DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 10.4 Interstitial chroma
filter for JPEG/JFIF averages
samples over a 2×2 block.
Shading represents the spatial
extent of luma samples. The
black dot indicates the effective
subsampled chroma position,
equidistant from the four luma
samples. The outline represents
the spatial extent of the result.
1 ⁄4
1 ⁄8
1 ⁄8
1 ⁄4 1 ⁄4
1 ⁄4 1 ⁄4
1 ⁄ 2 1 ⁄4
Figure 10.5 Cosited chroma
filter for Rec. 601, 4:2:2
causes each filtered chroma
sample to be positioned
coincident – cosited – with an
even-numbered luma sample.
1 ⁄4
1 ⁄4
1 ⁄8
1 ⁄8
Figure 10.6 Cosited chroma
filter for MPEG-2, 4:2:0
produces a filtered result
sample that is cosited horizontally,
but sited interstitially in
the vertical dimension.
Chroma subsampling filters
In chroma subsampling, the encoder discards selected
color difference samples after filtering. A decoder
approximates the missing samples by interpolation.
To perform 4:2:0 subsampling with minimum computation,
some systems simply average C B over a 2×2 block,
and average C R over the same 2×2 block, as sketched in
Figure 10.4 in the margin. To interpolate the missing
chroma samples prior to conversion back to R’G’B’,
low-end systems simply replicate the subsampled C B
and C R values throughout the 2×2 quad. This technique
is ubiquitous in JPEG/JFIF stillframes in
computing, and is used in M-JPEG, H.261, and
MPEG-1. This simple averaging process causes subsampled
chroma to take an effective horizontal position
halfway between two luma samples, what I call interstitial
siting, not the cosited position standardized for
studio video.
A simple way to perform 4:2:2 subsampling with horizontal
cositing as required by Rec. 601 is to use weights
of [ 1⁄ 4 , 1⁄ 2 , 1⁄ 4 ], as sketched in Figure 10.5. 4:2:2
subsampling has the advantage of no interaction with
interlaced scanning.
A cosited horizontal filter can be combined with
[ 1⁄ 2 , 1⁄ 2 ] vertical averaging, as sketched in Figure 10.6,
to implement 4:2:0 as used in MPEG-2.
Simple averaging filters like those of Figures 10.4, 10.5,
and 10.6 have acceptable performance for stillframes,
where any alias components that are generated remain
stationary, or for desktop-quality video. However, in
a moving image, an alias component introduced by
poor filtering is liable to move at a rate different from
the associated scene elements, and thereby produce
a highly objectionable artifact. High-end digital video
equipment uses sophisticated subsampling filters,
where the subsampled C B and C R of a 2×1 pair in 4:2:2
(or of a 2×2 quad in 4:2:0) take contributions from
several surrounding samples. The relationship of filter
weights, frequency response, and filter performance will
be detailed in Filtering and sampling, on page 141.
CHAPTER 10 INTRODUCTION TO LUMA AND CHROMA 93