[mythtv] FFmpeg 4.4.1 problems

[David Engel]

On Fri, Nov 12, 2021 at 08:39:58PM +0100, Klaas de Waal wrote:
> On Fri, 12 Nov 2021 at 15:46, Peter Bennett <pb.mythtv at gmail.com> wrote:
> 
> > David
> >
> > I saw your commit for reducing prebuffer.
> >
> > I am trying to get ffmpeg 4.4.1 in shape in the devel/ffmpeg-resync
> > branch. Klaas has reported picture corruption which sometimes happens at
> > start of play or after a jump and lasts up to 2 seconds. This happens
> > with VDPAU on interlaced H264 videos. Also to a lesser extent with NVDEC
> > on interlaced H264 videos (only lasts a small fraction of a second with
> > NVDEC as far as I have seen). it only happens on about one out of ten
> > jumps on VDPAU.
> >
> > The workaround I am looking at is to "prime" the decoder with an extra 2
> > seconds of data that are not shown, so that the first picture seen will
> > not be corrupted. This would cause a small delay after a jump, which may
> > be unacceptable, or may be less acceptable that seeing a couple of
> > seconds of corruption. I don't know if this is a good workaround.
> >
> > For fast forward or rewind this would not make sense so it would have to
> > be suppressed.
> >
> > Let me know if anybody has any suggestions, better ideas, etc.
> >
> > Peter
> >
> > The capability to skip very fast is the key feature that differentiates
> MythFrontend from other players and that I use extensively to skip past
> commercials in small increments. So I would rather not lose this feature.
> Skipping with VDPAU has worked perfectly for the last 15 years...  I prefer
> not upgrading to the latest FFmpeg if slower skipping is the consequence,
> also because I do not have any issues with the FFmpeg that we are currently
> using.

The same goes for me.  As an example, the local commercials inserted
by my cable company are often in stereo where as the main program is
in AC3 5.1.  There stereo commercials are noticeably and annoyingly
much louder than the normal program.  As far as I can tell, that'sdue
to a bug in my Nvidia Shield.  I finally found a work around by
turning on upconvert stero to 5.1 in MythTV even though I only have
stereo speakers.  I quickly turned off the work around beacuse
skipping went from 50-100ms to 200-250ms.  I know it doesn't sound
like much but to someone who skips a lot, it is a huge downgrade.

> About the possible cause, I can digress a little although it is mostly
> conjecture.
> The artifacts also appear with older recordings so it is purely a playback
> issue. This means that the keyframe index (if that is what it is called)
> generation in mythbackend is likely still OK. That limits the problem to
> playback.
> It could be that there is an issue in passing the jump position as read
> from the database to FFmpeg/decoder. Although I expect it is difficult to
> do this "a little bit" wrong; this kind of thing either crashes or works OK
> probably.
> 
> I think that I observed that when my mythfrontend is just powered up,
> skipping is OK at the start and that it does deteriorate after a while.
> This suggests that, when doing a skip, there must be something additional
> initialized/reset in FFmpeg/decoder.
> 
> For me the issue only appears with H264 interlaced. H265 progressive is OK.
> My guess is that H264 progressive would also be OK and that the issue is
> connected to the interlacing. This then could be related to top
> field/bottom field not being passed correctly or interpreted reverse or so.
> As said, mostly conjecture.... But I hope that this helps a bit.

Does h264 have true keyframes?  I vaguely recall something from long,
long ago that h264 (or perhaps some other new encoding) doesn't have
or doesn't have to have true keyframes where a complete, new frame is
started at a specific place in the stream.  Is is possible the source
of Klaas' recordings isn't including optional keyframes?

Alternativiely, maybe there's a bug in MythTV's h264 keyframe
detector.  There was in bug in MythTV several years ago where the
mpeg2 keyframe detector would automatically and incorrectly mark every
15th or 30th frame as a keyframe if a real keyframe wan't detected
"soon" enough.  One of my channels switched from the normal every
0.5s/15 frames, keyframe interval at the time to 2.5s/75 frames.  Most
but not all skips on recordings from that channel started showing
blocking and artifacts.

Klaas, does the same blocking occur when you play your recordings back
with something like mpv?

David

> 
> Thanks,
> Klaas.

-- 
David Engel
david at istwok.net