[Mp4-tech] Deriving the Chroma motion vectors from Luma motion vectors.

[Prasanna Sethuraman]

Dear all, 
The JVT draft for H.264 specifies that the chroma motion vectors should be
derived from the estimated Luma motion vectors by multiplying with 2:
"If the current macroblock is a frame macroblock, the horizontal and
vertical components of the chroma motion
vector mvCLX are derived by multiplying the corresponding components of luma
motion vector mvLX by 2,
through mapping one-quarter sample mvLX units to one-eighth sample mvCLX
units
mvCLX[ 0 ] = mvLX[ 0 ]      (8-174)
mvCLX[ 1 ] = mvLX[ 1 ]      (8-175)"
But logically, since the chroma components have only half the number of
samples compared to luma, should not we be diving the luma motion vectors by
2? Or am I missing something? 
--
Prasanna.
http://www.patni.com
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