[M4IF Technotes] Very basic question about video bitstream sy ntax and ES definitions

[Larry Pearlstein]

Dear Gary,
Thank you for your reply.
I think my main confusion on the ES question came from the labels in the
blocks in Fig. 6-12 like "Elementary Stream Visual Object 1 Layer 1".  Also,
perhaps the term "header" is used somewhat loosely?  The syntax is expressed
hierarchically, yet some of the syntax layers are described as headers.
                Thanks,
                Larry
 -----Original Message-----
From: Gary Sullivan [mailto:garysull   microsoft.com]
Sent: Friday, September 21, 2023 5:40 PM
To: Larry Pearlstein; technotes   lists.m4if.org
Subject: RE: [M4IF Technotes] Very basic question about video bitstream
syntax and ES definitions
This is addressed in 6.2.1 of MPEG-4 visual.  There is a distinct division
between
"configuration information" and "elementary stream data".  There is a clear
definition
of the "entry points for elementary streams, and entry into these functions
defines
the breakpoint between configuration information and elementary streams".
The elementary stream does not contain the visual object header or the
visual
object layer header or the video object layer header.  Those structures all
fall
above the defined entry points.
Configuration information may be carried separately or may be combined with
elementary
stream data.  Some configuration information is necessary for being able to
decode a stream.
This is why some files that contain only one elementary stream may contain
headers that are
not part of that elementary stream, and is why the issue is a tricky one.
I believe the visual spec is reasonably clear on this issue.  But you're
talking
about the contents of a file, and I believe file formats are outside the
scope
of the visual spec.  You need to get your information about the format of
your
file from somewhere else.
Best Regards,
-Gary Sullivan
-----Original Message-----
From: Larry Pearlstein [mailto:lpearlst   ati.com] 
Sent: Friday, September 21, 2023 11:24 AM
To: 'technotes   lists.m4if.org'
Subject: [M4IF Technotes] Very basic question about video bitstream syntax
and ES definitions
Dear M4IF community members, 
I'm wondering about the syntax that I've found in MPEG-4 video bitstreams. 
I see the following: 
0000 0101   video_object_start_code 
0000 0120   video_object_layer_start_code 
00c8        = "0000 0000 1100 1000"     >   random_acc = 0 
                                            video_object_type_indication =
0x01, simple object type 
                                            is_object_layer_id = 1 
                                            verid = 0010, 
                                            priority = 001 
                                            etc. 
My confusion comes from the fact that it seems that the syntax of the file
begins in the middle of the VisualObject syntax.  Perhaps I've missed some
text that suggests that this is allowed, suggested, etc?
In the conformance spec it says: 
        "A bitstream is the coded representation of one layer of a single
visual object." 
Based on this definition, I'd expect to find a visual_object_start_code at
the beginning of the file. 
Also, I'm confused about the definition about an MPEG-4 "Elementary Stream".
Figure 6-12 suggests that a Visual Object Layer is an elementary stream.  I
would imagine that a VideoObjectLayer is a type of Visual Object Layer,
right?
But the text suggests that a VideoObjectLayer contains configuration
information that is not part of an elementary stream.  Also,
VideoObjectLayer is hierarchically above the 5 functions that are enumerated
as "entry points" for elementary streams.
So - is a VideoObjectLayer an Elementary Stream?  Is an MPEG-4 video
bitstream file an elementary stream?  Is an MPEG-4 video bitstream file a
VideoObjectLayer, etc?
Thanks for any help.  Also, please forgive me if this isn't the appropriate
forum for this question, it's the first time I've posted.
                                Thanks, 
                                Larry Pearlstein 
                                ATI 
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