[Mp4-tech] encoder and decoder output different motion vectors

[Steinberg Erez-RM96322]

Perhaps one of them is not giving you the MV (instead your are getting
the MVD or MVP)
MVx = MVPx + MVDx
MVy = MVPy + MVDy
Regards,
Erez
________________________________
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Tomer Segal
Sent: Wednesday, August 02, 2023 5:33
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] encoder and decoder output different motion vectors
hi,
i'm a little confused about a set of results i got using the mpeg4
reference software (MoMuSys-1.0-001220_sony).
i encoded a yuv sequence having the encoder output the motion vectors
calculated during encoding. then decoded the resulting mpeg file with
the decoder, enabling a trace of the motion vectors extracted during
decoding. i repeated these steps twice, once for a very simple yuv
sequence (few blocks change throughout the sequence) and a second time
using a more complex sequence. for the simple sequence the motion
vectors output by the encoder and the decoder were exactly the same. for
the complex sequence however there were differences between the vectors
- in most blocks vectors differed by a single halfpel, but a few of them
differed by as much as 5 halfpels.
i thought the motion vectors extracted by the decoder were suppose to be
exactly the same as the ones calculated in the encoder. am i wrong? i
tried finding the answer in the mpeg4 iso and other related articles but
i was unable to.
if indeed the vectors are supposed to be identical, what could be
causing this difference? 
Thanks,
    Tomer.
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