[Sframe] Partial decodability and IDUs

Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com> Thu, 19 November 2020 21:40 UTC

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From: Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com>
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Date: Thu, 19 Nov 2020 22:39:57 +0100
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Subject: [Sframe] Partial decodability and IDUs
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Hi all,

As most of you already know, this morning I made a presentation in 
AVTCORE introducing the topic about the need to specify an agnostic 
video codec packetization format.


I got an AP for creating an initial draft so it could be reviewed and 

However, there were two main concerns that we should address in this 
this group:

  * Historically, avtcore has explicitly designed not to be payload
    agnostic and  declined to standardized codec agnostic payload
    formats in  number of cases.  If that is to be changed, needs to be
    done deliberately.
  * Need to define the "minimum decoding unit" or "independently
    decodable unit", that SFrame will work with.

Regarding the second one

  * Full video frames (just use whatever is the encoder output)
  * Spatial layer frames
  * "independend decodable subframes" like h264 slices, vp8 partitions
    or av1 tiles which allows partial decodability which is mainly aimed
    for enhancing packet loss resilience.

Spatial layer frames is the minimum we should target as if not it will 
just prevent SFUs for using SVC codecs. So the question is if we should 
go deeper and implement lower partitions of the frames or not.

AFAIK, currently, libwertc does not support partial decodability and I 
personally haven't seen any practical usage of this in the RTC world 
(while it makes a lot of sense in streaming/broadcasting world), but 
would like to hear what is the view and experience of the other members 
of this group. Also note that if we are going to support them on SFrame 
this will require a greater effort because we will need to explicitly 
define how the frames must be split before being encrypted y SFrame for 
*each* possible video codec (h264,h265,vp8,vp9,av1,...).

There was also the question about how/if we should support other codec 
features like DON/interleaved mode for h264, which I also think we 
should not support mainly because we are not currently using it on 
webrtc implementations.

What do you think?

Best regards