[FFmpeg-devel] [PATCH 3/3] WIP: lavf/utils: try to avoid decoding a frame to get the codec parameters

[Clément Bœsch]

On Wed, Oct 28, 2015 at 06:34:06PM +0100, Hendrik Leppkes wrote:
> On Sat, Oct 17, 2015 at 10:34 PM, Matthieu Bouron
> <matthieu.bouron at gmail.com> wrote:
> > From: Matthieu Bouron <matthieu.bouron at stupeflix.com>
> >
> > Avoid decoding twice images such as jpeg and png, once in the
> > avformat_find_stream_info and once when the actual decode is made.
> >
> > The decoder must honor the skip_frame option in order to skip
> > decoding. For now the AVDISCARD_ALL flag is only set for the mjpeg and
> > png decoders.
> > ---
> >  libavformat/utils.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> >
> > diff --git a/libavformat/utils.c b/libavformat/utils.c
> > index 689473e..67dfffc 100644
> > --- a/libavformat/utils.c
> > +++ b/libavformat/utils.c
> > @@ -2676,11 +2676,16 @@ static int has_codec_parameters(AVStream *st, const char **errmsg_ptr)
> >  static int try_decode_frame(AVFormatContext *s, AVStream *st, AVPacket *avpkt,
> >                              AVDictionary **options)
> >  {
> > +    int i;
> >      const AVCodec *codec;
> >      int got_picture = 1, ret = 0;
> >      AVFrame *frame = av_frame_alloc();
> >      AVSubtitle subtitle;
> >      AVPacket pkt = *avpkt;
> > +    int skip_frame;
> > +    static const enum AVCodecID no_decode_codecs[] = {
> > +        AV_CODEC_ID_MJPEG, AV_CODEC_ID_PNG,
> > +    };
> 
> Hardcoded lists of codecs in random places are never a good thing to have.
> If this is a feature we agree to have, its codecs should just get an
> internal capability that tells this code if it can parse all params
> without decoding.
> 

This list is supposed to be temporary (yes I know) until all other
decoders that currently support AVDISCARD_ALL set the information field
required for probing.

Not full frames decoding at probing but just filling the information will
also be useful for these codecs (I think there are about 20?).
Unfortunately, the changes are sensible so it's probably better to do it
progressively one by one, updating this white list progressively. A
blacklist system was also suggested earlier in the thread.

Maybe a FIXME/XXX should be added above? (And maybe even with a compiler
warning to not forget this?)

[...]

-- 
Clément B.
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