[FFmpeg-devel] [PATCH V1 0/2] Use avctx->framerate first for frame rate setting

[mypopy at gmail.com]

On Sun, Apr 28, 2019 at 5:30 PM Gyan <ffmpeg at gyani.pro> wrote:
>
>
>
> On 28-04-2019 07:19 AM, mypopy at gmail.com wrote:
> > On Sat, Apr 27, 2019 at 8:22 PM Gyan <ffmpeg at gyani.pro> wrote:
> >>
> >>
> >> On 27-04-2019 05:25 PM, Carl Eugen Hoyos wrote:
> >>> 2019-04-27 13:17 GMT+02:00, Jun Zhao <mypopydev at gmail.com>:
> >>>> perfer avctx->framerate first than use avctx->time_base when
> >>>> setting the frame rate to encoder. 1/time_base is not the
> >>>> average frame rate if the frame rate is not constant.
> >>> But why would the average framerate be a good choice to set
> >>> the encoder timebase?
> >>>
> >> Also, note that x264/5 RC looks at the framerate.
> >> See
> >> https://code.videolan.org/videolan/x264/commit/c583687fab832ba7eaf8626048f05ad1f861a855
> >>
> >> I can generate a difference with x264 by setting -enc_time_base to
> >> different values (with vsync vfr).
> >> Maybe check that this change does not lead to a significant change in
> >> output. Although I think this would be still an improvement for those
> >> cases where r_frame_rate >> avg_frame_rate
> >>
> >> Gyan
> > Yes, framerate and time_base is not close correlation in vfr case,
> > e,g, I can setting the framerate = 60fps, but time_base = 1/1000 s,
> > then setting pts like:
> >
> > time_base = 1/1000 s = 1 millisecond
> > framerate = 60 fps per second
> > PTS       0----16----33----50----66----83----100 ...
> >
> > PTS delta  16    17    17    16    17    17 ...
> >
> > we will get 16ms * 20 frames + 17 ms * 40 frames = 1000ms
>
> I'm aware of the relationship between TB and PTS. My point is x264's RC
> adjusts its quantizer based on fps. You're changing that value so the
> output bitrate will change for the same input with the same encoder
> config if (avg_frame_rate) != (ticks * 1/TB).
>
> Gyan
in fact，this is the purpose of this patch, we used FFmpeg API to
setting the time_base/pts/framerate like above to tuning the PTS.