[Mp4-tech] H264: Emulation prevention Bytes

[Gary Sullivan]

Samuel (et al),
I think you have made a good point about the treatment of the fourth
byte of the replacement pattern.  I think it would be useful to clarify
that aspect in 7.4.1.1 in a future revision of the standard.  I will try
to bring this to the attention of the JVT for action.  The timing of
this remark is good, as I am preparing an input document on the topic of
corrections and clarifications of the text for submission by Monday.
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: Friday, October 13, 2023 7:05 AM
+> To: mp4-tech lists.mpegif.org
+> Subject: Re: [Mp4-tech] H264: Emulation prevention Bytes
+> 
+> Gary,
+> 
+> > 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:
+> 
+>   I did not mean that the standard was ambiguous or incomplete. I is
+> just that that informative section confused me too the first 
+> time I read
+> it. Reading NAL unit syntax definition gave me an idea (that was
+> correct) that clashed with what I understood in the 
+> informative section.
+> Further reading in the reference code source made it 
+> perfectly clear. So
+> the specification is correct, I have nothing to say against it.
+> 
+>   Since Satendra seems to have the same problem I had, I 
+> tried to clarify
+> where the confusion took place. I pointed what misled me and why.
+> Whether it is my problem or the standard's is subject to discussion.
+> 
+>   I understand that I cited an excerpt of an informative 
+> section and I
+> did not say anything of the normative information which, for me, is
+> perfectly clear. Sorry if that annoyed you.
+> 
+>   Finally, just to make clear what I am missing, and then it may be
+> evaluated as an useful correction or just my misunderstanding:
+> 
+>   In section 7.4.1.1 it reads that a bit pattern 00000000 00000000
+> 00000011 000000xx must be laid to replace any 00000000 
+> 00000000 000000xx
+> pattern. For me, what is not clear is that for sequeneces 
+> such as 0x00
+> 0x00 0x00 0x00 ..., after the first substitution 0x00 0x00 0x03 0x00
+> 0x00, the fourth byte takes part of both 00000000 00000000 00000011
+> 00000000 pattern and the next 00000000 00000000 000000xx pattern.
+> 
+>   It is clear in NAL unit syntax though. But when reading 
+> the two parts
+> I was not able whether the problem was that I misunderstood 
+> the normative
+> part or the informative part.
+> 
+>   Regards.
+> -- 
+> 	Samuel
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