[Mp4-tech][H.264] Sliding window decoded reference picture marking

[Gary Sullivan]

Yaguang Xie,
It is not entirely clear what you mean by "two fields of a frame".  I
believe that the only way this concept appears in the decoding process
in the standard is in the concept of complementary field pairs.  But
both fields of a complementary field pair have the same value of
frame_num, while you are referring to "two fields of a frame ... with
different frame_num", so your question does not seem to make obvious
sense.
In any case I think the basic answer to your question is that if there
are two reference fields in the DPB and those two fields have different
values of frame_num, then those fields cannot be considered to be a
complementary reference field pair.  They would therefore, according to
your description, seem to be categorized as two non-paired reference
fields.  And they would therefore count as 2 rather than 1 in the
computation of numShortTerm.
This interpretation should be particularly clear if you read Annex C.
The concept is that the decoder has frame storage areas.  When two
fields are classified as a complementary field pair, they both go into
the same frame storage area.  When they are not complementary (such as
when they have different values of frame_num), they go into different
frame storage areas.  The sliding window process manages these frame
storage areas as its primitive units for reference picture marking, so
it counts two of them for two non-paired reference fields and it counts
one of them for 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 yaguang xie
	Sent: Thursday, June 22, 2023 9:00 PM
	To: mp4-tech lists.mpegif.org
	Subject: [Mp4-tech][H.264] Sliding window decoded reference
picture marking
	Dear experts,
	I have a question to 8.2.5.3(sliding window ref_pic marking) as
following.
	Let numShortTerm be the total number frames,complementary
reference field pairs and non-paired reference fields for which at least
one field is marked as "used for short-term reference".
	My question is: If the two fields of a frame are short-term
reference picture with different frame_num,  should the numShortTerm be
added by 1 or 2 for this frame?
	(since the frame_num is different, the two fields should not be
complementary,they are non-paired reference fields.)
	Thanks in advance!
	yaguang xie
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