[M4IF Technotes] GMC

[Sorin C. Cismas]

I don't think there are any restrictions on the motion vectors.  I have
the same concern about the need to fetch 64 non-contiguous 2x2 pels
to predict a macroblock.  To reduce complexity and bandwidth requirements,
it is highly desirable to put some limits on du[i] and dv[i] for i>0.
2 and 3 warping points are usefull for zoom-in and zoom-out, however, it is
not realistic to assume, for example, a 10x zoom-out on an S(GMC) prediction.
A 50% or even 25% zoom-out restriction will be more than sufficient.
For 50%, this will translate to du[i]<W    and dv[i]<H    for i>0.
For 25%, this will translate to du[i]<W//2 and dv[i]<H//2 for i>0.
The 50% restriction will limit the macroblock luma prediction to 25x25.
The 25% restriction will limit the macroblock luma prediction to 21x21.
Can these or similar restrictions be considered for a future corrigendum?
Thanks,
Sorin Cismas
> -----Original Message-----
> From: technotes-admin   lists.m4if.org
> [mailto:technotes-admin   lists.m4if.org]On Behalf Of Kasturi Rangam
> Sent: Friday, May 10, 2024 2:46 PM
> To: technotes   lists.m4if.org
> Subject: [M4IF Technotes] GMC
> 
> 
> I am trying to under GMC in MPEG4.
> Looks like you can have 0, 1, 2 or 3 sprite_warping_points.
> 
> To decode a 0 and 1 sprite_warping_points macroblock, you can
> fetch the whole macroblock data from the reference to do pixel
> prediction.
> 
> However, for 2 and 3 warping_points, the motion vector for each
> pixel can be anywhere in the reference frame. Thus we might need
> to perform 64 data fetches to predict one macroblock.
> 
> Is there a range that can be calculated for each macroblock, so that
> we can fetch data only once?
> 
> Thanks,
> 
> Kasturi
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