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

[Dave Singer]

At 12:42  -0700 22/09/06, Adam Potolsky wrote:
>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.

then you are wasting your time.  the mdat box is 
a place where bits are dumped in no particular 
order and without any framing.  audio, video, 
everything, all mixed together.  its is 
indecipherable by itself.  give up.
>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).

Hint, it is not in the mdat at all.
>
>Here is where I am getting lost: According to 
>the part-12: "Šand 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.

no, not at all.  The boxes in the movie atom can 
point at media data contained in arbitrarily 
formatted files (including those without any box 
structure).  that doesn't mean that those files 
are comprehensible without the movie atom.
>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  ................
>Š
>All 0x00 entries
>Š
>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.

Dear me.  The codec definitions are in the sample 
entries in the sample tables of each track.  Each 
sample entry has a 4CC (as registered by 
www.mp4ra.org) and in the sample entry are boxes 
that parameterize the codec.  For mpeg-4 part 2, 
this includes an elementary stream descriptor, 
which (eventually) includes a 
decoderspecificinfo, which in turn is defined for 
mpeg-4 part as including the visual headers.  In 
them you find p&l.
>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.

read the movie atom, and understand its structure.  it contains what you want.
>
>Thank you in advance for your continuing assistance,
>Adam Potolsky
>
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-- 
David Singer
Apple Computer/QuickTime
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