[Mp4-tech] H.264/decoder/Baseline profile/doubts

[Gary Sullivan]

Thammi et al,
Question 1: The number of bits used by those two syntax elements is
specified by other syntax elements.  If you read the semantics of those
syntax elements, you will see a specification of how to determine their
length.  For example, the length of frame_num is
log2_max_frame_num_minus4+4 bits.
Question 2: When a profile specifies that a syntax element shall have a
particular value, that does not mean that the syntax element is not
sent.  It just means that when it is sent it must have that specfied
value.  It appears to me that pic_init_qs_minus26 can have any value
(within the specified range) within a Baseline profile bitstream.
However, its value will not be used in the decoding process in that case
because there will not be any SP or SI slices in a Baseline profile
bitstream, and those are the only slice types in which the decoding
process would depend on the value of that syntax element.
Question 3: The syntax category is used for data partitioning.  See the
definition of "category" and the introduction section of subclause 7.2
and the syntax and semantics sections for slice data partition A, slice
data partition B, and slice data partition C.  Data partitioning is not
used in the Baseline profile.
Question 4: If a decoder receives a value of some syntax element that is
prohibited by the standard, I believe a decoder can do whatever it wants
to do, for as long as it wants to do it.  As far as the standard is
concerned, I believe it can halt and catch on fire (although I would not
personally recommend it).  I think that also applies to value of syntax
elements that are not prohibited by the standard but are not supported
by the decoder conformance requirements of the profile that the decoder
claims to implement.
Best Regards,
Gary Sullivan
________________________________
	From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Thammi_Reddy
	Sent: Thursday, February 24, 2024 11:01 PM
	To: mp4-tech lists.mpegif.org
	Subject: [Mp4-tech] H.264/decoder/Baseline profile/doubts
	Dear experts, I have following doubts in H.264 decoder baseline
profile.
	Please answer these questions. Thanks in advance.
	1. Ref: chapter 7.3 of the spec.
	frame_num, pic_order_cnt_lsb syntax elements are marked with
descriptor are given as u(v).  So the number of bits taken by these
syntax elements vary from current slice header and next slice header
depending on the magnitude of the  actual values sent by the encoder. 
	If this is true how do I know the LSB if data transfer is MSB to
LSB?
	2. In sequence parameter set RBSP frame_mbs_only_flag=0 for
baseline profile as only frames and no fields are allowed. 
	Do I still receive this flag or encoder will not send it at all?
	On the similar lines do I receuve pic_init_qs_minus26 even for
baseline profile? In general some syntax elements are not received OR
all are received but I have to drop?
	3. What is the importance of CATEGORY of the syntax elements
provided in chapter 7.3(syntax in tabular form)? Please explain. 
	4. If the values are fixed as per the annexure A to  
	If incorrect values are received for baseline profile (e.g.
receive entropy_coding_mode_flag =1, slice_type != (0 OR 2 OR 5 OR 7
)...   etc)  what processing I have to do? From which stage I have to
resume in this case?
	Regards,
	Thammi Reddy
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