Re: [Cellar] AV1 mapping update

["Timothy B. Terriberry" <[e-mail]>]

Andreas Rheinhardt wrote:
>> It's not up to the container to solve this.
>>
> It is not up to the container to decide whether the possibly damaged
> frames between the DRAP and the KFDRP should be displayed, but it very

I would argue that this does have implications for file trimming. The 
Matroska mapping is allowed to place more requirements on an 
implementation than the AV1 specification. So there is a choice to make. 
Just considering the default (no further restrictions) or requiring one 
of the three example behaviors in the spec has the following implications:

If you choose to leave the handling of frames between the DRAP and the 
KFDRP implementation-defined, then someone cutting a stream has no idea 
how they will be handled. So the only safe thing to do is to strip them 
all. This adds some minor complexity burden to the remuxer, but 
guarantees that all such useless data will get stripped by a correct 
implementation.

Choosing to require decoding and display even when the reference frames 
are not available would produce a similar result (since packet loss 
concealment is not normative).

Choosing to require decoding and display of all frames that are 
guaranteed to be correct might allow a remuxer to cut at slightly more 
places without including an entire extra keyframe interval, but given 
that such frames are not required to be consecutive, this would get 
pretty complicated to implement (but at least it is optional: you can 
always fall back to stripping all such frames as above).

Choosing to require that only frames starting from the KFDRP be decoded 
and displayed means that a remuxer does not need to strip such frames to 
guarantee consistent output from all players, allowing it to be slightly 
simpler, at the cost of allowing it to include some data that is useless.

None of this helps you for seeking, because you don't have a choice 
about which frames to include, so one must rely on the player not to 
display garbage (in the case that references are missing) or stuttering 
output (in the case that only some frames between the DRAP and KFDRP are 
correctly decodable).