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Chrominance Locations for 4:2:0 Interstitial/Co-Sited Interpolation

In the first format, called 4:2:0 interstitial, the interpolation positions are centered at locations that are halfway between two adjacent full-resolution pixels, both horizontally and vertically. In the second format, called 4:2:0 interstitial/co-sited, the interpolation locations are halfway between two pixels vertically and are aligned with original pixel locations horizontally. Table 3-2 shows the 4:2:0 variations used by the standard video conferencing codecs.

Table 3-2 4:2:0 Formats Used by Video Codecs

Codec Standard

Image Format

H.261

4:2:0 interstitial

H.263

4:2:0 interstitial

MPEG-4 Part 2

4:2:0 interstitial/co-sited

H.264

4:2:0 interstitial/co-sited

In addition, codecs must use a special variation of 4:2:0 when field coding interlaced video data. Figure 3-3 shows this variation.

For each individual field in an interlaced image, the encoder offsets the location of the chroma interpolation point up or down vertically, depending on whether the field is the top field or the bottom field. As a result, the chroma sampling positions are spatially uniform, both within each field and within the entire two-field frame.

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