[M4IF Technotes] GMC

[Kris Huber]

Soren,
This idea sounds interesting for limiting complexity of GMC.  Are any
limitations on the negative side of du[i] and dv[i] needed?  Once fully
fleshed out I think it possibly could be added as corrigendum.  It is one
more thing that encoder tools would have to worry about, though.  But the
constraints might be loose enough that nobody would be concerned.
Regards,
Kris
-----Original Message-----
From: "Sorin C. Cismas" <sorin   mobilygen.com>
To: <technotes   lists.m4if.org>
Subject: RE: [M4IF Technotes] GMC
Date: Sun, 12 May 2024 00:00:04 -0700
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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