[M4IF Technotes] Some puzzles about stuffing macroblock at end of VOP

[dsearles]

There are some posts related to this area from me on 10/15 and 10/18.
Dan Searles
Gary Sullivan wrote:
>+> -----Original Message-----
>+> From: Lefan Zhong [mailto:Lefan   mediaq.com] 
>+> Sent: Wednesday, February 05, 2024 5:44 PM
>+> To: technotes   lists.m4if.org
>+> Subject: [M4IF Technotes] Some puzzles about stuffing 
>+> macroblock at end of VOP
>+> 
>+> 
>+> Hi folks,
>+> 
>+> I have some questions about stuffing macroblocks in the end 
>+> of VOP. The official document (14496-2, 2001) and the 
>+> reference softwares conflict at the issue. I would like to 
>+> share my opinions here. I would like to hear your comments too.
>
>
>I don't know to what extent this issue was actually really considered
>in the drafting of the standard, but your analysis looks reasonable.
>If I were writing a decoder, I would allow for the stuffing
>to be there.  If I were writing an encoder, I would try to avoid using
>it, as I would be afraid some decoders would not be built to allow it.
>
>I do remember the topic coming up before.  I don't remember whether
>there was this much information provided analyzing the spec and
>reference software when the subject came up, and I don't remember
>whether MPEG decided to do anything to alter the document or the
>software.
>
>+> 
>+> 1) It is legal to insert some stuffing macroblocks after the 
>+> last non-stuffing macroblock in the VOP if the data 
>+> bitstream is not data partition format and not short video 
>+> header. Since in the document section 6.2.5.3, the function 
>+> combined_motion_shape_texture( ) uses do...while loop to 
>+> keep checking until resyn_marker of start_new_VOP. But 
>+> Momusys reference software doesn't skip the stuffing and go 
>+> to next VOP.
>
>So it sounds like there is a bug in that software or a
>problem in the document.  This is a case where the document
>and software appear to conflict.  Both are "normative", so
>it is up to the MPEG committee to decide which is right.
>
>
>+> 
>+> 2) If the bitstream is short video header, it is illegal to 
>+> insert the stuffing macroblocks after the last non-stuffing 
>+> macroblock in VOP. Since in the document section 6.2.5.2, 
>+> the function gob_layer( ) use for loop for macroblock 
>+> decoding. After the last non-stuffing macroblock is decoded, 
>+> the decoder should go out of the loop and shouldn't checking 
>+> any stuffing macroblock.  But Microsoft reference software 
>+> processes further skipping for the stuffing macroblocks.
>
>
>The fact that the Microsoft reference software skips over it
>does not necessarily mean anything in terms of whether it is
>allowed or not.  We would only consider something in the
>decoding software to be a bug if the decoder did not respond
>correctly to a conforming bitstream.  Here you're talking
>about something else - how the decoder responds to what appears
>to be a non-conforming bitstream.
>
>+> 
>+> 3) If the bitstream is data partitioned, it is illegal to 
>+> insert the stuffing macroblocks after the last non-stuffing 
>+> macroblock in VOP. Since in the document section 6.2.5.3, 
>+> function data_partitioned_i_vop() and 
>+> data_partitioned_p_vop( ), the stuffing macroblocks should 
>+> be in mcbpc, and should not at the end of packet. But both 
>+> Momusys and Microsoft reference software check the stuffing 
>+> macroblocks and skip them.
>
>Interesting.  Again, since there is no requirement for what
>the decoder should do when given a non-conforming bitstream,
>the status quo may be OK in this case.
>
>+> 
>+> Maybe there is some amendment on this issue but I don't know yet.
>+> 
>+> Thank you for your time.
>+> 
>+> --Lefan
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-- 
Dan Searles
Atmel Multimedia & Communications
3800 Gateway Centre, Suite 311
Morrisville, NC 27560
phone: (919)462-6553 fax: (919)462-0300