[FFmpeg-devel] [PATCH v2] ffmpeg: allow full range of dts_delta_threshold

[Michael Niedermayer]

On Wed, Apr 15, 2020 at 11:07:05PM +0530, Gyan Doshi wrote:
> 
> 
> On 15-04-2020 10:39 pm, Michael Niedermayer wrote:
> >On Wed, Apr 15, 2020 at 05:29:06PM +0530, Gyan Doshi wrote:
> >>
> >>On 15-04-2020 05:03 pm, Michael Niedermayer wrote:
> >>>On Wed, Apr 15, 2020 at 10:55:47AM +0530, Gyan Doshi wrote:
> >>>>On 15-04-2020 01:33 am, Michael Niedermayer wrote:
> >>>>>On Tue, Apr 14, 2020 at 02:48:47PM +0530, Gyan Doshi wrote:
> >>>>>>For inputs from demuxers with AVFMT_TS_DISCONT flag,
> >>>>>>the existing condition,
> >>>>>>
> >>>>>>   delta < -1LL*dts_delta_threshold*AV_TIME_BASE
> >>>>>>
> >>>>>>is rendered superflous due to the fixed threshold in
> >>>>>>
> >>>>>>pkt_dts + AV_TIME_BASE/10 < FFMAX(ist->pts, ist->dts)
> >>>>>>
> >>>>>>This prevents users from setting a high threshold to
> >>>>>>avoid discontinuity correction due to errant timestamps.
> >>>>>>
> >>>>>>Now, the maximum of the two thresholds is used.
> >>>>>>
> >>>>>>fate-mpeg4-resolution-change call changed to preserve existing
> >>>>>>timestamp correction by ffmpeg.c
> >>>>>>---
> >>>>>>Tested with multiple satellite MPEG-TS inputs.
> >>>>>>
> >>>>>>  fftools/ffmpeg.c     | 2 +-
> >>>>>>  tests/fate/mpeg4.mak | 2 +-
> >>>>>>  2 files changed, 2 insertions(+), 2 deletions(-)
> >>>>>breaks:
> >>>>>./ffmpeg -i 'concat:angels.mpg|angels.mpg'  -vsync 0  file.avi
> >>>>>
> >>>>>...
> >>>>>frame=   24 fps=0.0 q=12.5 Lsize=     215kB time=00:00:07.50 bitrate= 234.7kbits/s speed=75.5x
> >>>>>video:84kB audio:110kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 10.697786%
> >>>>>
> >>>>>vs.
> >>>>>
> >>>>>...
> >>>>>[avi @ 0x5601caccc180] Non-monotonous DTS in output stream 0:1; previous: 219, current: 86; changing to 220. This may result in incorrect timestamps in the output file.
> >>>>>[avi @ 0x5601caccc180] Non-monotonous DTS in output stream 0:1; previous: 220, current: 87; changing to 221. This may result in incorrect timestamps in the output file.
> >>>>>[avi @ 0x5601caccc180] Non-monotonous DTS in output stream 0:1; previous: 221, current: 88; changing to 222. This may result in incorrect timestamps in the output file.
> >>>>>[avi @ 0x5601caccc180] Non-monotonous DTS in output stream 0:1; previous: 222, current: 89; changing to 223. This may result in incorrect timestamps in the output file.
> >>>>>[avi @ 0x5601caccc180] Non-monotonous DTS in output stream 0:1; previous: 223, current: 90; changing to 224. This may result in incorrect timestamps in the output file.
> >>>>>[avi @ 0x5601caccc180] Non-monotonous DTS in output stream 0:1; previous: 224, current: 91; changing to 225. This may result in incorrect timestamps in the output file.
> >>>>>[mpeg4 @ 0x5601cac87540] Invalid pts (72) <= last (83)
> >>>>>Video encoding failed
> >>>>Yes, if the input includes a mid-stream dts reset of gap less than the
> >>>>default value of dts_delta_threshold then timestamps won't be made
> >>>>continuous. Default value is 10 seconds but till now it wasn't enforced for
> >>>>negative jumps, so your command worked. With this change, user will have to
> >>>>set it manually if they expect smaller jumps.
> >>>Negative jumps should always be a discontinuity more or less.
> >>>What are you trying to fix exactly ?
> >>>
> >>>is this about bitstream errors in the timestamps ?
> >>Yes. This is when there's a backward jump in one of the streams due to
> >>corruption in the input and that leads to a new ts_offset. This new offset
> >>when applied to other streams with error-free timestamps leads to async as
> >>well as muxing errors due to (now) increased interval in the muxing queue.
> >>dts_delta_threshold (in combination with filtering) should allow to prevent
> >>these offset adjustments at the cost of filtering out or adjusting a few
> >>packets, but the existing check prevents that by making    'delta <
> >>-1LL*dts_delta_threshold*AV_TIME_BASE'    irrelevant  . With this patch, the
> >>user can set a threshold and avoid unwanted offset adjustments due to
> >>negative jumps.
> >iam still not sure we talk about the same thing
> >if there is corruption in the timestamp field then on average
> >the timestamp delta will be large and a threshold will not work
> >in seperating that reliably from discontinuities.
> >
> >to detect / filter such issues looking at more than just the next
> >timestamp would be needed
> >as in
> >5,6,7,79861 you dont know if this is
> >5,6,7,79861,79862,79863
> >or
> >5,6,7,79861,9,10,
> >without seeing the next values
> 
> Right. So I'm talking about scenarios where the user *knows* that the input
> is supposed to be
> 
> 85006,85007,85008,85009,...95443,0,1,2,3,4,5...
> 
> but is received as
> 
> 85006,85007,24104,85009...95443,0,1,2,14131,4,5...
> 
> Without this patch, user can set a threshold to ignore 2,14131 but not 85007,24104.

dts_delta_threshold is not there for detecting bitstream errors in timestamps
also its not able to be used for that.
This just works in the one sample you show here.
A bitstream error would generally be bigger than a discontinuity so it would
be the other way around

If you want to filter out such individual timestamps that are bad that should
be done by a filter maybe a bitstream filter that has access to more future
timestamps.

Also theres another issue, lets just assume there is a error and it does 
get detected as a timestamp discontinuity. The next frame afterwards too
should be detected as a discontinuity and these 2 discontinuity when being
handled should remove the error with no ill effects
That IIUC does not happen in your case, finding out why not is something
thats probably worth investigating

thx


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Nations do behave wisely once they have exhausted all other alternatives. 
-- Abba Eban
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