On Wed, Nov 20, 2019 at 6:17 PM peichuan zhang
> Thank you, James
> It's very good to know that the level information is transmitted out of band, and that's where I was confused about. But I think Profile and Level
> place constrants on both sides, the encoder and the decoder. The decoder needs to know if it's capable of decoding a particular bitstream
> at certain level. That's why I was confused when I found there's no level syntax.
You're right some decoders may want this information, which as noted
can be provided out of band. For inspecting files offline you can use
webm_info from libwebm  to calculate the level (use -vp9_level or
> 在 2019年11月21日星期四 UTC+8上午8:08:25，James Zern写道：
>> On Tue, Nov 19, 2019 at 3:56 AM peichuan zhang
>> > Hi there, I found that there is no syntax for level id in vp9 spec( vp9-bitstream-specification-v0.6-20160331-draft.pdf), but there is level defination both online(https://www.webmproject.org/vp9/levels/
) and in libvpx. So, how does the level constraint work?
>> Profiles and levels are just a description of the properties of a
>> particular bitstream, they don't affect the decode process, they're
>> used to constrain the bitstream at encode time. In vp9's case this
>> information is transmitted out of band, e.g. via the container or dash
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