[FFmpeg-devel] Question about edit-list+fragment explanation from " [PATCH] mov.c: read fragment start dts from fragmented mp4"

[Bryan Huh]

Sorry I am not posting to the same thread directly, I joined the list after
this discussion finished a few weeks ago.

I was reading Yusuke's explanation (pasted below) regarding the
relationship between PTS, CTS, and DTS in the context of fragments, and
there are some parts that I don't understand or don't agree with (I am also
a little new to these concepts).

The part where I get confused is:

The presentation starts with CTS=2002 because of media_time of the second
edit, so the PTS of the sync sample corresponds to X - 1001.
>*From this, X is equal to 49001, and the DTS of trun.sample[0] is equal to X
*- trun.sample[0].sample_duration = 49001 - 1001 = 48000.

It looks like X is supposed to represent the DTS of the sync-sample. I see
that (1001 + X) - 2002 = (X - 1001) represents the amount of time, in
mdhd.timescale, into the presentation of actual media. Earlier, we had
computed that the 48000 'time' from tfra corresponded to 600 into the
presentation of actual media, in mvhd.timescale. So the way I would compute
X would be:

600*mdhd.timescale/mvhd.timescale = X-1001

So X = 600*24000/600 + 1001 = 25001

Which does not agree with the original calculation of X = 49001. Do I have
a misunderstanding?

Thanks,
Bryan

>>* Would you mind explaining how edit lists and fragment times are
*>* supposed to work together?
*>>The tfra box is designed as the player seeks and finds sync sample on
presentation timeline i.e. by PTS in units of mdhd.timescale.
PTS comes from CTS via edit list, and CTS comes from DTS. So, basically you
can't get DTS directly from the 'time' field in the tfra box.

Let's say mvhd.timescale=600, mdhd.timescale=24000 and the edit list
contains two edits (edit[0] and edit[1]),
  edit[0] = {segment_duration=600, media_time=-1, media_rate=1}; // empty
edit
  edit[1] = {segment_duration=1200, media_time=2002, media_rate=1};
and the track fragment run in a track fragment which you get from an entry
in the tfra box, where 'time' in that entry is equal to 48000 is as follows.
  trun.sample[0].sample_is_non_sync_sample = 1
  trun.sample[0].sample_duration=1001
  trun.sample[0].sample_composition_time_offset=1001
  trun.sample[1].sample_is_non_sync_sample = 0
  trun.sample[1].sample_duration=1001
  trun.sample[1].sample_composition_time_offset=1001
Then, time/mdhd.timescale*mvhd.timescale=1200, that is, the PTS of the sync
sample is equal to 1200 in mvhd.timescale.
And, the first edit is an empty edit, so the presentation of actual media
starts with 1200 - 600 = 600 in mvhd.timescale.
The CTS of the second sample in the trun.sample[1] is equal to 1001 + X,
where the X is the sum of the duration of all preceding samples.
The presentation starts with CTS=2002 because of media_time of the second
edit, so the PTS of the sync sample corresponds to X - 1001.
>*From this, X is equal to 49001, and the DTS of trun.sample[0] is equal to X
*- trun.sample[0].sample_duration = 49001 - 1001 = 48000.

|*<--edit[0]-->|<---------edit[1]--------->|
*|*-------------|-------------|-------------|---->presentation timeline
*0             D             T'
              |-------------|------------------>composition timeline
          media_time        T
        |-----|-------|-----|------------------>decode timeline
        0 media_time  X     T
                      |<--->|
                     ct_offset

D = edit[0].segment_duration = 600
T' = time/mdhd.timescale*mvhd.timescale = 1200
media_time = edit[1].media_time = 1001
ct_offset = trun.sample[1].sample_composition_time_offset = 1001
T=ct_offset + X
T-media_time = (T'-D)*mdhd.timescale/mvhd.timescale