Re: [FFmpeg-devel] [PATCH v3 2/2] ffmpeg_opt: consider HW acceleration method when selecting decoder

[Soft Works <softworkz@hotmail.com>]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> Xiang, Haihao
> Sent: Monday, August 1, 2022 3:53 AM
> To: ffmpeg-devel@ffmpeg.org
> Subject: Re: [FFmpeg-devel] [PATCH v3 2/2] ffmpeg_opt: consider HW
> acceleration method when selecting decoder
> 
> On Fri, 2022-07-29 at 09:46 +0000, Soft Works wrote:
> > > -----Original Message-----
> > > From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> > > Ronald S. Bultje
> > > Sent: Friday, July 29, 2022 10:50 AM
> > > To: FFmpeg development discussions and patches <ffmpeg-
> > > devel@ffmpeg.org>
> > > Subject: Re: [FFmpeg-devel] [PATCH v3 2/2] ffmpeg_opt: consider
> HW
> > > acceleration method when selecting decoder
> > >
> > > Hi,
> > >
> > > On Fri, Jul 29, 2022 at 4:38 PM Xiang, Haihao <
> > > haihao.xiang-at-intel.com@ffmpeg.org> wrote:
> > >
> > > > libdav1d is the preferred AV1 decoder in FFmpeg, libdav1d is
> always
> > >
> > > used
> > > > when
> > > > running the command below even if user expects vaapi or other
> HW
> > > > acceleration
> > > > methods.
> > > >
> > >
> > > I think that is a pretty serious issue. Don't we always want to
> > > prefer a hw
> > > decoder by default? I agree there should also be ways to force-
> select
> > > specific hw/sw decoders, but shouldn't hw be the default?
> >
> > I don't think that this would be reasonably possible in any way.
> > There are a lot of questions which ffmpeg cannot answer, e.g.:
> >
> > - Which hwaccel to choose?
> > - Which hwaccel is available?
> > - Which parameters are required for selecting a device
> >   that is working?
> > - Is the auto-selected device even capable to decode a certain
> >   input?
> >   (pixel format, bit depth, codec profile, frame size, ...)
> >
> > For the user who is creating the command line, it is important to
> > be able to rely on what is going to happen. If they can't command
> > lines will fail:
> >
> > - The outputs of hw decoders vary. Some output to hw format, some
> >   to sw format by default. You may need to specify
> hwaccel_output_format
> >   or use hwdownload
> > - Depending on the hwaccel, a totally different set of filters
> >   may be required (not to speak of encoders)
> > - How would you specify that you want a sw decoder instead?
> >
> > IMO, predictability is the topmost important behavioral aspect
> > for users (be it humans of machines.. ;-)
> 
> 
> I agree with you it would be better not to make hw the default if
> user doesn't
> ask for. However '-hwaccel arg' is used to specify hw acceleration
> method in the
> examples. According to doc (
> https://github.com/FFmpeg/FFmpeg/blob/master/doc/ffmpeg.texi#L1260-
> L1262), a hwdecoder is expected.
> 
> "@item -hwaccel[:@var{stream_specifier}] @var{hwaccel}
> (@emph{input,per-stream})
> Use hardware acceleration to decode the matching stream(s). "
> 
> This patch is to fix the issue that a SW decoder is chosen even if
> user is
> expecting a hw method, not to make hw the default in any case.
> 
> Thanks
> Haihao
> 

Oh, I'm in no way opposed to your patch, I just talked about the
idea of a general automatic selection of hardware decoders.

I agree that the behavior for AV1 should be the same as for the other
decoders.

Best wishes,
softworkz


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