Re: [FFmpeg-devel] [PATCH 1/2] avformat/matroskaenc: Don't create wrong packet durations

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

James Almer:
> On 9/29/2023 1:01 PM, Andreas Rheinhardt wrote:
>> We have to write an explicit BlockDuration element (and use
>> a BlockGroup instead of a SimpleBlock) in case the Track
>> has a DefaultDuration that is inconsistent with the duration
>> of the packet.
>>
>> The matroska-h264-remux test uses a file with coded fields
>> where the duration of a Block is the duration of a field,
>> not of a frame, therefore this patch writes said BlockDuration
>> elements.
>>
>> (When using a BlockGroup, one has to add ReferenceBlock elements
>> to distinguish keyframes from non-keyframes. Unfortunately,
>> the AV1 codec mapping [1] requires us to reference all references
>> and to really use the real references, which requires a lot of
>> effort for basically no gain. When BlockGroups are used with AV1,
>> the created files are most likely invalid, both before and after
>> this patch, but this patch makes this more likely to happen.)
> 
> AV1 has no fields, and the vast majority of samples will also have fixed
> frame duration across the entire coded stream, so i doubt this will make
> a difference.
> 
> There's also the fact it's unlikely parsers will care about
> ReferenceBlock elements at all to begin with, and instead just have the
> decoder handle things relying on the bitstream level information.
> 

That's why I consider this requirement to be nonsense. In fact, I would
only require one thing from the ReferenceBlock elements: If one
recursively goes back the ReferenceBlock elements until one has reached
a keyframe, then decoding these blocks is sufficient to be able to
decode the actually desired block. That way a simple muxer like ours
could always reference the preceding block (in case the current packet
is not a keyframe) and create a valid stream.

- Andreas

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