[FFmpeg-devel] yadif frame doubling - incorrect closed captioning

[Michael Niedermayer]

On Mon, Jan 14, 2019 at 11:11:59AM -0500, Devin Heitmueller wrote:
> > > In short, a centralized function would be good, but we probably need
> > > to think through what the API looks like so we don't have to introduce
> > > a new API in libavutil and then deprecate it once we want to make the
> > > splitting/combining logic work according to the spec.
> >
> > I fear that no matter how hard we try we will likely eventually run
> > into cases that it cannot handle
> 
> Oh, I completely agree.  Some captions are just going to be too
> screwed up to render.  However I think there are cases we can recover
> from, and in particular I would like to make sure that streams created
> with versions of ffmpeg before this patch continue to play properly
> (i.e. where the stream only has caption data on every other frame and
> the cc_count is 2x what it is supposed to be).
> 
> > also maybe 2 functions would keep this simpler
> > one to deal with temporal transformations (frame drop, duplicate, interpolate,
> > combine)
> > one to deal with spatial transformations, crop, pad, scale, rotate
> 
> There shouldn't be any need for a function for spatial
> transformations.  The expected cc_count is unrelated to the resolution
> of the video.  It's tied exclusively to the framerate and whether you
> are doing frame or field-level encoding when the video is interlaced.
> The underlying goal was to ensure that the bitrate of the caption data
> is a constant 9600bps and thus they wanted it to be spread evenly
> across the frames/fields.  When people encounter problems going from
> 1080i to 720p for example, it's because that also involves framerate
> conversion, not because the resolution of the individual video frames
> is being changed.

well, there are other types of side data too. for example motion
vectors would need to be updated on crop depending on which rectangle
is croped out

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I know you won't believe me, but the highest form of Human Excellence is
to question oneself and others. -- Socrates
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