[FFmpeg-devel] [PATCH] avfilter/vsrc_ddagrab: add option to control pts offset removal

[Marton Balint]

On Tue, 11 Apr 2023, Jan Ekström wrote:

> This keeps the default behavior as-is by defaulting to true,
> while allowing to start the timestamps with the offset from
> av_gettime_relative epoch start. This allows for synchronization
> with other capture modules that utilize the same epoch start, such
> as DShow audio capture.

I don't think an option is needed. The zero based pts should simply be 
changed to wallclock based pts, most capture devices already work that 
way. ffmpeg.c can make pts 0 based if needed.

Also, there were arguments in the past for using av_gettime() for pts 
values and av_gettime_relative() for elapsed time measurement, and most 
capture devices tend to follow that unless I am mistaken.

Regards,
Marton


> ---
> libavfilter/vsrc_ddagrab.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/libavfilter/vsrc_ddagrab.c b/libavfilter/vsrc_ddagrab.c
> index 042842104f..14fb4abd10 100644
> --- a/libavfilter/vsrc_ddagrab.c
> +++ b/libavfilter/vsrc_ddagrab.c
> @@ -100,6 +100,7 @@ typedef struct DdagrabContext {
>     int        out_fmt;
>     int        allow_fallback;
>     int        force_fmt;
> +    int        start_pts_at_zero;
> } DdagrabContext;
>
> #define OFFSET(x) offsetof(DdagrabContext, x)
> @@ -123,6 +124,8 @@ static const AVOption ddagrab_options[] = {
>                                                    OFFSET(allow_fallback), AV_OPT_TYPE_BOOL,   { .i64 = 0    },       0,       1, FLAGS },
>     { "force_fmt",  "exclude BGRA from format list (experimental, discouraged by Microsoft)",
>                                                    OFFSET(force_fmt),  AV_OPT_TYPE_BOOL,       { .i64 = 0    },       0,       1, FLAGS },
> +    { "start_pts_at_zero", "start timestamps at PTS=0, removing any initial offset",
> +                                                   OFFSET(start_pts_at_zero), AV_OPT_TYPE_BOOL, { .i64 = 1   },       0,       1, FLAGS },
>     { NULL }
> };
>
> @@ -913,7 +916,7 @@ static int ddagrab_request_frame(AVFilterLink *outlink)
>         av_usleep(delay);
>     }
>
> -    if (!dda->first_pts)
> +    if (dda->start_pts_at_zero && !dda->first_pts)
>         dda->first_pts = now;
>     now -= dda->first_pts;
>
> -- 
> 2.40.0
>
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