[Mp4-tech] RE: global motion compensation (GMC)

[Sandip Ray]

I am aware of "mcsel" available as macroblock parameter in S(GMC)-VOP of
MPEG 4. It basically decides whether mb type is "inter" or "gmc". My
question is that whether there is any mb type which uses both of gmc as well
as motion vectors in the decoding process.
Regards,
Sandip
  -----Original Message-----
  From: Subhadeep Chatterjee [mailto:subhac noida.interrasystems.com]
  Sent: Wednesday, July 20, 2023 11:55 AM
  To: sandipkr dgbmicro.com
  Subject: Re: global motion compensation (GMC)
  Hi Sandip,
  There is one field that comes in the macroblock level called mcsel. That
flag is used to switch between global motion compensation and local motion
compensation. If that flag is set GMC is employed for that MB, and if it is
0 then motion vector comes for that MB and motion compensation is carried
out like normal inter MB.
  Hope that clarifies your doubt. Please feel free to write back if you need
any further clarification in this regard.
  Regards,
  Subhadeep Chatterjee
  Senior Member of Technical Staff
  Interra Systems Ind. Pvt. Ltd.
Hi,
The global motion estimation generates VOP level parameters for S(GMC)-VOP
in MPEG 4 bitstream. I don't think it generates any macroblock level
parameters (e.g. motion vector). The macroblock type within a S(GMC)-VOP can
be intra, inter, gmc or not coded. I understand GMC parameters capture
camera motion rather than object motion.
Are there any macroblock types where motion vectors are used along with
global motion compensated VOP to capture both of camera motion and object
motion ? Please clarify my doubt and whether any of my understanding is not
correct.
Thanks,
Sandip
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