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w3c / webrtc-svc

W3C Scalable Video Coding (SVC) Extension for WebRTC
https://w3c.github.io/webrtc-svc/
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L3T1_KEY is missing from the table, but not the diagram section #81

Closed henbos closed 1 year ago

henbos commented 1 year ago

This seems like a mistake rather than intentional. See https://w3c.github.io/webrtc-svc/#L3T1_KEY*

If it was not for this semi-missing value, layer reduction maps well with the scalabilityMode string.

henbos commented 1 year ago

Ready for PR?

aboba commented 1 year ago

I have submitted PR https://github.com/w3c/webrtc-svc/pull/82 but there are some questions we need to answer before merging.

Looking at the AV1 bitstream specification, I do not see a value assigned for SCALABILITY_L3T1_KEY on page 123, nor is there a diagram for L3T1_KEY on page 125. This makes me wonder whether L3T1_KEY is actually a valid "canned" AV1 mode, supported in libaom. Is L3T1_KEY supported in Chrome?

We should probably check with Danil and/or Michael Horowitz to figure this out.

DanilChapovalov commented 1 year ago

L3T1_KEY is a valid scalability mode. Chrome should already support it both for AV1 and VP9.

AV1 spec doesn't have dedicated scalability_mode_idc for it in Metadata OBU, so encoder that wish to produce such optional OBU would have to use generic SCALABILITY_SS.

It was probably missed because L3T1_KEY_SHIFT is not a valid structure, and in the AV1 spec there is 1:1 relation between _KEY and _KEY_SHIFT structures.

aboba commented 1 year ago

Closed after merger of PR https://github.com/w3c/webrtc-svc/pull/82