Re: [FFmpeg-devel] [PATCH v4] avfilter/setpts: add option to decide framerate handling

["Ronald S. Bultje" <rsbultje@gmail.com>]

Hi,

On Sat, Jan 25, 2025 at 11:06 PM Gyan Doshi <ffmpeg@gyani.pro> wrote:

>
>
> On 2025-01-26 12:49 am, Marton Balint wrote:
> >
> >
> > On Sat, 25 Jan 2025, Gyan Doshi wrote:
> >
> >> In f121d95, the outlink framerate was unconditionally unset.
> >> This breaks/bloats outputs from CFR muxers unless the user explicitly
> >> sets a sane framerate. And the most common invocation for setpts seen in
> >> workflows, our docs and across the web is `PTS-STARTPTS` or others of
> >> the
> >> general form `PTS+constant` which preserves the input framerate.
> >>
> >> Fixes #11428
> >> ---
> >> v4: negated option sense and renamed to vfr
> >>
> >> doc/filters.texi          | 6 ++++++
> >> libavfilter/setpts.c      | 6 +++++-
> >> tests/fate/hevc.mak       | 2 +-
> >> tests/fate/mov.mak        | 2 +-
> >> tests/filtergraphs/setpts | 2 +-
> >> 5 files changed, 14 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/doc/filters.texi b/doc/filters.texi
> >> index b926b865ae..ea11d045ec 100644
> >> --- a/doc/filters.texi
> >> +++ b/doc/filters.texi
> >> @@ -31478,6 +31478,12 @@ This filter accepts the following options:
> >> @item expr
> >> The expression which is evaluated for each frame to construct its
> >> timestamp.
> >>
> >> +@item vfr (@emph{video only})
> >> +Boolean option which determines if the original framerate metadata
> >> is unset.
> >> +If set to true, be advised that a sane frame rate should be explicitly
> >> +specified if output is sent to a constant frame rate muxer.
> >
> > I propose a more understandable variant for the first sentence:
> >
> > Sets the filter output to variable frame rate by dropping the original
> > constant framerate information if present. If set to true...
>
> But that's not actually the case. This option does not make the output
> VFR. If it did, this option would not be needed.
> Once FR is unset, if the output goes to a CFR muxer and fps_mode is not
> specified, ffmpeg will emit a  CFR stream using the time_base.
> What the option does is a single narrow technical thing which has
> implications depending on context. And that was the basis for the
> original option name and description.
>
> 'strip_fps' and a corresponding description seems more accurate.


(I'm the bug reporter.) I actually agree. Setting FPS doesn't mean CFR, it
could just mean average frame rate or something. But please decide on
something, we're currently stripping said metadata and there's no way to
reinstate it except with lengthy hacks that we don't want people to start
posting on stackoverflow - because then we'll never hear the end of it.

Ronald
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