.

> 
> - Hendrik
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