Re: [Cellar] AV1 mapping Matroska

Steve Lhomme <slhomme@matroska.org> Wed, 27 June 2018 09:30 UTC

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From: Steve Lhomme <slhomme@matroska.org>
Date: Wed, 27 Jun 2018 11:30:40 +0200
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To: Codec Encoding for LossLess Archiving and Realtime transmission <cellar@ietf.org>
Cc: Frank Galligan <fgalligan@google.com>
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Subject: Re: [Cellar] AV1 mapping Matroska
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2018-06-27 10:58 GMT+02:00 Moritz Bunkus <moritz=40bunkus.org@dmarc.ietf.org>rg>:
> Hey,
>
> I've only had time to skim over it. It's definitely understandable.
>
> For me the most important thing is to have the mapping in Matroska and WebM
> be identical. As the reference encoder already produces WebM files, I
> highly suggest we communicate with their developers regarding the
> mapping. I cannot find a document for AV1/WebM bindings at the moment,
> though…

Ah I didn't know that. Now I can see the code. Especially webmenc.c
with a TODO for Frank Galligan in there.

It way for some Wip In Progress code, because I have been appointed by
the powers that be at AOM to do the proper/official mapping.

At least this code doesn't write any CodecPrivate which could work,
except, as for VP9 we wouldn't know the decoding profile just from
reading the TrackEntry which is a major problem to select the proper
hardware decoder. And the MP4 mapping also integrates a kind of
CodecProvate with the Sequence Header OBU in there.

>> I will dig deeper but we may have to tweak CodecDelay to make it signed.
>
> You cannot change the type of an existing element. It would potentially
> break the interpretation of existing files created with the old
> semantics. Let's take the value 144, which as an unsigned EBML integer
> would be stored as 0x81 0x90. If you read that back as a signed EBML
> integer, you'd get the value -128.

Given a codec delay sounds like some time the codec needs when fed
data before it outputs data. So I have hope it matches and I did the
math wrong.

In any case I don't know if we should include the flags like MP4 does.
Because it seems very MP4 specific (and it's a draft and may go away).
Unlike the rest of their "Codec Private" it should not be fed to the
decoder (the OBUs we have can be fed as-is to the decoder).

> Kind regards,
> mosu
>
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-- 
Steve Lhomme
Matroska association Chairman