[M4IF Technotes] About the "validity" in BME

[Zhou Guanfeng]

Hi,everyone,
  I get a question in shape coding BME.
  In MPEG-4 VM18, when finding the MPVs, "by looking into MVs1,MVs2,MVs3,MV1,MV2,MV3 in this order, MVPs is determined by taking the first encountered MV that is valid". 
  But what is "valid"? Suppose the neighborhood BAB is all_0 or all_255 (i.e. type 3 or 4), they have no MVs, are they  valid? What's the value of MVs if valid? 
  Then how about the case of type 4(i.e. intraCAE)?
  Thanks in advance.
  Z.G.F.
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