[M4IF Technotes] Quarter pel issues.

[Sachin S Gangaputra]

Hi all,
The MPEG4 documentation that I have describes "how" to implement quarter
pel interpolation but fails to describe "why" we choose this 8-tap
symmetric filter. Can someone point me towards references in this
direction that suggest - using this filter is better than the alternative
method of doing a simple linear interpolation.  
Further I'm not sure why one needs to perform the mirroring during motion
compensation of a given 8x8 or 16x16 block. In compensating for the next
block from the current block, we do have the whole decoded current frame -
so then do we really need this mirroring - why not just use neighboring
blocks in the interpolation. At the edges of the frame mirroring does make
sense. 
Moreover during motion compensation at the encoder mirroring is never
performed as one can interpolate the whole frame in the search of the
right quarter pel motion vector. Shouldn't both sides be consistent in
using or not using mirroring?
Thanks,
Sachin Gangaputra
Graduate Student,
Johns Hopkins Univ.
Baltimore.