[Mp4-tech] Re: Mp4-tech Digest, Vol 57, Issue 9

[pankaj bajpai]

Dear Shevach,
Sequence header and Frames are different.
Sequence header is separate entity, and it is not
necessary to present with first frame. The only restriction
is that it should be passed before first frame.
Now, standard allows duplication of header. Read below 2 paragraphs
from standard
===================================================
Visual object sequence is the highest syntactic structure of the visual bitstream.
A visual object sequence commences with a visual_object_sequence_start_code which is followed by one
visual object (when used in a visual bitstream), or one or more visual objects (when used in a systems
bitstream with separate configuration information). The visual object sequence is terminated by a
visual_object_sequence_end_code. To enable random access into the visual bitstream and recovery from
errors, the visual object sequence header may be repeated before termination, as described in clause 6.2.1.
=============================================================
Section 6.2.1
2. Combined Configuration / Elementary Streams
The elementary stream data associated with a single layer may be wrapped in configuration information defined in
accordance with the syntax below. A visual bitstream may contain at most one instance of each of
ISO/IEC 14496-2:2004(E)
42 © ISO/IEC 2004 – All rights reserved
VisualObjectSequence(), VisualObject() and VideoObjectLayer(), with the exception of repetition of the Visual
Object Sequence Header, the Visual Object Header and the Video Object Layer Header as described below. The
Visual Object Sequence Header must be identical for all visual streams input simultaneously to a decoder. The
Visual Object Headers for each layer of a multilayer object must be identical.
The Visual Object Sequence Header, the Visual Object Header and the Video Object Layer Header may be
repeated in a single visual bitstream. Repeating these headers enables random access into the visual bitstream
and recovery of these headers when the original headers are corrupted by errors. This header repetition is used
only when visual_object_type in the Visual Object Header indicates that visual object type is video. (i.e.
visual_object_type==”video ID”) All of the data elements in the Visual Object Sequence Header, the Visual Object
Header and the Video Object Layer Header repeated in a visual bitstream shall have the same value as in the
original headers, except that first_half_vbv_occupancy and latter_half_vbv_occupancy may be changed to specify
the VBV occupancy just before the removal of the first VOP following the repeated Video Object Layer Header.
====================================================================
With Regards
Pankaj Bajpai
> 
> From: Shevach Riabtsev <sriabtsev broadcom.com>
> To: mp4-tech lists.mpegif.org
> Subject: [Mp4-tech] How provide random access feature in MPEG4 Simple profile
> Date: Thu, 10 Apr 2024 06:05:15 -0700
> 
> 
> Dear experts
> 
> According to my understanding of MPEG4 standard, the profile and the
> level are signalled as profile_and_level_indication syntax element  in
> Visual Object Sequence Start header. This header is sent once per
> sequence thus this fact actually disables random access. Indeed, if a
> decoder skip the very first picture of a sequence how it can derive
> profile and level.
> 
> Is it correct to put Visual Object Sequence Start header at the start of
> each I-picture?
> 
> Regards, Shevach
> Broadcom
> 

Pankaj Kumar Bajpai
Mulitimedia Engineer
India
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