[Mp4-tech] Question about definition of Profiles@Levels for VisualEncoder

[Vinay Sathe]

I think this line of questioning is deviating away from a purist MPEG
perspective.
It is appropriate to talk about compliance of the bitstream
produced by an encoder, not the encoder's compliance itself. For example,
quality and capability of
two different encoders producing Main Profile/ Main Level compliant
bitstreams could be
vastly different.
Regards,
Vinay
-----Original Message-----
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org]On Behalf Of Allen H.Y. Chin
Sent: Friday, August 01, 2023 6:50 AM
To: MP4-Tech
Subject: Re: [Mp4-tech] Question about definition of Profiles Levels for
VisualEncoder
Rob,
Thanks a lot for your explanation, but I still have a question regarding
"no lower bound".
>>And what are the lower bounds for certain Profile Level? it
>>it the upper bound of the lower level?
>>
>>
>
>There are no lower bounds for encoders / bitstreams. A compliant decoder
>must at minimum be able to decode all compliant bitstreams. In general,
>again, a conformance point gives a maximum complexity for bitstreams (and
>thus encoders), and minimum performance measures for decoders. Think of
>it as a bar - the decoder must be above the bar, the bitstreams (and
>encoder) must remain below the bar.
>

The CP L2 has a VCV decoder rate of 23760 MB/s,
which corresponds to twice CIF (its typical visual session size) at 30 Hz.
If encoder A can achieve real-time encoding rate higher than 23760 MB/s
(but it produce bitstream at a rate lower than 23760 MB/s for compliance),
can we say the encoder supports CP L2?
Encoder B is able to encode at a rate slightly lower than 23760 MB/s,
can we say it supports CP L2? Is it over-designed since it can not achieve
typical session size at 30Hz for CP L3?
Encoder C is able to encode at a rate slightly higher than 11880 MB/s,
can we say it supports CP L2?
Finally, if an encoder's max encoding rate is only 2970 MB/s or even worse,
can we say it supports CP L2?
Since encoders aren't standardized, and thus no lower bounds specified,
I'm confused about the meaning of "support" and why is there a "typical
session size".
I thought that "typical session size" was a lower bound (for encoder).
Perhaps it's not a good way to classify encoders using Profile Level?
Instead, taking max encoding rate as a measure when comparing
implementations might be a better choice?
Thank you again for your kind help.
Best regards,
Allen Chin
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