Video - A Practical Guide to Video and Audio Compression - page 109

The second digit (which is often the value “2”) means that during the same sampling
period, only 2 samples are taken for each of the color-difference signals in the horizontal
axis.

The third digit (which is also often the value “2”) describes the sampling in the ver-
tical axis but this sub-sampling involves some additional computation to halve the amount
of data.
There is sometimes a fourth digit that represents an alpha channel-blending value.
This becomes important for the more advanced codecs described by MPEG-4 and must be
sampled at the same rate as the luma signal or else some unpleasant fringing effects will
appear around the edges of the alpha-blended video object.
Table 5-8 lists some related formats that are relevant: In the table below, the first sam-
pling pattern is called 4:4:4. The full range of values is preserved and this is essentially an
uncompressed pixel map. Luma and chroma are both sampled at full resolution.
Some data reduction is achieved with 4:2:2 sampling, which reduces the amount of
chroma information by only sampling half the pixels. The other pixels are ignored. Note
that this is not an average value of two samples; it is selecting one of two samples.
A further reduction to a quarter of the chroma information is achieved with the 4:1:1
sampling pattern. This ignores 3 out of 4 color values and only records the fourth.

Table 5-8 Luma and Chroma Sample Sizes

Format Description Cell size Chroma Bytes Factor Fidelity

4:4:4 Luma and chroma sampled 2 × 2 2 × 2 12 1:1 Uncompressed

at full resolution

4:2:2 Luma sampled at 4 times 2 × 2 1 × 2 8 1.5:1 Lossy

the sub-carrier frequency.
Chroma sampled at half
the resolution on the
horizontal axis and
down-sampled in the
vertical axis

4:1:1 Double-sized aperture cell 4 × 2 1 × 2 6 2:1 Lossy

to allow higher
compression ratio for
chroma. 8:2:2 is a more
functional description
of the sampling.

4:2:0 Only one chroma sample 2 × 2 1 × 1 6 2:1 Lossy

for the whole block of
luma samples