[Mp4-tech] [H.264] One question regarding P_8x8ref0

[Gary Sullivan]

The P_8x8ref0 prediction mode is just a short-hand way of expressing a
combined selection of P_8x8 and ref_idx_l0 = 0 together using a more
efficient entropy code.  When using CAVLC entropy coding, this turned
out to be a more efficient way to signal this joint selection than
sending ref_idx_l0 separately for this case.  However, CABAC is a more
sophisticated entropy coder and so it doesn't need this short-hand
trick.  Therefore we don't allow mb_type equal to 4 in P slices with
CABAC.
This saves us the need to devote a bin string to represent this case in
Table 9-28.  If we didn't prohibit that case in CABAC operation, we
would need a bin string for it.
It can still be used with CAVLC.
Best Regards,
Gary Sullivan
________________________________
	From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of
EricChuang via.com.tw
	Sent: Sunday, June 11, 2023 8:09 PM
	To: mp4-tech lists.mpegif.org
	Subject: [Mp4-tech] [H.264] One question regarding P_8x8ref0
	Dear experts,
	In the standard, it reads that P_8x8ref0 has the same meaning as
P_8x8, except that all refIdx of P_8x8ref0 blocks are inferred as 0.
When CABAC is used, the standard describes that P_8x8ref0 macroblock
type is not allowd. Does it mean P_8x8ref0 can only be used when CABAC
is not used, or P_8x8ref0 has been deprecated ? And is there any reason
that P_8x8ref0 cannot be used when CABAC is used ?
	Appreciate for your comment in advance,
	Eric
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