[FFmpeg-devel] Re: [PATCH] libavcodec: support frame dropping in libvpxenc

[James Zern via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

Anton,

On Wed, Nov 26, 2025 at 10:29 AM Dariusz Marcinkiewicz via ffmpeg-devel <
ffmpeg-devel@ffmpeg.org> wrote:

> vp8 encoder can be configured to drop frames, when e.g. bitrate
> overshoot is detected. At present the code responsible for
> managing an internal fifo assumes that we will get an output frame per
> each frame fed into encoder. That is not the case if the encoder can
> decide to drop frames.
>
> Running:
> ffmpeg -stream_loop 100 -i dash_video3.webm -c:v libvpx -b:v 50k
> -drop-threshold 20 -screen-content-mode 2 output.webm
>
> results in lots of warnings like:
> [libvpx @ 0x563fd8aba100] Mismatching timestamps: libvpx 2187 queued
> 2185; this is a bug, please report it
> [libvpx @ 0x563fd8aba100] Mismatching timestamps: libvpx 2189 queued
> 2186; this is a bug, please report it
>
> followed by:
> [vost#0:0/libvpx @ 0x563fd8ab9b40] [enc:libvpx @ 0x563fd8aba080] Error
> submitting video frame to the encoder
> [vost#0:0/libvpx @ 0x563fd8ab9b40] [enc:libvpx @ 0x563fd8aba080] Error
> encoding a frame: No space left on device
> [vost#0:0/libvpx @ 0x563fd8ab9b40] Task finished with error code: -28
> (No space left on device)
> [vost#0:0/libvpx @ 0x563fd8ab9b40] Terminating thread with return code
> -28 (No space left on device)
>
> The reason for the above error is that each dropped frame leaves an
> extra item in the fifo, which eventually overflows.
>
> The proposed fix is to keep popping elements from the fifo until the
> one with the matching pts is found. A side effect of this change is that
> the code no longer considers pts mismatch to be a bug.
>
> This has likely regressed around 5bda4ec6c3cb6f286bb40dee4457c3c26e0f78cb
> when fifo started to be universally used.
>

If you're around could you have a look at this?


>
> Signed-off-by: Dariusz Marcinkiewicz <darekm@google.com>
> ---
>  libavcodec/libvpxenc.c | 31 ++++++++++++++++++++++++-------
>  1 file changed, 24 insertions(+), 7 deletions(-)
>
> diff --git a/libavcodec/libvpxenc.c b/libavcodec/libvpxenc.c
> index 05c34a6857..c537c96603 100644
> --- a/libavcodec/libvpxenc.c
> +++ b/libavcodec/libvpxenc.c
> @@ -383,6 +383,18 @@ static int frame_data_submit(AVCodecContext *avctx,
> AVFifo *fifo,
>      }
>
>      ret = av_fifo_write(fifo, &fd, 1);
> +    if (ret == AVERROR(ENOSPC)) {
> +      FrameData fd2;
> +
> +      av_log(avctx, AV_LOG_WARNING, "Fifo full, will drop a front
> element\n");
> +
> +      ret = av_fifo_read(fifo, &fd2, 1);
> +      if (ret >= 0) {
> +        frame_data_uninit(&fd2);
> +        ret = av_fifo_write(fifo, &fd, 1);
> +      }
> +    }
> +
>      if (ret < 0)
>          goto fail;
>
> @@ -398,13 +410,18 @@ static int frame_data_apply(AVCodecContext *avctx,
> AVFifo *fifo, AVPacket *pkt)
>      uint8_t *data;
>      int ret = 0;
>
> -    if (av_fifo_peek(fifo, &fd, 1, 0) < 0)
> -        return 0;
> -    if (fd.pts != pkt->pts) {
> -        av_log(avctx, AV_LOG_WARNING,
> -               "Mismatching timestamps: libvpx %"PRId64" queued
> %"PRId64"; "
> -               "this is a bug, please report it\n", pkt->pts, fd.pts);
> -        goto skip;
> +    while (1) {
> +      if (av_fifo_peek(fifo, &fd, 1, 0) < 0)
> +          return 0;
> +
> +      if (fd.pts == pkt->pts) {
> +        break;
> +      }
> +
> +      av_log(avctx, AV_LOG_DEBUG, "Dropped frame with pts %"PRId64"\n",
> +             fd.pts);
> +      av_fifo_drain2(fifo, 1);
> +      frame_data_uninit(&fd);
>      }
>
>      pkt->duration = fd.duration;
> --
> 2.52.0.487.g5c8c507ade-goog
>
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