[XviD-devel] A little more rounding

[Christoph Lampert]

Hi,

I had a look at MPEG-4 working documents, "List of Problems"

http://mpeg.telecomitalialab.com/working_documents/mpeg-04/visual/visual_implementers_guide.zip

Id 4.8 is rather clearly connected to "our" problem: 

Problem:
-------------------------
A truncation error of the chrominance field MV with quarter_pel_mode
P/S-VOP: ISO/IEC 14496-2 Section 7.6.5 says,
 "Motion vector MVDCHR for both chrominance blocks is derived by
calculating the sum of the K luminance vectors, that corresponds to K 8x8
blocks that do not lie outside the VOP shape and dividing this sum by
2*K; in quarter sample mode the vectors are divided by 2 before
summation."



Solution : 
---------------------------------
 Line 4354 in mot_comp.c L4354(line 3768 of OptSimple version)
replace 
     dy = (Int)(2*yvec[0]);
with 
     dy = (Int)(2*yvec[0])/2*2;

 Line 4534 in mot_comp.c(line 3948 of OptSimple version) replace
     dy = (Int)(2*yvec[1]);
with 
     dy = (Int)(2*yvec[1])/2*2;



other topic may, too...

gruel