[Mp4-tech] [Video][H.264] Decoded Reference Picture Marking Process

[Reshma Prasanna]

Dear H.264 Experts,
I would really appreciate it if anyone could answer my questions.
Qn 1) Decoded reference picture marking.
Clause 8 of the H.264 std specifies that the only restriction on invoking
the decoded reference picture marking process is that the decoded picture
must be a reference picture, there are no constraints on the type of frame
i.e. I, P, IDR etc.
Can the adaptive memory control decoded reference picture marking process
be invoked if the decoded picture is an I frame but not IDR?
If yes, then consider the case when there are decoded reference pictures
stored in the DPB, the current decoded picture is an I frame(not IDR) and
MMCO = 1 is decoded. This means that some short term ref picture with
picNum = CurrPicNum - (difference_of_pic_nums_minus_1 + 1) is to be marked
as "unused for reference". Since the decoded picture has only I slices,
the reference lists have not been constructed and hence picNum =
FrameNumWrap has not been calculated for any short tm reference frames
w.r.t the current picture's frame_num. Then how will the mmco = 1
operation be properly executed? The same question holds with any other
mmco operation that requires comparison to reference lists, if the current
decoded picture is an I frame.
Qn 2) Consider this case in a H.264 encoder:
The DPB is full and a reconstructed(decoded) picture (not IDR) is to be
inserted into the DPB. If adaptive_ref_pic_marking_mode_flag = 0, then
sliding window process will mark the short tm reference frm with smallest
value of FrameNumWrap as "unused". But if say, the encoder wishes to mark
the current picture as "used for long term reference", since the picture
is not IDR, then adaptive_ref_pic_marking_mode_flag must be set equal to 1
and mmco = 6 must be used. In this case, sliding window process is not
invoked and hence none of the frames in the DPB will be marked as "unused
for reference". There will not be any space in the DPB for the current
frame.
My understanding is that the encoder must then find which short term frame
has smallest frameNumWrap and send another mmco = 1 to set this short tm
reference frame as "unused" so that the current reconstructed frame can be
inserted into the DPB. Is this correct?
Thanks in advance,
Reshma.
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