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AOMediaCodec / av1-spec

AV1 Bitstream & Decoding Process Specification
https://aomedia.org/
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Level bitstream conformance requirement conditions are ambiguous #314

Open rzumer opened 4 years ago

rzumer commented 4 years ago

It is specified at section A.3, page 643, that:

When the mapped level is contained in the tables above, it is a requirement of bitstream conformance that the following constraints hold: (...)

  • FrameWidth is greater than or equal to 16
  • FrameHeight is greater than or equal to 16

Firstly, the wording of the first sentence is ambiguous, because some tables include undefined levels, reserved levels and maximum parameters. It is not clear whether the conformance requirements apply only to levels <= 6.3, or to all levels in the first table. I propose rewording the sentence to clarify which levels the list of requirements apply to.

Secondly, the frame size requirements are inconsistent with the reference encoder behavior. aomenc will encode frames below this size and set a level <= 6.3. See this comment for a sample. I assume that this is a bug in aomenc rather than a spec issue, but would like a confirmation.

xzhaao commented 3 years ago

For the first issue, "in the tables above" is indeed ambiguous. Maybe labels and captions should be added for tables to avoid such ambiguity when tables are being referenced in the spec, but it require a lot of changes. For this specific issue, clarifying the relevant levels sounds good enough.