RTCRtpEncodingParameters.scaleResolutionDownTo

[henbos]

Request for Mozilla Position on an Emerging Web Specification

• Specification title: WebRTC Extensions (section "RTCRtpEncodingParameters extensions", attribute "scaleResolutionDownTo")
• Specification or proposal URL (if available): https://w3c.github.io/webrtc-extensions/#dom-rtcrtpencodingparameters-scaleresolutiondownto
• Proposal author(s) (@-mention GitHub accounts): @henbos
• Mozillians who can provide input (optional): @jan-ivar
• WebKit standards-position: RTCRtpEncodingParameters.scaleResolutionDownTo WebKit/standards-positions#396

Other information

This adds a new encoding parameter to WebRTC allowing the app to say which resolution it wants to send in absolute terms ("send 360p") instead of relative terms ("downscale by 2" + frame is 720p), avoiding race conditions when the track dynamically changes size.

Example use case is sending {360p, 720p} simulcast and when dynamically disabling the 720p layer also adjusting the track size to 360p in order to avoid expensive effects processing in 720p when only 360p is sent.

See https://www.w3.org/2024/08/27-webrtc-minutes.html#t06 where this was discussed and approved by the W3C WebRTC WG.

[jan-ivar]

I propose a positive position (disclaimer: I was involved with specifying this). Having apps express desired outcome more directly without relying on track constraints seems good for interop in the long term.

Main concerns like rotation (the specified functionality is rotation agnostic) and interaction with scaleResolutionDownBy (overrides it and throws if not present on all layers) have been addressed.

I understand this has shipped in M131.