Description

This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.

88 How are the subsampled chroma samples cited ? (MPEG-2)

It is moderately important to properly co-site chroma samples,
otherwise a sort of chroma shifting effect (exhibited as a halo) may
result when the reconstructed video is displayed. In MPEG-1 video, the
chroma samples are exactly centered between the 4 luminance samples
(Fig 1.) To maintain compatibility with the CCIR 601 horizontal
chroma locations and simplify implementation (eliminate need for phase
shift), MPEG-2 chroma samples are arranged as per Fig.2.

  Y   Y   Y   Y             Y   Y   Y   Y            YC  Y   YC  Y
   C       C                C         C
  Y   Y   X   Y             Y   Y   Y   Y            YC  Y   YC  Y
  
  Y   Y   Y   Y             Y   Y   Y   Y            YC  Y   YC  Y
    C       C               C         C
  Y   Y   Y   Y             Y   Y   Y   Y            YC  Y   YC  Y
  
  Fig.1 MPEG-1               Fig.2  MPEG-2           Fig.3 MPEG-2 and 
 4:2:0 organization         4:2:0 organization         CCIR Rec. 601
                                                     4:2:2 organization

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