[Mp4-tech] [video] Followup: interlaced chrominance bug in MS reference?

[Sven.Siggelkow sci-worx.com]

After discussion with Vqual experts it seems the question rather is: Why
does MS reference have that factor 2 treatment, i.e. perform rounding on
field instead of frame vectors? That may be reasonable but is it really
standardized that way?
>-----Original Message-----
>From: mp4-tech-bounces lists.mpegif.org 
>[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of 
>Siggelkow Sven (SCI)
>Sent: Mittwoch, 2. Februar 2005 11:53
>To: mp4-tech lists.mpegif.org
>Subject: [Mp4-tech] [video] interlaced chrominance bug in MS reference?
>
>
>Hi there,
>
>I'm wondering whether we've found a bug in the MS reference SW which is
>not reported yet in "list of problems" (n6363 version). The chrominance
>MV calculation in case of interlaced involves div2round which 
>is clearly
>defined in the standard:
>
>Div2Round(x) = (x >> 1) | (x & 1) )
>
>However the MS reference SW (microsoft-v2.5-040207-NTU) seems to run
>through another implementation in mc.cpp mvLookupUV:
>      xRefUV1 = ((dx & 3) ? ((dx >> 1) | 1) : (dx>>1));
>      yRefUV1 = ((dy & 6) ? ((dy >> 1) | 2) : (dy>>1));  
>Probably the vertical has its own factor 2 treatment but looking at the
>following example it is illogical, e.g. a value of 2 would become 3,
>i.e. BIGGER after this function:
>
>MS "x" implementation looks ok
>halfpel vectors:  x00	-> x0
>                  x01   -> x1 
>                  x10   -> x1
>                  x11   -> x1                  
>                  
>MS "y" implementation is strange
>halfpel vectors: x00x	-> x00
>                 x01x   -> x11    <= THIS SEEMS WRONG I'D ASSUME x10 !
>                 x10x   -> x10
>                 x11x   -> x11
>
>
>--
>Dr. Sven Siggelkow
>Senior Design Engineer
>
>
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