Re: [FFmpeg-devel] [PATCH v3 2/2] avcodec: add external dec libvvdec for H266/VVC - Rémi Denis-Courmont

From: "Rémi Denis-Courmont" <remi@remlab.net>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH v3 2/2] avcodec: add external dec libvvdec for H266/VVC
Date: Mon, 20 May 2024 21:03:26 +0300
Message-ID: <14303208.qZsRhMg2gi@basile.remlab.net> (raw)
In-Reply-To: <0101018f96da0e11-fcea07ae-0303-4aec-886f-69f734765228-000000@us-west-2.amazonses.com>

Le maanantaina 20. toukokuuta 2024, 19.33.43 EEST Cosmin Stejerean via ffmpeg-
devel a écrit :
> > Adding a disabled-by-default decoder is adding bloat, if there is not a
> > specific known reason why that is needed.

> > This is especially true for video decoders, where the external decoders
> > typically will have no or worse optimisations for DSP functions, no
> > thread support (or not accessible via libavcodec), and no integration
> > with hwaccel.
> 
> Is this an actual problem with vvdec?

Yes?

> To me it seems like a reasonably optimized decoder with support for
> threading, etc.

It seems to be vendoring SIMDe, which is 1) a bad thing according to most 
distro policies, 2) not as good as FFmpeg optimisations generally speaking and 
3) lacking several ISAs featured by FFmpeg.

> hwaccel decoding seems somewhat orthogonal

How exactly will that work then? Either vvdec is the default, and hwaccel 
won't work, or vvdec is not the default and it's essentially dead code.

> > Providing a non-default choice is a poor excuse for a bug. Most users
> > won't know how to do that, and many FFmpeg reverse dependencies don't
> > even allow it.
> 
> Need better documentation then?

Well, good luck with that. Documentation for working around as yet unknown 
bugs sounds a bit difficult to write, and even then, requiring manual decoder 
selection seems very user-hostile to me.

> Not sure that designing for the lowest common denominator of reverse
> dependencies is the best way to do things, but at a minimum one can rebuild
> ffmpeg with only one decoder enabled to get around this.

To the contrary, gstreamer, mpv, VLC et al. are better equipped to handle 
this, starting with a proper plugin architecture.

-- 
レミ・デニ-クールモン
http://www.remlab.net/

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