[M4IF Technotes] Rounding of motion vectors for chroma?

[Mithun N]

hi christoph,,
when quarter pel is active, the max accuracy u can
seek is
0.25 ..right???
for eg..if the four motion vectors are
 8.25, 6.25,3.25.and 5......for the luma blocks
then the chrominance motion vector is calculated
as
 mv_chr = sum/4 = 22. 75/4 = 5.6875
but since the chroma vector is calculated as the
above given formula...the value can go upto 0.24/4
= 1/16
which is i/4 of quarter pel
similarly if there are 3,2or 1 non transparent
luma blocks then the motion vectors shall be
1/12,1/8 or 1/4 of quarter pel respectively
Now these vectors which can go to the accuracy of
1/16 (0.0625) have to be rounded to the nearest
halfpel resolution
as given in the table 7.6 - 7.9.... since the
result needs to be at a half pixel
i hope this answers ur query...
regds
mithun
-----Original Message-----
From: technotes-admin   lists.m4if.org
[mailto:technotes-admin   lists.m4if.org]On Behalf
Of Christoph Lampert
Sent: Wednesday, October 30, 2023 8:21 PM
To: technotes   lists.m4if.org
Subject: [M4IF Technotes] Rounding of motion
vectors for chroma?
Hi,
I have a question about rounding motion vectors,
especially for
chroma if quarterpel is active. I'll use a simple
example.
Consider a macro block in inter4v mode with
luminance motion vectors
mv[0] to mv[3] in halfpel or quarterpel
resolution.
How do I calculate the motion vector for chroma
from them?
I know of course that it's
mv[0]+mv[1]+mv[2]+mv[3] / (2*4)
but how exactly is rounding supposed to be done?
What I read from the
standard about this is as unclear as possible.
Thank you in advance,
Christoph
--
Christoph H. Lampert chl   math,uni-bonn,de | Diese
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