On 10/05/2017 09:45 AM, John Stebbins wrote: > On 10/04/2017 03:21 PM, Michael Niedermayer wrote: >> On Wed, Oct 04, 2017 at 10:58:19AM -0700, John Stebbins wrote: >>> On 10/04/2017 10:13 AM, Michael Niedermayer wrote: >>>> On Wed, Oct 04, 2017 at 08:18:59AM -0700, John Stebbins wrote: >>>>> On 10/04/2017 03:50 AM, Michael Niedermayer wrote: >>>>>> On Fri, Sep 29, 2017 at 08:54:08AM -0700, John Stebbins wrote: >>>>>>> When keyframe intervals of dash segments are not perfectly aligned, >>>>>>> fragments in the stream can overlap in time. Append new "trun" index >>>>>>> entries to the end of the index instead of sorting by timestamp. >>>>>>> Sorting by timestamp causes packets to be read out of decode order and >>>>>>> results in decode errors. >>>>>>> --- >>>>>>> libavformat/mov.c | 4 ++-- >>>>>>> 1 file changed, 2 insertions(+), 2 deletions(-) >>>>>>> >>>>>>> diff --git a/libavformat/mov.c b/libavformat/mov.c >>>>>>> index 899690d920..c7422cd9ed 100644 >>>>>>> --- a/libavformat/mov.c >>>>>>> +++ b/libavformat/mov.c >>>>>>> @@ -4340,8 +4340,8 @@ static int mov_read_trun(MOVContext *c, >>>>>>> AVIOContext *pb, MOVAtom atom) >>>>>>> MOV_FRAG_SAMPLE_FLAG_DEPENDS_YES)); >>>>>>> if (keyframe) >>>>>>> distance = 0; >>>>>>> - ctts_index = av_add_index_entry(st, offset, dts, sample_size, >>>>>>> distance, >>>>>>> - keyframe ? AVINDEX_KEYFRAME : >>>>>>> 0); >>>>>>> + ctts_index = add_index_entry(st, offset, dts, sample_size, >>>>>>> distance, >>>>>>> + keyframe ? AVINDEX_KEYFRAME : 0); >>>>>> can this lead to timestamps being out of order not just changing >>>>>> from strictly monotone to monotone ? >>>>>> >>>>>> Maybe iam missing somehing but out of order could/would cause problems >>>>>> with av_index_search_timestamp() and possibly others >>>>>> >>>>>> >>>>> I'm not sure I understand the question. But I think I can answer. The >>>>> new fragment can start before the last fragment >>>>> ends. I'll make a concrete example. Lets say the new fragment's first >>>>> DTS is 10 frames before the end of the previous >>>>> fragment. So the first DTS of the new fragment is before the timestamp of >>>>> 10 entries in the index from the previous >>>>> fragment. av_add_index_entry searches the existing index and inserts the >>>>> first sample of the new fragment in position >>>>> nb_index_entries - 10 (and shifts the existing entries). The next 9 >>>>> samples of the new fragment get intermixed with the >>>>> remaining 9 samples of the previous fragment, sorted by DTS. When the >>>>> samples are read out, you get samples from the >>>>> last fragment and the new fragment interleaved together causing decoding >>>>> errors. >>>>> >>>>> Using add_index_entry will result in the timestamps in the index going >>>>> backwards by 10 frames at the fragment boundary >>>>> in this example. In the other patch that accompanied this one, I've >>>>> marked the samples from the new fragment that >>>>> overlap previous samples with AVINDEX_DISCARD. ff_index_search_timestamp >>>>> appears to be AVINDEX_DISCARD aware. So I >>>>> think av_index_search_timestamp will do the right thing. >>>> yes, that makes sense now. >>>> Please correct me if iam wrong but then patch 1 would introduce a >>>> issue that the 2nd fixes. So both patches should be merged to avoid >>>> this >>>> >>>> But theres another problem, trun can be read out of order, when one >>>> seeks around, so the next might have to be put elsewhere than after the >>>> previous >>>> >>>> thanks >>>> >>> Hmm, can you describe the circumstances where this would happen. I looked >>> at the seek code and can't see any way for it >>> to seek to the middle somewhere without first reading previous trun. It >>> looks to me like if avformat_seek_file or >>> av_seek_frame fails to find the desired timestamp in the index it falls >>> back to seek_frame_generic which seeks to the >>> position of the last sample in the index and performs av_read_frame until >>> it gets to the timestamp it wants. Is there a >>> path I've missed where it can skip to the middle of the file somehow? >> I used >> -rw-r----- 1 michael michael 66908195 Dec 11 2015 buck480p30_na.mp4 >> ./ffplay buck480p30_na.mp4 >> >> (i can upload this if needed, i dont know where its from exactly) >> >> and when seeking around by using the right mouse buttonq it sometimes read >> trun chunks with lower times than previous (seen from the av_logs in >> there) >> >> I hope i made no mistake and would assume this happens with any file >> with these chunks >> >> ... >> [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f3884000940] AVIndex stream 0, sample 151, >> offset 60134, dts 450000, size 194, distance 25, keyframe 0 >> ... >> Seek to 68% ( 0:07:11) of total duration ( 0:10:34) >> ... >> [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f3884000940] AVIndex stream 0, sample 152, >> offset 2b74fd6, dts 38757000, size 8284, distance 0, keyframe 1 >> ... >> Seek to 14% ( 0:01:29) of total duration ( 0:10:34) >> ... >> [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f3884000940] AVIndex stream 0, sample 152, >> offset 959164, dts 7749000, size 55027, distance 0, keyframe 1 >> >> > When seeking mov_read_trun is getting called repeatedly for the same fragment > which has a number of undesirable side > effects, even without my patch. The following things get updated to > incorrect values when seeking backward and the trun > is re-read: > > sc->data_size > sc->duration_for_fps > sc->nb_frames_for_fps > sc->track_end > > The trun is getting re-read in mov_switch_root because headers_read in > MOVFragmentIndex has not yet been set for the > fragment. I think a solution to this is to set headers_read for the > appropriate entry in MOVFragmentIndex when the trun > is read the first time. Does this sound like the right approach? >
I got the analysis wrong above. It's not re-reading the trun. What's happening is that while seeking forward it can skip one or more trun. Then seeking back, it will read that trun. So, as you said, re-ordering of the index will be necessary to handle seeking past a trun. It can seek forward past a trun because the sidx has the offset to each moof and is used by mov_seek_stream. I missed this earlier. Since the trun can overlap, reordering shouldn't be done by simply sorting by the index_enties timestamps though. I'm thinking a good way would be to add a index_entry member to MOVFragmentIndexItem that records where in index_entries the samples for the trun were written. The position in index_entries of a *new* trun would be determined by looking at the position of the MOVFragmentIndexItem that corresponds to that trun and finding for the next MOVFragmentIndexItem that has a valid index_entry set (which means it's trun was read and samples inserted into the index). If no next valid index_entry is found, the samples of the new trun get appended. If a valid index_entry is found, open a hole in index_entries before that entry and populate the samples from the new trun in the hole. Then fix up the index_entry members of MOVFragmentIndexItems to account for the hole. Looking again at sc->* most of these are accurate I think. I question sc->track_end though. It seem like is should not be going backwards when seeking backwards. Am I on the right track now? -- John GnuPG fingerprint: D0EC B3DB C372 D1F1 0B01 83F0 49F1 D7B2 60D4 D0F7
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