[M4IF Technotes] Motion estimation + SAD

[Peter Haighton]

Graham,
I am not sure exactly what you mean below. SAD is used as to to help find
the best matching block that minimizes the difference between the current
and the predictor frame. There are other methods besides SAD. If you have
already determined that the motion vector is at (0,0) then there is little
reason to do a SAD.
MPEG-4 does support higher bits/pixel in the N-bit profile, but the rest
of the video profiles only support 8bits/pixel/component.
Peter
--
Peter Haighton
VideoSpheres Inc.
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See http://www.m4if.org for the latest on MPEG-4
-----Original Message-----
From: technotes-admin   lists.m4if.org
[mailto:technotes-admin   lists.m4if.org]On Behalf Of the_ether
Sent: Wednesday, November 28, 2023 10:54 PM
To: Technotes   lists.m4if.org
Subject: [M4IF Technotes] Motion estimation + SAD
I have some basic questions:
1- What is the formula for calculating the sum of absolute differences
(SAD)
for a block, for which the motion vector (MV) has been determined to be
(0,0)?
I have seen:
    SAD = SAD(0,0) - (Nb / 2 + 1)
but with two interpretations of Nb:
a) Nb = the number of pixels inside the VOP
b) same as (a) but multiplied by 2^(bits per pixel - 8) and number of
pixels
inside the macroblock instead of VOP
I have a feeling that MPEG-4 only supports 8 bits per pixel, or at least
only 8 bits each pixel in each of the Y, Cb and Cr components. In which
case
2^0 = 1 so (a) = (b). (Except for no. of pixels  inside VOP vs MB which I
think is a typo)
2- Shouldn't Nb be the number of pixels in a *block* (ie Nb = 64) instead
of
a macroblock since DCT is calculated on each of the 6 blocks within a
macroblock?
3- Why have this special case for MV(0,0) anyway? What if we have no
movement between two frames (ie SAD(0,0) = 0), why subtract (Nb / 2 + 1)
prior to performing DCT on that block ?
Regards
graham
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