[FFmpeg-devel] [PATCH 2/4] lavf/utils: avoid decoding a frame to get the codec parameters

[Ronald S. Bultje]

Hi,

On Mon, Nov 2, 2015 at 5:45 AM, Matthieu Bouron <matthieu.bouron at gmail.com>
wrote:

> From: Matthieu Bouron <matthieu.bouron at stupeflix.com>
>
> Avoid decoding a frame to get the codec parameters while the codec
> supports FF_CODEC_CAP_SKIP_FRAME_FILL_PARAM. This is particulary useful
> to avoid decoding twice images (once in avformat_find_stream_info and
> once when the actual decode is made).
> ---
>  libavformat/utils.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/libavformat/utils.c b/libavformat/utils.c
> index 5c4d452..ba62f2b 100644
> --- a/libavformat/utils.c
> +++ b/libavformat/utils.c
> @@ -2695,6 +2695,8 @@ static int try_decode_frame(AVFormatContext *s,
> AVStream *st, AVPacket *avpkt,
>      AVFrame *frame = av_frame_alloc();
>      AVSubtitle subtitle;
>      AVPacket pkt = *avpkt;
> +    int do_skip_frame = 0;
> +    enum AVDiscard skip_frame;
>
>      if (!frame)
>          return AVERROR(ENOMEM);
> @@ -2733,6 +2735,12 @@ static int try_decode_frame(AVFormatContext *s,
> AVStream *st, AVPacket *avpkt,
>          goto fail;
>      }
>
> +    if (st->codec->codec->caps_internal &
> FF_CODEC_CAP_SKIP_FRAME_FILL_PARAM) {
> +        do_skip_frame = 1;
> +        skip_frame = st->codec->skip_frame;
> +        st->codec->skip_frame = AVDISCARD_ALL;
> +    }
> +
>      while ((pkt.size > 0 || (!pkt.data && got_picture)) &&
>             ret >= 0 &&
>             (!has_codec_parameters(st, NULL) ||
> !has_decode_delay_been_guessed(st) ||
> @@ -2768,6 +2776,10 @@ static int try_decode_frame(AVFormatContext *s,
> AVStream *st, AVPacket *avpkt,
>          ret = -1;
>
>  fail:
> +    if (do_skip_frame) {
> +        st->codec->skip_frame = skip_frame;
> +    }
> +
>      av_frame_free(&frame);
>      return ret;
>  }
> --
> 2.6.2


I think we need an assert in the try_decode loop to ensure that it indeed
did fill all the params. This is to prevent the case where someone adds a
new "thing" to the list of things required for find_stream_info to succeed,
and forgets to update the codecs.

(These features break easily and subtly, and ideally we'd detect such
breakage in fate, not 6 months after the next release.)

Ronald