Is there more to this? If there is, it's not evident to me.
If there are several B-frames in a row, then adjust your mind to 'see' larger shifts across the
multiple B-frames and adjust PTSs & DTSs to match.
Cutting off a leading open GOP:
<————open GOP————> <—————closed GOP——————>
PTS order [_I_][_B_][_P_][_B_][_I_][_B_][_P_][_B_][_P_][_I_]
__/ ___¦___/ ___¦___/ ___¦___/ ___¦___/ __/
/ / ¦ / ¦ / ¦ / ¦ /
DTS order [_I_][_P_][_B_][_I_][_B_][_P_][_B_][_P_][_B_][_I_]
X X X X
PTS order [_I_][_B_][_P_][_B_][_P_][_I_]
__/ ___¦___/ ___¦___/ __/
/ / ¦ / ¦ /
DTS order [_I_][_P_][_B_][_P_][_B_][_I_]
Cut on PTS, then fix up the 1st DTS & packet SCRs.
Or, I don't think it's going to hurt anything to leave this:
PTS order [_I_][_B_][_P_][_B_][_P_][_I_]
_______/ ___¦___/ ___¦___/ __/
/ / ¦ / ¦ /
DTS order [_I_] [_P_][_B_][_P_][_B_][_I_]
and just fix-up the packet SCRs.
=====
Cutting off a leading closed GOP:
<—————closed GOP——————> <————open GOP————>
PTS order [_I_][_B_][_P_][_B_][_P_][_I_][_B_][_P_][_B_][_I_]
__/ ___¦___/ ___¦___/ __/ ___¦___/ ___¦___/
/ / ¦ / ¦ / / ¦ / ¦
DTS order [_I_][_P_][_B_][_P_][_B_][_I_][_P_][_B_][_I_][_B_]
X X X X X
PTS order [_I_][_B_][_P_][_B_][_I_]
__/ ___¦___/ ___¦___/
/ / ¦ / ¦
DTS order [_I_][_P_][_B_][_I_][_B_]
Just cut it and fix-up the packet SCRs.
=====
Cutting off a trailing GOP from a closed GOP:
<—————closed GOP——————>
PTS order [_I_][_B_][_P_][_B_][_P_][_I_]..
__/ ___¦___/ ___¦___/
/ / ¦ / ¦
DTS order [_I_][_P_][_B_][_P_][_B_]
Just cut at the end of the closed GOP.
=====
Cutting off a trailing GOP from an open GOP:
<————open GOP————>
PTS order [_I_][_B_][_P_][_B_][_I_]..
__/ ___¦___/ ___¦___/
/ / ¦ / ¦
DTS order [_I_][_P_][_B_][_I_][_B_]
PTS order [_I_][_B_][_P_][_B_][_I_]
__/ ___¦___/ ___¦___/
/ / ¦ / ¦
DTS order [_I_][_P_][_B_][_I_][_B_]
Cut retaining an ending I-frame, only.
Honestly, I don't know why the MPEG folks don't supply illustrations instead of
their torturous text.
This isn't magic, it's Legos.
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