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Re: [webrtc-svc] how to specify L3T3? (#61)

From: Sergio Garcia Murillo via GitHub <sysbot+gh@w3.org>
Date: Mon, 10 Jan 2022 08:41:04 +0000
To: public-webrtc-logs@w3.org
Message-ID: <issue_comment.created-1008648759-1641804062-sysbot+gh@w3.org>
In webrtc svc we only support SRST https://www.w3.org/TR/webrtc-svc/#dfn-srst so the second snipped is the correct one. There should be no need to specify the rid at all (as we don't support MRST). Regarding controlling the downscale factor, I am not sure if it is supported by all of the codecs, or if they are fixed values in the codec spec. 

The first code snippet would set up 3 simulcast layers, each one with L3T3, which is something allowed by the spec IIRC:

When sendEncodings is used to request the sending of multiple simulcast streams using addTransceiver(), it is not possible to configure the sending of "S" scalability modes. The browser may only be configured to send simulcast encodings with multiple SSRCs and RIDs, or alternatively, to send all simulcast encodings on a single RTP stream. Attempting to simultaneously utilize both simulcast transport techniques MUST return OperationError in setParameters() or addTransceiver().

But not sure if we should only allow using temporal scalability modes when simulcast is used. For example:

To set up 3 simulcast layers with 3 temporal layers each, the code should be:

pc.addTransceiver(stream.getVideoTracks()[0], {
  direction: 'sendonly',
  sendEncodings: [
    {rid: 'q', scaleResolutionDownBy: 4.0, scalabilityMode: 'L1T3'}
    {rid: 'h', scaleResolutionDownBy: 2.0, scalabilityMode: 'L1T3'},
    {rid: 'f', scalabilityMode: 'L1T3'},

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