Re: [FFmpeg-devel] [PATCH] ffmpeg: add per-stream input option drop_changed

[Marton Balint <cus@passwd.hu>]

On Sat, 15 Mar 2025, Gyan Doshi wrote:

> This is a replacement in ffmpeg for the deprecated avcodec flag AV_CODEC_FLAG_DROPCHANGED.
>
> This option is meant to be used when the filtergraph should not be
> reinited upon input parameter changes as that leads to loss of state
> in the filtergraph potentially leading to broken or aborted output,
> e.g. inserting of silence with first_pts specified in aresample.
>
> Generally useful to avoid corrupted yet decodable packets in live
> streaming inputs.
>
> This option when enabled takes precedence over reinit_filters
> ---
> doc/ffmpeg.texi         |  6 ++++++
> fftools/ffmpeg.h        |  2 ++
> fftools/ffmpeg_demux.c  | 13 ++++++++++++-
> fftools/ffmpeg_filter.c | 10 ++++++++++
> fftools/ffmpeg_opt.c    |  3 +++
> 5 files changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi
> index fca220a334..a73db79f94 100644
> --- a/doc/ffmpeg.texi
> +++ b/doc/ffmpeg.texi
> @@ -1373,6 +1373,12 @@ The properties where a change triggers reinitialization are,
> for video, frame resolution or pixel format;
> for audio, sample format, sample rate, channel count or channel layout.
>
> +@item -drop_changed[:@var{stream_specifier}] @var{integer} (@emph{input,per-stream})
> +This boolean option determines whether a frame with differing frame parameters mid-stream
> +gets dropped instead of leading to filtergraph reinitialization, as that would lead to loss
> +of filter state. Generally useful to avoid corrupted yet decodable packets in live streaming
> +inputs. Default is false.
> +
> @item -filter_threads @var{nb_threads} (@emph{global})
> Defines how many threads are used to process a filter pipeline. Each pipeline
> will produce a thread pool with this many threads available for parallel processing.
> diff --git a/fftools/ffmpeg.h b/fftools/ffmpeg.h
> index 86a3e10c6b..5869979214 100644
> --- a/fftools/ffmpeg.h
> +++ b/fftools/ffmpeg.h
> @@ -232,6 +232,7 @@ typedef struct OptionsContext {
>     SpecifierOptList filter_scripts;
> #endif
>     SpecifierOptList reinit_filters;
> +    SpecifierOptList drop_changed;
>     SpecifierOptList fix_sub_duration;
>     SpecifierOptList fix_sub_duration_heartbeat;
>     SpecifierOptList canvas_sizes;
> @@ -262,6 +263,7 @@ enum IFilterFlags {
>     IFILTER_FLAG_REINIT         = (1 << 1),
>     IFILTER_FLAG_CFR            = (1 << 2),
>     IFILTER_FLAG_CROP           = (1 << 3),
> +    IFILTER_FLAG_DROPCHANGED    = (1 << 4),
> };
>
> typedef struct InputFilterOptions {
> diff --git a/fftools/ffmpeg_demux.c b/fftools/ffmpeg_demux.c
> index b8171e336e..292b718f14 100644
> --- a/fftools/ffmpeg_demux.c
> +++ b/fftools/ffmpeg_demux.c
> @@ -67,6 +67,7 @@ typedef struct DemuxStream {
>     int                      reinit_filters;
>     int                      autorotate;
>     int                      apply_cropping;
> +    int                      drop_changed;
>
>
>     int                      wrap_correction_done;
> @@ -1099,7 +1100,8 @@ int ist_filter_add(InputStream *ist, InputFilter *ifilter, int is_simple,
>         return AVERROR(ENOMEM);
>
>     opts->flags |= IFILTER_FLAG_AUTOROTATE * !!(ds->autorotate) |
> -                   IFILTER_FLAG_REINIT     * !!(ds->reinit_filters);
> +                   IFILTER_FLAG_REINIT     * !!(ds->reinit_filters) |
> +                   IFILTER_FLAG_DROPCHANGED* !!(ds->drop_changed);
>
>     return 0;
> }
> @@ -1410,6 +1412,15 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st, AVDictiona
>     ds->reinit_filters = -1;
>     opt_match_per_stream_int(ist, &o->reinit_filters, ic, st, &ds->reinit_filters);
>
> +    ds->drop_changed = 0;
> +    opt_match_per_stream_int(ist, &o->drop_changed, ic, st, &ds->drop_changed);
> +
> +    if (ds->drop_changed && ds->reinit_filters) {
> +        if (ds->reinit_filters > 0)
> +            av_log(ist, AV_LOG_ERROR, "drop_changed and reinit_filters both set, which is contradictory; ignoring reinit_filters.\n");

I'd rather simply return an error, instead of continuing, unless there is 
a very good reason to allow conflicting options and fall back to one or 
the other.

> +        ds->reinit_filters = 0;
> +    }
> +
>     ist->user_set_discard = AVDISCARD_NONE;
>
>     if ((o->video_disable && ist->st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) ||
> diff --git a/fftools/ffmpeg_filter.c b/fftools/ffmpeg_filter.c
> index ef35c3988f..fddfbceec1 100644
> --- a/fftools/ffmpeg_filter.c
> +++ b/fftools/ffmpeg_filter.c
> @@ -126,6 +126,7 @@ typedef struct InputFilterPriv {
>
>     int                 eof;
>     int                 bound;
> +    uint64_t            nb_dropped;
>
>     // parameters configured for this input
>     int                 format;
> @@ -2898,6 +2899,13 @@ static int send_frame(FilterGraph *fg, FilterGraphThread *fgt,
>     } else if (ifp->downmixinfo_present)
>         need_reinit |= DOWNMIX_CHANGED;
>
> +    if (need_reinit && (ifp->opts.flags & IFILTER_FLAG_DROPCHANGED)) {
> +            ifp->nb_dropped++;
> +            av_log(fg, AV_LOG_DEBUG, "Avoiding reinit; dropping frame pts: %s bound for %s\n", av_ts2str(frame->pts), ifilter->name);

Maybe it would make sense to use AV_LOG_WARNING for the first dropped 
frame.

> +            av_frame_unref(frame);
> +            return 0;
> +    }
> +
>     if (!(ifp->opts.flags & IFILTER_FLAG_REINIT) && fgt->graph)
>         need_reinit = 0;
>
> @@ -3140,6 +3148,8 @@ read_frames:
>         ret = read_frames(fg, &fgt, fgt.frame);
>         if (ret == AVERROR_EOF) {
>             av_log(fg, AV_LOG_VERBOSE, "All consumers returned EOF\n");
> +            if (ifp->opts.flags & IFILTER_FLAG_DROPCHANGED)
> +                av_log(fg, AV_LOG_VERBOSE, "Total changed input frames dropped : %"PRId64"\n", ifp->nb_dropped);

I think you should make this AV_LOG_INFO and it is enough to print if the 
number of dropped frames is non-zero.

>             break;
>         } else if (ret < 0) {
>             av_log(fg, AV_LOG_ERROR, "Error sending frames to consumers: %s\n",
> diff --git a/fftools/ffmpeg_opt.c b/fftools/ffmpeg_opt.c
> index 1f5e6050c8..6ec325f51e 100644
> --- a/fftools/ffmpeg_opt.c
> +++ b/fftools/ffmpeg_opt.c
> @@ -1718,6 +1718,9 @@ const OptionDef options[] = {
>     { "reinit_filter",          OPT_TYPE_INT, OPT_PERSTREAM | OPT_INPUT | OPT_EXPERT,
>         { .off = OFFSET(reinit_filters) },
>         "reinit filtergraph on input parameter changes", "" },
> +    { "drop_changed",          OPT_TYPE_INT, OPT_PERSTREAM | OPT_INPUT | OPT_EXPERT,
> +        { .off = OFFSET(drop_changed) },
> +        "drop frame instead of reiniting filtergraph on input parameter changes", "" },
>     { "filter_complex",         OPT_TYPE_FUNC, OPT_FUNC_ARG | OPT_EXPERT,
>         { .func_arg = opt_filter_complex },
>         "create a complex filtergraph", "graph_description" },
> -- 
> 2.46.1

Thanks,
Marton
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