[Mp4-tech] Question on pyramidal coding of a GOP in MPEG-4 AVC Main Profile

[daniele renzi (bsoft)]

Dear experts,
I have to encode with MPEG-4 AVC Main Profile the following pyramidal sequence of frames:
I0  B1  B2  B3  P4  B5  B6  B7  P8        (in increasing order of POC)
I0  P4  B2  B1  B3  P8  B6  B5  B7        (in decoding order, increasing order of PicNum)
What I'd like to do is to code the picture P4 using as reference for prediction the picture I0, the picture B2 using as references pictures I0 and P3, the picture B1 using as references pictures I0 and B2 and the picture B3 using as references pictures B2 and P4, and the same for the next GOP...in a PYRAMIDAL way.
I'd like to mark the pictures B1 and B3 with nal_ref_idc = 0, as they are never used as reference, and keep the size of the reference lists L0 and L1 always to 1 (num_ref_idx_lX_active_minus1 = 0).
I have this problem when I have to encode the picture P8: the reference lists construction process for P-pictures put in first (and only) position of the reference list L0 the picture B2, that's the picture, previously marked as "used for reference", with the highest value of PicNum.
I should instead use the picture P4 as reference.
This problem doesn't exist for B pictures, as for them the reference lists are ordered according to POC (not PicNum).
I thought about using a memory management control operation = 1 to mark the picture B2 as "unused for reference" after the decoding of picture B3, but this is not possible if I mark the picture B3 with nal_ref_idc = 0, because this condition prevents the call to any reference picture marking process.
Does anyone know whether there is a simple way (i.e. memory management control operation or reference picture list reordering, etc.) to tell the decoder that the picture P4 has to be used as reference for P8, instead of B2?
Thanks in advance for your patience
Best regards,
Daniele Renzi
bSoft -- www.bsoft.info
+39-0733-57707  (tel/fax)
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