[FFmpeg-devel] [PATCH 1/2] mov: fix decode of fragments that overlap in time

John Stebbins stebbins at jetheaddev.com
Thu Oct 5 19:45:32 EEST 2017

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:


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?

John      GnuPG fingerprint: D0EC B3DB C372 D1F1 0B01  83F0 49F1 D7B2 60D4 D0F7

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 488 bytes
Desc: OpenPGP digital signature
URL: <http://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20171005/bb1c2bbb/attachment.sig>

More information about the ffmpeg-devel mailing list