[Mp4-tech] Question regarding reference list in B Slice

[EricChuang via.com.tw]

Dear experts,
In standard, it mentions the non-existing reference frame would be put
into DPB when frame number gap is allowed. I have 3 questions about
this.
1.       What's the usage of frame_num gap ? Why it's introduced in
standard ? Is there any document on the reason ?
2.       When constructing reference list for B Slice, the standard
describes that reference pictures marked as non-existing are not
included in RefList0 or RefList1, is this exclusion operaton is done
specially by decoder, or encoder has make it impossible to happen by
using reference picture list reordering ?
3.       Since non-existing reference picture cannot be put into
RefList0 or RefList1 in B slice, why standard specifies the default
value of delta_pic_order_cnt[0] and delta_pic_order_cnt[1] to them in
section 8.2.5.2. Is there any place to reference the POC of non-existing
pictures ?
Thanks in advance,
Eric
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