[Mp4-tech] Follow-up about 'mdat' data and "profile_and_level"

[Girish Shenoy]

Hi Adam,
Have you explored the ES Descriptor in stsd box?
I believe the "Decoder specific info" field contained in the ES Descriptor
(inside the stsd) would be able to pinpoint the exact encoding being used
without too much of a decoding effort.
14496-1 would give you the details of the DecoderSpecificInfo field.
Hope this helps.
Regards,
Girish
-----Original Message-----
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org]On Behalf Of Adam Potolsky
Sent: Saturday, September 23, 2023 1:12 AM
To: Mp4-Tech Mailing List
Subject: [Mp4-tech] Follow-up about 'mdat' data and "profile_and_level"
Good day,
Although I still hold out hopes of finding a method of discovering the
profile and level of a given MP4, I am trying to dig through the actual
'mdat' to find what profile and level is required to use the media. I've
been trying to piece together information from parts 1, 2, and 12 to figure
out how to decode the data contained in the 'mdat' so I can find the
VisualObjectSequence() and ultimately the profile_and_level_indication and
compare it to Annex G (part 2).
Here is where I am getting lost: According to the part-12: ".and files
without any box structure may also be referenced and used." (p15) implies
that I can just dive into the 'mdat' and start hunting w/o needing any
information from the box data. When I look at the 'mdat' for a file I have
this is what that section looks like:
0002700: 8700 09d9 8800 09db 4a00 09b6 7a6d 6461  ........J...zmda
0002710: 7420 0020 0000 800d fb60 0000 0000 0000  t . .....`......
0002720: 0000 0000 0000 0000 0000 0000 0000 0000  ................
.
All 0x00 entries
.
00027f0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0002800: 0000 1c07 2625 b840 3ff0 11ff e58a 0002  ....&%.@?.......
0002810: 05f0 0382 6240 c69a 606d aba2 824a 65fb  ....b ..`m...Je.
What I expected to see (and I am still trying to learn what's going on in
this part,) is a stream of meaningful objects that set up the interface. The
0x00s are throwing me off, as is the 0x20 followed by the 0x00 right after
the 'mdat'. I am trying to use the spec to break this down, but I am having
some difficulties following it.
If there is anyone that can provide further insight into this I would
greatly appreciate it.
Thank you in advance for your continuing assistance,
Adam Potolsky
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