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

[Tourapis, Alexis]

Dear Prasanna,
It may be good to read this in context with the section entitled
"Fractional Sample Interpolation Process". More specifically see
derivation of xIntc and yIntc.
Btw, apparently you are using an old version of the text since it
appears what you are reading does not consider FRExt profiles (i.e.
including support of 4:2:2 and 4:4:4).
Best regards,
Alexis
  _____  
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Prasanna
Sethuraman
Sent: Thursday, February 23, 2024 4:53 AM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] Deriving the Chroma motion vectors from Luma
motionvectors.
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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