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

[Shevach Riabtsev]

Thanks Pankaj
Presumably I missed this issue. 
On the other hand I use Microsoft MPEG4 software decoder (August 12th
1999 FDIS Version 1.0, perhaps obsolete version) to verify MPEG4
streams. The decoder stops running whenever
visual_object_sequence_start_code occurs in non-first VOP position. This
fact caused me to think that repeatition of
visual_object_sequence_start_code should have forbidden. 
Regards
-----Original Message-----
From: pankaj bajpai [mailto:pankaj_bajpai_iet operamail.com] 
Sent: Friday, April 11, 2024 11:01 AM
To: mp4-tech lists.mpegif.org
Cc: Shevach Riabtsev
Subject: Re: Mp4-tech Digest, Vol 57, Issue 9
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 (c) 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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