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

[Hendrik Leppkes]

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.

>
>      if (!frame)
>          return AVERROR(ENOMEM);
> @@ -2719,6 +2724,14 @@ static int try_decode_frame(AVFormatContext *s, AVStream *st, AVPacket *avpkt,
>          goto fail;
>      }
>
> +    skip_frame = st->codec->skip_frame;
> +    for (i = 0; i < FF_ARRAY_ELEMS(no_decode_codecs); i++) {
> +        if (st->codec->codec_id == no_decode_codecs[i]) {
> +            st->codec->skip_frame = AVDISCARD_ALL;
> +            break;
> +        }
> +    }
> +
>      while ((pkt.size > 0 || (!pkt.data && got_picture)) &&
>             ret >= 0 &&
>             (!has_codec_parameters(st, NULL) || !has_decode_delay_been_guessed(st) ||
> @@ -2753,6 +2766,8 @@ static int try_decode_frame(AVFormatContext *s, AVStream *st, AVPacket *avpkt,
>      if (!pkt.data && !got_picture)
>          ret = -1;
>
> +    st->codec->skip_frame = skip_frame;
> +
>  fail:
>      av_frame_free(&frame);
>      return ret;
> --
> 2.6.1
>
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