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Re: Temporal and spatial scalability support on RTCRtpEncodingParameters

From: Cullen Jennings (fluffy) <fluffy@cisco.com>
Date: Thu, 21 Sep 2017 16:30:07 +0000
To: Bernard Aboba <Bernard.Aboba@microsoft.com>
CC: Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com>, "public-webrtc@w3.org" <public-webrtc@w3.org>
Message-ID: <43A2DCC5-27BF-41A8-9A6D-31843557E280@cisco.com>

The different ontexts for how SVC is used, and what is trying to do at that point are so different for different applications that there never any way for the browser to guess what what the right thing to do it. The app needs to be able to have control of the encodings and browser needs to just enforce that the the boundaries placed by other envelopes such as SDP and congestion controls are not exceeded. 

> On Sep 16, 2017, at 3:54 PM, Bernard Aboba <Bernard.Aboba@microsoft.com> wrote:
> Currently support for scalable video coding (SVC) is out of scope of the WebRTC 1.0 API.  Nevertheless, it should be observed
> that support for decoding of SVC is supported in multiple browsers (Edge, Chrome, Firefox) without any new API surface, and
> SVC encoding is supported in Edge and Chrome (and perhaps other browsers) with minimal additional API support. 
> Since in several video codecs (VP8, VP9, AV1), decoders are required to be able to decode anything that the encoder can encode,
> it not clear that additional API functionality is required to support decoding of SVC streams.   For example, browsers that support
> VP8 (such as Edge) can decode a temporally encoded VP8 stream, even if they cannot support temporal encoding. 
> Therefore, the question is really about an API to control SVC encoding.  The ORTC API provides this via addition of
> RTCRtpEncodingParameters.dependencyEncodingIds. 
> While in theory this provides the application with granular control of SVC encoding, in practice implementers have observed that the
> decision on how many layers to encode is intimately connected to bandwidth estimation and congestion control, so it is really best left to the browser. 
> Just as it wouldn't make much sense for an application to try to implement its own bandwidth estimation using the Statistics API and then try to control congestion by continuously adjusting RTCRtpEncodings.maxBitrate,
> in practice, it makes little sense for an application to attempt to continuously calculate how many layers should be encoded, and then attempt to control that via RTCRtpEncodingParameters.dependencyEncodingIds.
> So in practice, implementations supporting SVC encoding tend to fall into two camps: 
> 1. APIs that say "Turn on SVC encoding, let the browser figure out how many layers to send".   For example, in Edge the selection of the H.264UC codec (a variant of H.264/SVC) represents a request for the 
> encoder to consider (but not necessarily to use at any given moment) temporal scalability.  
> 2. Interpretation of RTCRtpEncodingParameters.dependencyEncodingIds as a hint, meaning "Encode up to this many layers, but no more, with the browser given the flexibility to figure out how many layers can be encoded at a given instant."
> I believe that ORTC Lib takes this approach. 
> ________________________________________
> From: Sergio Garcia Murillo [sergio.garcia.murillo@gmail.com]
> Sent: Friday, September 1, 2017 1:45 AM
> To: public-webrtc@w3.org
> Subject: Temporal and spatial scalability support on RTCRtpEncodingParameters
> Hello all,
> I assume this topics has been already been discussed, but I have been
> digging on the mailing list and on the issue tracker and I found no
> reference to it.
> Are there any plans to support setting temporal and spatial scalability
> properties on the RTCRtpEncodingParameters, at least for SRST? Currently
> VP8 temporal scalability is supported by chrome (not sure about FF) when
> doing simulcast, but there will be no way of  controlling it from the
> app side once when transceivers are implemented.
> Best regards
> Sergio
Received on Thursday, 21 September 2017 16:30:32 UTC

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