[Mp4-tech][video][MPEG4] MV limitation and bounding rectangle of a rectangular VOP

[J. Miles]

Hi everyone!
While working with MPEG-4 (Part 2, not AVC) decoding, I have been looking at
the use of unrestricted motion vectors in relation to the MV limitation to
bounding rectangles and find that the Standard (sections 7.6.1-7.6.4) is not
entirely clear on that. Let me try to describe the case:
The video bitstreams handled are purely coded as usual rectangular objects
(no sprites or shapes). In the situation, where the video_object_layer_width
and/or video_object_layer_height are not dividable by 16, the coded size is
extended so that it is dividable by 16 and fits with complete MBs. For
instance, a VOL size of 800×600 would have the coded size being 800×608 to
fit with an integral number of MBs in the height. As motion vectors are
allowed to be unrestricted, they may point outside the actual VOL area and
into the extended VOL area and actually also outside that area. This has an
impact on the MV limitation described in section 7.6.4 for unrestricted
motion vectors because the MV is limited "...to the last full pel position
inside the decoded VOP area". In that section and the other related sections
of the Standard, there are mostly references to arbitrarily shaped objects
and not that many to rectangular objects. Although in section 7.6.4, the
bounding rectangle of a rectangular VOP is defined as being equal to the
coded (and extended) area.
 1) What is the 'decoded VOP area' of a rectangular VOP? Is it the area of
the so-called 'displayable part' of the VOP (800×600), or is it the full
coded VOP area (800×608)?
 2) If the 'decoded VOP area' refers to the full coded VOP area including
extensions, what would the extended parts contain? Would they contain the
actual coded information (I assume that there are no requirements to what
that would be), or should the so-called repetitive padding process of
section 7.6.1 be applied to the extended parts (although that section begins
with that the padding process is defined for "...samples outside the VOP for
prediction of arbitrarily shaped objects")?
I do hope someone can help me in understanding this better.
Thanks.
Jay
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