[M4IF Technotes] MPEG-4 quarter-pel motion compensation

[Deepali Arya]

In both half sample and quarter sample mode, you need one extra pixel value
for interpolation - 
For eg. for generating 16 interpolated pixels, you will need 17 reference
pixels. If any of these 17 pixels lies outside the decoded VOP, you need to
use the edge pixel to recreate that. 
In the quarter sample mode, once you have obtained the reference pixels
(with edge repeatition if needed), the next step is to mirror the first 3
and the last 3 pixels on each side: 
For eg. if your 17-pixel row is :
	a b c d e f g h i j k l m n o p q
Then the mirrored row is:
	c b a a b c d e f g h i j k l m n o p q q p o
You use this mirrored row for filtering in the quarter sample mode.
Hope this helps.
-----Original Message-----
From: SACHIN GANGAPUTRA [mailto:SSG14   JHUNIX.HCF.JHU.EDU]
Sent: Thursday, June 20, 2023 11:31 PM
To: technotes   lists.m4if.org
Subject: [M4IF Technotes] MPEG-4 quarter-pel motion compensation
Hi,
I have a small bug in my implementation of quarter pel compensation and 
I suspect its something to do with unrestricted motion compensation. In 
reference to sec 7.6.4 in the standard 
"...In case of half sample mode, when a sample referenced by a motion 
vector stays outside the decoded VOP area, an edge sample is used. 
This edge sample is retrieved by limiting the motion vector to the last full
pel position inside the decoded VOP area. Limitation of a motion vector is 
performed on a sample basis and separately for each component of the 
motion vector, as depicted in Figure 7-19. In case of quarter sample 
mode, when a sample needed for interpolation (see subclause 7.6.2.2) 
stays outside the decoded VOP area, an edge sample is used prior to 
block boundary mirroring."
I'm not sure what the difference between half and quarter is based on this 
statement. I don't completely understand what the last line implies. Any 
help in this direction would be greatly appreciated.
Thanks,
Sachin Gangaputra
Grad Student, 
Johns Hopkins UIniversity,
Baltimore
_______________________________________________
Technotes mailing list
Technotes   lists.m4if.org
http://lists.m4if.org/mailman/listinfo/technotes