[M4IF Technotes] GMC

[Yoshinori Suzuki]

Dear Dr. Cismas and all,
> GMC is usefull to transmit only once a motion vector that applies or
> can be extrapolated by simple scaling to several macroblocks.  Beyond
> that, I'm not sure about its real life usefullness.  Without apriori
> knowledge, it will be quite difficult for the encoder to figure out how
> to use this feature.  And even if it does, I'm not sure how many bits
> are saved.  But on the downside, an MPEG4 decoder which claims to be
> ASP compliant, has to support these features even if all commercial
> encoders will never use affine based prediction in all its generality.

In my experience on the MPEG-4 standard activities, the affine based 
prediction provides 10-30% bit saving compared with the Version 1 tool sets 
of MPEG-4 Video at low bitrate if the sequence has zooming scene and 
the actual global motion is detected. And some methods for the global motion 
estimetion have been developed and made public. I understand that 
the encoding techniques on GMC are not familier to the hardware implementers, 
since its features are not found in the MPEG-2 and MPEG-4 Version 1 standard.
However, I believe that it will be available in the near future, and 
the functions for affine tranform in the ASP decoder will be useful.
Best regards,
Yoshinori Suzuki
Hitachi, Ltd.