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

[Paul B Mahol <onemda@gmail.com>]

On 9/8/22, jyrkive@nekonyansoft.com <jyrkive@nekonyansoft.com> wrote:
> From: Jyrki Vesterinen <jyrkive@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 records the presentation timestamp of the first frame and
> determines after that if the new frame is the first frame (possible after
> seeking to the start) by comparing its pts to the stored pts.
> ---
>  libavcodec/vorbisdec.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/libavcodec/vorbisdec.c b/libavcodec/vorbisdec.c
> index 4d03947c49..38a5367be3 100644
> --- a/libavcodec/vorbisdec.c
> +++ b/libavcodec/vorbisdec.c
> @@ -131,6 +131,7 @@ typedef struct vorbis_context_s {
>
>      FFTContext mdct[2];
>      uint8_t       first_frame;
> +    int64_t       initial_pts;
>      uint32_t      version;
>      uint8_t       audio_channels;
>      uint32_t      audio_samplerate;
> @@ -1847,6 +1848,10 @@ static int vorbis_decode_frame(AVCodecContext *avctx,
> AVFrame *frame,
>
>      if (!vc->first_frame) {
>          vc->first_frame = 1;
> +        vc->initial_pts = frame->pts;
> +    }
> +
> +    if (frame->pts == vc->initial_pts) {
>          *got_frame_ptr = 0;
>          av_frame_unref(frame);
>          return buf_size;
> @@ -1881,7 +1886,6 @@ 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
>


LGTM

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