[Mp4-tech] H264: Emulation prevention Bytes

[Gary Sullivan]

Samuel et al,
You may say that the standard is somewhat confusing on this topic, and
it may be true that this part is not especially easy to read, but I
believe it is sufficiently clear (and if you find some particular
missing thing, please let us know and we'll try to fix it). When reading
it, it may be helpful to keep the following things in mind:
1) For reasons of trying to allow maximum freedom to implementers while
achieving interoperability goals, the standard is written almost
entirely from the perspective of a decoder, not an encoder.  Do not read
it as a recipe for encoder design.  It does contain a limited quantity
of remarks intended for that purpose, but that is not its major focus.
2) Certain sections of the text contain requirements that constrain the
content of conforming bitstreams.  Certain other sections contain
"informative" remarks only (statements intended to be helpful but not
saying something mandatory).  It can sometimes be important to pay
attention to which is which.  In theory it should be possible to
completely remove the "informative" sections without affecting the
sufficiency of the document as a specification.
3) The standard is generally written in a way that avoids saying things
redundantly.  This means that you need to read every part of the
standard that touches on the subject you're investigating.  It is not
sufficient to focus on one or two sections and hope that everything you
need will be mentioned there.  You need to look for every place that
touches your topic of interest and think about the full implications of
what each statement says.  For example, if something is fully specified
by the syntax diagram of the NAL unit syntax structure, it may not be
reiterated anywhere in words in the text.
Best Regards,
Gary Sullivan 
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org 
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Samuel Rivas
+> Sent: Thursday, October 12, 2023 12:31 AM
+> To: mp4-tech lists.mpegif.org
+> Subject: Re: [Mp4-tech] H264: Emulation prevention Bytes
+> 
+> Satendra wrote:
+> >    1)in normal scenes this is how we will use emulation 
+> prevention bytes.
+> >    0x000000   ------------------> 0x00000300
+> >    0x000001   ------------------> 0x00000301
+> >    0x000002   ------------------> 0x00000302
+> >    0x000003   ------------------> 0x00000303
+> > 
+> >    2) but how this byte sequence will map
+> >    0x000000000005
+> >                   ---------------------> 0x000003 000003 0005
+> >                     or
+> >                   ---------------------> 0x00000300 000005
+> 
+>  The first one. For the second one, 0x000003 000000 05 has an illegal
+>                                              ^^^^^^
+> sequence of bytes.
+> 
+> 
+>   The standard is a bit confusing on this subject:
+> 
+> "[...]
+>  and a byte equal to 0x03 is inserted to replace these bit 
+> patterns with
+> the patterns
+> 
+> '00000000 00000000 00000011 000000xx',
+> [...]"
+> 
+> But it does not make clear is that 000000xx takes part in the next
+> 0x000000
+> sequence if it is present.
+> 
+> Regards
+> 
+> -- 
+> 	Samuel
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