[M4IF Technotes] 'invalid' motion vector

[Biswajit Acharya]

Hi Hirakawa,
Motion vector components of Intra MBs (type 3 or 4) are considered to be
zero. Hecne, if these MBs are valid (in the sense mentioned in the
spec.), then assume the corresponding motion vector components to be
zero.
--
Bach
Keigo Hirakawa wrote:
> 
> Hi all.  This is probably a question that has been answered before.
> 
> In section 7.6.5 of the MPEG4 spec part 2, vector decoding processing is
> described.  Clause 1) says
> 
> "If a candidate predictor MVi is in a transparent spatial neighborhood
> macroblock or in a transparent block of the current macroblock it is not
> valid, otherwise, it is set to the corresponding block vector"
> 
> In the paragraph immediately following, the spec says
> 
> "note that any neighborhood macroblock outside the current VOP or video
> packet or outside the current GOB for which gob_header_empty is "0" is
> treated as transparent in the above sense."
> 
> Now, suppose we are coding a P-VOP.  If macroblock corresponding to MVi is
> type 3 or 4 (ie, no motion vector), what happens to MVi?  The spec does not
> specifically say "make it invalid".
> 
> Thanks.
> Keigo
> 
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