[Mp4-tech] Motion Compensation Quarter Pixel Interpolation

[Erik Schlanger]

I am looking at the way quarter pixel interpolation is performed for 
Mpeg4 motion compensation. The way the interpolation is performed in 
ffmpeg is different than the way it is done by the ISO MPEG4 reference 
decoder. Specifically, the ffmpeg code will do all neccesary 8-tap 
filtering prior to doing any bilinear interpolation. However, in the ISO 
MPEG4 reference decoder all 8-tap filtering for the horizontal direction 
will be performed followed by bilinear interpolation in the horizontal 
direction. The results of this are then fed to the vertical 8-tap 
filter, followed by the vertical bilinear filter.
Can anyone comment on what effect this different implementaion may have? 
  I have consulted the specification and it is unclear to me. Looking at 
numerical outputs the two implementations do produce different results.
Thanks,
Erik