Re: [Cellar] AV1 mapping update

["Timothy B. Terriberry" <[e-mail]>]

Andreas Rheinhardt wrote:
> "The presentation times of AV1 samples are given by the Matroska
> container. The [timing_info_present_flag] in the `Sequence Header OBU`
> (in the `CodecPrivate` or in the bitstream) SHOULD be set to 0. If set

I'll ask the usual question: when is it reasonable to violate this SHOULD?

> width as [max_frame_width_minus_1]+1 (similar for height). But what I'd
> like to know is how much of AV1 will probably be excluded from Matroska
> by these requirements? Could we contact some of the codec designers and
> ask them about their opinions on this? (Honestly, "we" probably means

Hi, I am one of the codec designers. I think the expectation is that 
DisplayWidth and DisplayHeight will remain constant for a coded video 
sequence (and these should match [max_frame_width_minus_1] + 1 and 
[max_frame_height_minus_1] + 1). If the output dimensions of a frame 
change, it should be scaled to the display resolution before being 
displayed. Spatial scalability depends on this (at least if you ever 
intend to drop some of the layers, and if you don't then there is no 
point to using scalability). Even without spatial scalability, it is 
sometimes desirable to reduce the coded resolution to maintain quality 
while fitting within the instantaneous channel bandwidth. That mostly 
applies to live/low-latency streams, but I think an important use case 
for Matroska is to be able to capture such streams for long-term storage 
(for example, via WebRTC and the MediaStream Recording API in web browsers).

> [operating_parameters_info]. Furthermore the dimensions of all output
> frames MUST be equal."

As such, I disagree with this part (as an individual).

> "A SimpleBlock MUST be marked as a Keyframe only if the first Frame OBU
> in the Block has a [frame_type] of KEY_FRAME and the SimpleBlock
> contains a Sequence Header OBU or if the Sequence Header OBU is
> correctly omitted (see above)."

This is mathematically correct (assuming you really mean "only if" and 
not "if and only if), but I think it could be easily misread. Perhaps 
"MUST NOT... unless..."?

> Seeking to a non-RAP is undefined and not recommended.

RECOMMENDED is an RFC 2119 keyword (but "NOT RECOMMENDED" is not 
explicitly listed as one). You might want to rephrase to avoid confusion.