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

[Cosmin Stejerean via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

On May 18, 2024, at 10:48 PM, Rémi Denis-Courmont <remi@remlab.net> wrote:

Le 18 mai 2024 21:55:04 GMT+03:00, Cosmin Stejerean via ffmpeg-devel <ffmpeg-devel@ffmpeg.org <mailto:ffmpeg-devel@ffmpeg.org> > a écrit :


On May 18, 2024, at 7:04 AM, Nuo Mi <nuomi2021@gmail.com <mailto:nuomi2021@gmail.com> > wrote:

This happened many years ago. See the discussion here:
https://patchwork.ffmpeg.org/project/ffmpeg/patch/20201221060710.12230-6-nuomi2021@gmail.com/#60589 <https://patchwork.ffmpeg.org/project/ffmpeg/patch/20201221060710.12230-6-nuomi2021@gmail.com/#60589> 
Now that we have an internal vvc decoder, we can focus on improving it.
As for the encoder, it is far more complex than the decoder. Reasonable to
wrapper other libraries just like libx264 and libx265...

I'm all for improving the internal decoder, and I agree that the internal decoder should be preferred and be used by default, but it's not clear why that should preclude adding an external decoder as an option.

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), 

Is this an actual problem with vvdec? To me it seems like a reasonably optimized decoder with support for threading, etc.

and no integration with hwaccel.

The later point has become a problem even with dav1d...

hwaccel decoding seems somewhat orthogonal


Sometimes bugs are found in the internal decoders

Providing a non-default choice is a poor excuse for a bug. Most users won't know how to do that, 

need better documentation?

and many FFmpeg reverse dependencies don't even allow it.

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


or some functionality may be missing, and having the option to fallback to an external decoder as a workaround is very useful in practice.

That I can agree, but what do you find missing in the VVC decoder compared to libvvcdec?
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This I'm not sure, perhaps there is no feature gap? Does the internal VVC decoder supports all features of the Main10 profile?

- Cosmin
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