[Mp4-tech] compute colocated macroblock

[Tourapis, Alexis]

The answer is yes.
Best regards,
Alexis 
-----Original Message-----
From: ChenJianjun [mailto:cjj730 hotmail.com] 
Sent: Tuesday, March 14, 2024 12:53 AM
To: Tourapis, Alexis; mp4-tech lists.mpegif.org
Subject: RE: [Mp4-tech] compute colocated macroblock
Dear Alexis:
First, thank you for your reply.
I still have doubt for B direct mode: if direct_8x8_inference_flag=1, does four 4x4 blocks in a 8x8 block share the same MV? Since their colocated MV seems same:select the same "x" 4x4 block in the following table: 
xo|ox
oo|oo
--|--
oo|oo
xo|ox
Thanks again!
ChenJianjun
----------------------------------------
> Subject: RE: [Mp4-tech] compute colocated macroblock
> Date: Mon, 13 Mar 2024 02:19:34 -0800
> From: Alexis.Tourapis dolby.net
> To: cjj730 hotmail.com; mp4-tech lists.mpegif.org
> 
> Dear ChenJianjun
> 
> The RSD() function relates to the consideration of the direct_8x8_inference_flag. I would recommend studying JVT-C115r1.doc if you are curious about more details on this and to see a visual representation of what is done and why. This is basically what is done in section 8.4.1.2.1 (Derivation process for the co-located 4x4 sub-macroblock partitions) although without any of the explanations.
> 
> dpb_split_field() only prepares motion vectors, that is it generates the corresponding field motion vectors from frame information (which can be quite complex for the MBAFF case). Note that this is done only once after a picture is encoded. Compute_colocated() on the other hand is called before a B slice is generated and prepares/selects the appropriate motion information to be used for the encoding of this slice. It is quite important to make this separation since, due to reordering or the capability of specifying any coding order in H.264/AVC, it is possible that any picture is used for the generation of these parameters. These parameters may also be used differently if there is a certain relationship between top and bottom fields and such are obviously unknown during preparation of the field motion info. Btw you could of course do the decisions in dpb_split_field within compute_colocated but it was found a bit cleaner to prepare the motion information in a field vs frame format after a picture is encoded than redoing everything for every new slice that you have to encode (remember also that you may have to do many slices).
> 
> The code is by far not optimized (especially relating to memory operations), but well ... it works... 
> 
> Best regards,
> 
> Alexis
> 
> -----Original Message-----
> From: mp4-tech-bounces lists.mpegif.org 
> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of ChenJianjun
> Sent: Sunday, March 12, 2024 6:55 PM
> To: mp4-tech lists.mpegif.org
> Subject: [Mp4-tech] compute colocated macroblock
> 
> Dear All:
> 
> In JM reference decode code, it will use  "RSD()" in compute_colocated() function, I dont know the meaning of RSD(), why should it do like this, I cant find the reason in SPEC. Could anyone tell me about it or is there any document I can read?
> Another question: In dpb_split_field() function, it will compute the MVs of colocated macroblocks for top_field and bottom_field pictures, but in compute_colocated() function, it will compute again, Is there any difference?
> 
> Thanks very much.
> ChenJianjun.
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