[Mp4-tech] Question on the mb_field_decoding_flag

[Alan Yan - SH]

Dear Experts,
When MbaffFrameFlag==1, there is one syntax "mb_field_decoding_flag" in
MB layer for each MB pair.
In case of entropy_coding_mode_flag==0, when the first MB (top MB) in
the MB pair is skipped, mb_field_decoding_flag (MBFDF) is also skipped
for the top MB. For following decoding process like B_DIRECT_16X16(for
B_SLICE) and Boundary Strength calculation, MBFDF is necessary. And
according to the spec 7.4.4, the derivation of MBFDF is performed after
we know both MB of an MB pair are skipped. Does it mean that if the top
MB is skipped, the spec do not have any default or temporary value
defined for MBFDF and so we can not start following decoding process
like B_DIRECT_16X16 and BS calculation for this top MB? We must wait
till the bottom MB is parsed and the final MBFDF is decoded or derived?
And when entropy_coding_mode_flag==1, I see the derivation process is
always performed before decoding any syntax element, is this statement
correct? And it is possible that the before-anything-derived value for
MBFDF can be different from the value parsed later from the bit-stream,
is it correct? In this situation and if mb_skip_flag is 1 for the top
MB, can we use this temporarily derived MBFDF to start following
decoding process like B_DIRECT_16X16 and BS calculation for this
top-skipped MB or should we wait and use the final MBFDF?
Thanks for your kindly help! (To Mr. Gary Sullivan: Thank you!)
Best Regards,
alan
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