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

[Xiang, Haihao]

On Fri, 2022-07-29 at 09:46 +0000, Soft Works wrote:
> > -----Original Message-----
> > From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> > Ronald S. Bultje
> > Sent: Friday, July 29, 2022 10:50 AM
> > To: FFmpeg development discussions and patches <ffmpeg-
> > devel at 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 at 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