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

[Xiang, Haihao]

> On Wed, 2021-09-22 at 15:42 +0800, Haihao Xiang wrote:
> > Usually a HW decoder is expected when user specifies a HW acceleration
> > method via -hwaccel option, however the current implementation doesn't
> > take HW acceleration method into account, it is possible to select a SW
> > decoder.
> > 
> > For example:
> > 
> > $> ffmpeg -hwaccel vaapi -i av1.ivf -f null -
> > ...
> > Stream #0:0 -> #0:0 (av1 (libdav1d) -> wrapped_avframe (native))
> > 
> > libdav1d is selected in this case even if vaapi is specified.
> > 
> > After applying this patch, the native av1 decoder (with vaapi support)
> > is selected.
> > ...
> > Stream #0:0 -> #0:0 (av1 (native) -> wrapped_avframe (native))
> 
> Any comment for this patchset? We may simplify the command line using HW
> acceleration method after applying this patchset.

Ping again. 

According to the help string for -hwaccel option, HW accelerated decoding is
used if -hwaccel arg is specified. However in the current code, hwaccel is not
taken into account when choosing a codec for the video stream, (see 
https://github.com/FFmpeg/FFmpeg/blob/master/fftools/ffmpeg_opt.c#L792-L805).

For example, libdav1d is selected (if libdav1d is available) when running the
command below because libdav1d has higher priority than the native av1 decoder
in the current FFmpeg. 

$> ffmpeg -hwaccel vaapi -i av1.ivf -f null -

(Note `ffmpeg -hwaccel vaapi -i input.h264 -f null -` works as expected, the
native h264 decoder is selected)

This patch enabled the automatic codec selection for hw accelerated decoding,
-hwaccel option can work as expected after applying this patch. 

Thanks
Haihao


> > ---
> >  fftools/ffmpeg_opt.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 43 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fftools/ffmpeg_opt.c b/fftools/ffmpeg_opt.c
> > index 2d9a8a2ea0..914078add9 100644
> > --- a/fftools/ffmpeg_opt.c
> > +++ b/fftools/ffmpeg_opt.c
> > @@ -804,6 +804,48 @@ static const AVCodec *choose_decoder(OptionsContext *o,
> > AVFormatContext *s, AVSt
> >          return avcodec_find_decoder(st->codecpar->codec_id);
> >  }
> >  
> > +static const AVCodec *choose_decoder2(InputStream *ist, OptionsContext *o,
> > AVFormatContext *s, AVStream *st)
> > +{
> > +    char *codec_name = NULL;
> > +
> > +    MATCH_PER_STREAM_OPT(codec_names, str, codec_name, s, st);
> > +    if (codec_name) {
> > +        const AVCodec *codec = find_codec_or_die(codec_name, st->codecpar-
> > > codec_type, 0);
> > 
> > +        st->codecpar->codec_id = codec->id;
> > +        if (recast_media && st->codecpar->codec_type != codec->type)
> > +            st->codecpar->codec_type = codec->type;
> > +        return codec;
> > +    } else {
> > +        if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&
> > +            ist->hwaccel_id == HWACCEL_GENERIC &&
> > +            ist->hwaccel_device_type != AV_HWDEVICE_TYPE_NONE) {
> > +            const AVCodec *p;
> > +            void *i = 0;
> > +
> > +            while ((p = av_codec_iterate(&i))) {
> > +                int j;
> > +
> > +                if (p->id != st->codecpar->codec_id ||
> > +                    !av_codec_is_decoder(p) ||
> > +                    !avcodec_get_hw_config(p, 0))
> > +                    continue;
> > +
> > +                for (j = 0; ;j++) {
> > +                    const AVCodecHWConfig *config =
> > avcodec_get_hw_config(p,
> > j);
> > +
> > +                    if (!config)
> > +                        break;
> > +
> > +                    if (config->device_type == ist->hwaccel_device_type)
> > +                        return p;
> > +                }
> > +            }
> > +        }
> > +
> > +        return avcodec_find_decoder(st->codecpar->codec_id);
> > +    }
> > +}
> > +
> >  /* Add all the streams from the given input file to the global
> >   * list of input streams. */
> >  static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
> > @@ -932,7 +974,7 @@ static void add_input_streams(OptionsContext *o,
> > AVFormatContext *ic)
> >              ist->hwaccel_pix_fmt = AV_PIX_FMT_NONE;
> >          }
> >  
> > -        ist->dec = choose_decoder(o, ic, st);
> > +        ist->dec = choose_decoder2(ist, o, ic, st);
> >          ist->decoder_opts = filter_codec_opts(o->g->codec_opts, ist->st-
> > > codecpar->codec_id, ic, st, ist->dec);
> > 
> >  
> >          ist->reinit_filters = -1;
> 
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