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

[Gary Sullivan]

Comments:
1) You're looking at an old draft.  Get a new one.  (You can get it free
from ITU-T.)
2) Pay attention to the equations rather than the words in this case.
3) There is no actual multiplication operation involved here since the
various unit conversions cancel it out.
Best Regards,
Gary Sullivan
________________________________
	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
	World-Wide Partnerships. World-Class Solutions. 
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