[M4IF Technotes] information regarding halfpel interpolation in the quartersample mode

[Vinod Kumar]

Hello Gary
	Thankyou for clearing some doubts of mine. There is one more
clarification regarding 1. that you have replied. Since according to the
standard(as I can understand) for every interger pel the calculation of
half pel is done by filtering horizontally and vertically and the
resultant halfpel values that i get is positioned like the way shown
below
	I  H
        H  H
	where H is the halfpel value and I is the integer pel in focus
isnt it almost every time enough if I grab 1 more column to the left and
one more row above without bothering about whether the Integerpel
motionvector is + or - 
Am I missing something over here? please enlighten me.
regards
vinod
Gary Sullivan wrote:
> 
> Vinod,
> 
> 1) The remark about biasing means that if the
>    fractional part of the motion vector is positive,
>    you need to grab one more column of samples to
>    the right or one more row below the samples of
>    the block at the location pointed to by the motion vector.
>    If that fractional part is negative, you need
>    a column to the left or a row above.
>    If you need to interpolate to predict a 16x16
>    area, you will need 17x17 samples in the reference
>    picture.  The same is true for half-pel motion.
> 
> 2) As reported a few days ago on this reflector, there
>    is a problem with the specification of MPEG-4's
>    quarter pel motion.  It's too early to predict
>    how that will be resolved, but most people seem
>    in favor of the method in the reference software.
> 
> 3) Your remark about trying to calculate eight
>    quarter pel positions around an integer pel vector
>    gives me the impression that you're talking about
>    how do a search in the encoder.  The spec only
>    covers how the decoder operates, and the decoder
>    doesn't need to do anything that fits that description.
>    Again I would point to the reference software if
>    you want an illustration of how to design an encoder.
> 
> Best Regards,
> 

> Gary Sullivan
> 
> +> -----Original Message-----
> +> From: Vinod Kumar [mailto:kmr   iis.fhg.de]
> +> Sent: Thursday, October 25, 2023 4:52 AM
> +> To: technotes   lists.m4if.org
> +> Subject: [M4IF Technotes] information regarding halfpel
> +> interpolation in the quartersample mode
> +>
> +>
> +> Hello
> +>      i have a confusion regarding halfpel interpolation in
> +> the quartersample
> +> mode. The standard says "a reference block of size
> +> (M+1)x(N+1) biased in
> +> the direction of the half or quarter sample position is read from the
> +> reconstructed and padded reference VOP." what does biased
> +> hear mean? Do
> +> I have to calculate the half pel values 8 times for the
> +> eight adjacent
> +> positions around the Integerpel mv? If anybody have any web reference
> +> regarding the principle of quarterpel interpolation please do let me
> +> know.
> +> --
> +> Vinod
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