[M4IF Technotes] 'invalid' motion vector

[Keigo Hirakawa]

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