[vlc-devel] [PATCH] vout: fix low framerate stuttering

[Romain Vimont]

On Tue, Aug 06, 2019 at 03:58:54PM +0200, Steve Lhomme wrote:
> It seems that you are overriding more cases than the "refresh" one. What are
> the consequences when first and drop_next_frame are false ?

(you mean when first or drop_next_frame are _true_)

In those cases, Thread() will wait until the deadline written (if it is
lower than 100ms) instead of 100ms (which is arbitrary).

For "first", it seems better (the following frame can be before 100ms).

For "drop_next_frame", I don't know: waiting 100ms or until the next
frame date before calling ThreadDisplayPicture() again is arbitrary.
Moreover, it seems that the current code immediately renders the next
frame (whatever its date) if drop_next_frame is set anyway (so dates are
not respected).

I did it that way because it made the code more straightforward, but
this is a weak reason. Would it be better to keep 100ms if
drop_next_frame is true?
> On 2019-08-06 15:07, Romain Vimont wrote:
> > In ThreadDisplayPicture(), when "refresh" was true, the output parameter
> > deadline was not written and the function returned a non-zero value.
> > 
> > As a consequence, in video_output.c:Thread(), the next loop iteration
> > waited for the max deadline (100ms). When the following frame target
> > date was before this deadline, the video was stuttering.
> > 
> > To avoid the problem, write the deadline before returning from
> > ThreadDisplayPicture(), so that Thread() does not wait more than
> > expected.
> > 
> > Since an existing frame is refreshed only every 80ms
> > (VOUT_REDISPLAY_DELAY), this happened only on low framerate videos
> > (<12.5 fps). Otherwise, "refresh" was always false and the problem never
> > occurred.
> > ---
> > You can test with this video at 10fps: https://tmp.rom1v.com/10fps.mp4
> > 
> >   src/video_output/video_output.c | 13 +++++++------
> >   1 file changed, 7 insertions(+), 6 deletions(-)
> > 
> > diff --git a/src/video_output/video_output.c b/src/video_output/video_output.c
> > index 4a2cde9f98..ef0d9488ae 100644
> > --- a/src/video_output/video_output.c
> > +++ b/src/video_output/video_output.c
> > @@ -1243,13 +1243,14 @@ static int ThreadDisplayPicture(vout_thread_t *vout, vlc_tick_t *deadline)
> >       }
> >       bool force_refresh = !drop_next_frame && refresh;
> > +    if (!frame_by_frame) {
> > +        if (date_refresh != VLC_TICK_INVALID)
> > +            *deadline = date_refresh;
> > +        if (date_next != VLC_TICK_INVALID && date_next < *deadline)
> > +            *deadline = date_next;
> > +    }
> > +
> >       if (!first && !refresh && !drop_next_frame) {
> > -        if (!frame_by_frame) {
> > -            if (date_refresh != VLC_TICK_INVALID)
> > -                *deadline = date_refresh;
> > -            if (date_next != VLC_TICK_INVALID && date_next < *deadline)
> > -                *deadline = date_next;
> > -        }
> >           return VLC_EGENERIC;
> >       }
> > -- 
> > 2.23.0.rc1
> > 
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