[Mp4-tech] Motion Compensation Quarter Pixel Interpolation

[Piyush Kaul]

Seems to be a problem to me too.
As I understand the specs , only full-pel or other half-pel locations should be used for interpolating  half-pel positions. The quarter-pel positions are
then obtained by bilenear interpolation using the half-pel and quarter pel positions . The microsoft decoder seems to do it in the wrong order.  The values
for any combination of XFrac=1,3 and YFrac=1,2,3 should be wrong.
Regards
Piyush
Erik Schlanger wrote:
> 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
>
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