[M4IF Technotes] Question about chrominance MVs in a 4MV Macroblock

[Luc Martel]

Tkx, 
just to make sure that I understand what's going on.
What's the rounding I should make if I have only one
motion vector in a MB? Should it be done using table
7-9?
Tkx.
Luc
> -----Original Message-----
> From: technotes-admin   lists.m4if.org
> [mailto:technotes-admin   lists.m4if.org]On Behalf Of Dan Searles
> Sent: Tuesday, March 18, 2024 6:18 PM
> To: lmartel   atsana.com
> Cc: technotes   lists.m4if.org
> Subject: Re: [M4IF Technotes] Question about chrominance MVs in a 4MV
> Macroblock
> 
> 
> 
> Change:
> 
> temp_int = temp >> 8; //  temp_int = 6
> 
> 
> to:
> 
> temp_int = temp / 16; //  temp_int = 3
> ('>> 8' is probably a typo of '/8')
> 
> and
> 
> MVx = 2*temp_int + roundtab16[temp_rem] = 2*6 + 1; = 13
> 
> to:
> 
> MVx = 2*temp_int + roundtab16[temp_rem] = 2*3 + 1; = 7
> 
> and you are done. MVx is the Chrom X vector. The way I look
> at it is the integer parts are handled separately from the
> fraction parts. Be careful to treat a negative sum as positive
> during the middle of the calc and restore to negative after.
> 
> 
> 
> Luc Martel wrote:
> 
> >Hi all,
> >
> >here is my understanding of what's in the standard but obviously I
> >understand it wrong because I'm getting very strange results.
> >
> >Let's suppose I have these MVs for a macroblock (in half 
> sample accuracy),
> >coordinates are (X,Y)
> >
> >There is no transparent macroblock so I'm using table 7-6 
> for 16th pixel
> >position.
> >
> >(15, 20) (12, -11)
> >(13, 15) (11, -9)
> >
> >
> >Then I calculate the MV for both chrominance blocks
> >X :
> >roundtab16[16] = {0,0,0,1,1,1,1,1,1,1,1,1,1,1,2,2};
> >temp = (15+12+13+11) // sum of x component in hp accuracy
> >temp_int = temp >> 8; //  temp_int = 6
> >temp_rem = temp % 16; // temp_rem = 3
> >MVx = 2*temp_int + roundtab16[temp_rem] = 2*6 + 1; = 13
> >
> >this result would be in half sample accuracy in the luminance image.
> >MVx = 13//2 -> 7
> >
> >same thing for Y.
> >
> >What I'm I doing wrong and is there any explanation on this 
> clearer that
> >what is written in the standard?
> >
> >Tkx in advance
> >
> >Luc
> >
> >
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> >
> >
> >  
> >
> 
> -- 
> 
> Dan Searles
> Atmel Multimedia & Communications
> 3800 Gateway Centre, Suite 311
> Morrisville, NC 27560
> phone: (919)462-6553 fax: (919)462-0300
> 
> 
> 
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