[FFmpeg-devel] [PATCH] avcodec: Vorbis decode: don't use a flag to determine if frames have been output

[Paul B Mahol]

On Thu, Sep 8, 2022 at 10:26 AM <jyrkive at nekonyansoft.com> wrote:

> From: Jyrki Vesterinen <jyrkive at nekonyansoft.com>
>
> If a developer using FFmpeg libraries seeks into an earlier position and
> calls
> avcodec_flush_buffers() afterwards as recommended, the Vorbis decoder will
> drop
> the next frame, since buffer flushing clears the first_frame flag. As a
> result,
> the audio samples the calling code receives may be ahead of the requested
> seek
> position, which is unacceptable in some use cases such as playing a looping
> sound effect.
>
> This commit removes the first_frame flag entirely and instead uses the
> presentation timestamp to determine if it's the first frame.
>

Proper solution is to fetch initial/first pts and use that one instead
using of using
fragile pts < 0.


> ---
>  libavcodec/vorbisdec.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/libavcodec/vorbisdec.c b/libavcodec/vorbisdec.c
> index 4d03947c49..d4b030d7b9 100644
> --- a/libavcodec/vorbisdec.c
> +++ b/libavcodec/vorbisdec.c
> @@ -130,7 +130,6 @@ typedef struct vorbis_context_s {
>      AVFloatDSPContext *fdsp;
>
>      FFTContext mdct[2];
> -    uint8_t       first_frame;
>      uint32_t      version;
>      uint8_t       audio_channels;
>      uint32_t      audio_samplerate;
> @@ -1845,8 +1844,7 @@ static int vorbis_decode_frame(AVCodecContext
> *avctx, AVFrame *frame,
>      if ((len = vorbis_parse_audio_packet(vc, channel_ptrs)) <= 0)
>          return len;
>
> -    if (!vc->first_frame) {
> -        vc->first_frame = 1;
> +    if (frame->pts < 0) {
>          *got_frame_ptr = 0;
>          av_frame_unref(frame);
>          return buf_size;
> @@ -1881,7 +1879,6 @@ static av_cold void
> vorbis_decode_flush(AVCodecContext *avctx)
>                               sizeof(*vc->saved));
>      }
>      vc->previous_window = -1;
> -    vc->first_frame = 0;
>  }
>
>  const FFCodec ff_vorbis_decoder = {
> --
> 2.37.2.windows.2
>
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