x264cli marks 'key-frames' as sync points even if they are not IDR-frames.
('key-frame' is a random accessible frame in x264's term.)
This will work but, of course, this is spec violation.
It can say there are two types of random access.
1) IDR (Instantaneous Decoding Refresh)
2) GDR (Gradual Decoding Refresh)
Open-GOP is the special case of GDR, where recovery_frame_cnt (the number of frames necessary to be correct in output order from the SEI) is 0.
It is discussed and declared on
93rd meeting that we prefer a new sample group (‘rap ‘?), which documents random access points which might not be sync points.
Each of them is signaled as random accessible point by different ways in MP4/ISO Base Media file format.
IDR: sync sample table ('stss')
GDR: roll recovery grouping ('roll')
IDR and Open-GOP: random access point grouping ('rap ')
'stss' and 'roll' are already defined publicly in ISO/IEC 14496-12.
However, amendment for 'rap ' is still on
a draft.
So the implementation of random access for Open-GOP in MP4 is dangerous in the present stage.