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

[SACHIN GANGAPUTRA]

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