[FFmpeg-devel] Re: [PATCH] lavfi/mpdecimate: fix three bugs in keep_count logic

[Dana Feng via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

Ah got it, thank you! Do I just create a pull request?

-----Original Message-----
From: hassan hany via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>
Sent: Wednesday, February 18, 2026 5:00 PM
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Cc: hassan hany <hassanhanyrashad@gmail.com>
Subject: [FFmpeg-devel] Re: [PATCH] lavfi/mpdecimate: fix three bugs in keep_count logic

Hi dana
you probably want to submit this to https://code.ffmpeg.org/ since thats where FFmpeg development happens nowadays (yes I am aware the readme says you should submit patches to the ML but its outdated)

On Wed, Feb 18, 2026 at 11:55 PM Dana Feng via ffmpeg-devel < ffmpeg-devel@ffmpeg.org> wrote:

> Hello! Just following up on this.
>
> From: Dana Feng
> Sent: Friday, February 6, 2026 4:38 PM
> To: 'ffmpeg-devel@ffmpeg.org' <ffmpeg-devel@ffmpeg.org>
> Subject: [PATCH] lavfi/mpdecimate: fix three bugs in keep_count logic
>
> Fix the following issues with the keep option:
>
> 1. No similarity check during keep period: The code returned early
>    during the keep period without checking if the frame was actually
>    similar to the reference.
>
> 2. keep_count doesn't reset on different frames: The counter should
>    track "consecutive similar frames," but when a different frame
>    arrived mid-count, keep_count didn't reset to 0. It only reset
>    after going negative (after a drop), causing the counter to
>    accumulate across non-consecutive similar frames and drop frames
>    earlier than expected.
>
> 3. Reference frame drift: When similar frames were kept due to
>    keep_count, they became the new reference. Over time, tiny
>    differences (sensor noise, compression artifacts) accumulated.
>    Frame N got compared to frame N-1 instead of the last different
>    frame, allowing gradual scene changes to slip through undetected.
>
> Now decimate_frame() checks similarity first, then applies keep_count
> logic, returning distinct values:
>    1 = drop (similar, over keep threshold)
>    0 = keep (different frame) - updates reference, resets keep_count
>   -1 = keep (similar, under keep threshold) - preserves reference
>
> Signed-off-by: Dana Feng danaf@twosigma.com<mailto:danaf@twosigma.com>
> ---
> libavfilter/vf_mpdecimate.c | 49 +++++++++++++++++++++++++------------
> 1 file changed, 34 insertions(+), 15 deletions(-)
>
> diff --git a/libavfilter/vf_mpdecimate.c b/libavfilter/vf_mpdecimate.c
> index 0fc95556b7..5b1654f671 100644
> --- a/libavfilter/vf_mpdecimate.c
> +++ b/libavfilter/vf_mpdecimate.c
> @@ -109,19 +109,18 @@ static int diff_planes(AVFilterContext *ctx,
> /**
>   * Tell if the frame should be decimated, for example if it is no much
>   * different with respect to the reference frame ref.
> + *
> + * @return 1 if frame should be dropped (similar, over keep threshold),
> + *         0 if frame should be kept (different),
> + *        -1 if frame should be kept (similar, under keep threshold)
>   */
> static int decimate_frame(AVFilterContext *ctx,
>                            AVFrame *cur, AVFrame *ref) {
>      DecimateContext *decimate = ctx->priv;
> +    int is_similar;
>      int plane;
>
> -    if (decimate->max_keep_count > 0 &&
> -        decimate->keep_count > -1 &&
> -        decimate->keep_count < decimate->max_keep_count) {
> -        decimate->keep_count++;
> -        return 0;
> -    }
>      if (decimate->max_drop_count > 0 &&
>          decimate->drop_count >= decimate->max_drop_count)
>          return 0;
> @@ -129,6 +128,7 @@ static int decimate_frame(AVFilterContext *ctx,
>          (decimate->drop_count-1) > decimate->max_drop_count)
>          return 0;
>
> +    is_similar = 1;
>      for (plane = 0; ref->data[plane] && ref->linesize[plane]; plane++) {
>          /* use 8x8 SAD even on subsampled planes.  The blocks won't
> match up with
>           * luma blocks, but hopefully nobody is depending on this to
> catch @@ -141,8 +141,21 @@ static int decimate_frame(AVFilterContext *ctx,
>                          cur->data[plane], cur->linesize[plane],
>                          ref->data[plane], ref->linesize[plane],
>                          AV_CEIL_RSHIFT(ref->width,  hsub),
> -                        AV_CEIL_RSHIFT(ref->height, vsub)))
> -            return 0;
> +                        AV_CEIL_RSHIFT(ref->height, vsub))) {
> +            is_similar = 0;
> +            break;
> +        }
> +    }
> +
> +    if (!is_similar)
> +        return 0;  /* Frame is different - keep it */
> +
> +    /* Frame is similar - apply keep_count logic */
> +    if (decimate->max_keep_count > 0 &&
> +        decimate->keep_count > -1 &&
> +        decimate->keep_count < decimate->max_keep_count) {
> +        decimate->keep_count++;
> +        return -1;  /* Similar but under keep threshold - keep
> + without
> updating ref */
>      }
>
>      return 1;
> @@ -200,29 +213,35 @@ static int filter_frame(AVFilterLink *inlink,
> AVFrame *cur)
>      DecimateContext *decimate = inlink->dst->priv;
>      AVFilterLink *outlink = inlink->dst->outputs[0];
>      int ret;
> +    int result = decimate->ref ? decimate_frame(inlink->dst, cur,
> decimate->ref) : 0;
>
> -    if (decimate->ref && decimate_frame(inlink->dst, cur, decimate->ref))
> {
> +    if (result == 1) {
> +        /* Drop: similar frame, over keep threshold */
>          decimate->drop_count = FFMAX(1, decimate->drop_count+1);
> -        decimate->keep_count = -1; // do not keep any more frames until
> non-similar frames are detected
> +        decimate->keep_count = -1;
> +    } else if (result == -1) {
> +        /* Keep: similar frame, under keep threshold - don't update ref */
> +        decimate->drop_count = FFMIN(-1, decimate->drop_count-1);
> +        if ((ret = ff_filter_frame(outlink, cur)) < 0)
> +            return ret;
>      } else {
> +        /* Keep: different frame - update ref and reset keep_count */
>          av_frame_free(&decimate->ref);
>          decimate->ref = cur;
>          decimate->drop_count = FFMIN(-1, decimate->drop_count-1);
> -        if (decimate->keep_count < 0) // re-enable counting similar
> frames to ignore before dropping
> -            decimate->keep_count = 0;
> -
> +        decimate->keep_count = 0;
>          if ((ret = ff_filter_frame(outlink, av_frame_clone(cur))) < 0)
>              return ret;
>      }
>
>      av_log(inlink->dst, AV_LOG_DEBUG,
>             "%s pts:%s pts_time:%s drop_count:%d keep_count:%d\n",
> -           decimate->drop_count > 0 ? "drop" : "keep",
> +           result > 0 ? "drop" : "keep",
>             av_ts2str(cur->pts), av_ts2timestr(cur->pts,
> &inlink->time_base),
>             decimate->drop_count,
>             decimate->keep_count);
>
> -    if (decimate->drop_count > 0)
> +    if (result > 0)
>          av_frame_free(&cur);
>
>      return 0;
> --
> 2.39.5
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