[Mp4-tech] [H.264]Question about the UnusedShortTermFrameNumvariable

[Gary Sullivan]

Regarding the first question, I think you can interpret the second
"that" as "and".
Regarding the second question, the value of frame_num must be equal to
PrefRefFrameNum when the current picture is an IDR picture and so is the
preceding reference picture, and the value of frame_num may be equal to
PrevRefFrameNum when all of the following conditions are true:
-             the current picture and the preceding reference picture
belong to consecutive access units in decoding order
-             the current picture and the preceding reference picture
are reference fields having opposite parity 
-             one or more of the following conditions is true
-     the preceding reference picture is an IDR picture
-     the preceding reference picture includes a
memory_management_control_operation syntax element equal to 5
NOTE - When the preceding reference picture includes a
memory_management_control_operation syntax element equal to 5,
PrevRefFrameNum is equal to 0.
-     there is a primary coded picture that precedes the preceding
reference picture and the primary coded picture that precedes the
preceding reference picture does not have frame_num equal to
PrevRefFrameNum
-     there is a primary coded picture that precedes the preceding
reference picture and the primary coded picture that precedes the
preceding reference picture is not a reference picture
Mostly those conditions are about whether the current picture and the
preceding reference picture together form a complementary reference
field pair.
Best Regards,
Gary Sullivan
________________________________
	From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Tan Rui --
Huawei
	Sent: Thursday, July 20, 2023 6:30 PM
	To: mp4-tech lists.mpegif.org
	Subject: [Mp4-tech] [H.264]Question about the
UnusedShortTermFrameNumvariable
	Hi, All experts,
	When I read the protocol, I find it is difficult to understand
the description about the UnusedShortTermFrameNum variable.
------------------------------------------------------------------------
------------------------------------------------
	In the protocol, the variable is defined as the following:
------------------------------------------------------------------------
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	When the value of frame_num is not equal to PrevRefFrameNum, the
following applies.
	- There shall not be any previous field or frame in decoding
order that is currently marked as "used for short-term reference" that
has a value of frame_num equal to any value taken on by the variable
UnusedShortTermFrameNum in the following: [[ I can not understand the
sentence, because there are two 'that..' clauses. What is the main
idea?]] 
	UnusedShortTermFrameNum = ( PrevRefFrameNum + 1 ) % MaxFrameNum 
	while( UnusedShortTermFrameNum != frame_num )
( 7-21)
	    UnusedShortTermFrameNum = ( UnusedShortTermFrameNum + 1 ) %
MaxFrameNum
------------------------------------------------------------------------
------------------------------------------------
	Bay the way, Can some one explain that when will the
PrevFrameNum variable be equal to the frame_num variable?   
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