DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

digital subsampling schemes. In 4:2:2 subsampling,
after filtering, alternate color difference samples are
discarded at the encoder. In 4:2:0, vertical chroma
detail is removed as well. At the decoder, the missing
samples are approximated by interpolation.
In analog chroma bandlimiting, and in digital subsampling,
some color detail is lost. However, owing to the
poor color acuity of vision, the loss cannot be detected
by a viewer at normal viewing distance.
Some low-end digital video systems simply drop
chroma pixels at the encoder without filtering, and
replicate chroma pixels at the decoder. Discarding
samples can be viewed as point sampling; that operation
runs the risk of introducing aliases. Proper decimation
and interpolation filters should be used; these
should be designed according to the principles
explained in Filtering and sampling, on page 141.
Luma/color difference summary
When luma and color difference coding is used for
image interchange, it is important for the characteristics
of red, green, and blue to be maintained from the
input of the encoder to the output of the decoder. The
chromaticities of the primaries were detailed in Color
science for video, on page 233, and mentioned in this
chapter as they pertain to the encoding and decoding
of luma. I have assumed that the characteristics of the
primaries match across the whole system. The primaries
upon which luma and color difference coding are
based are known as the interchange (or transmission)
primaries.
In practice, a camera sensor may produce RGB components
whose chromaticities do not match the interchange
primaries. To achieve accurate color
reproduction in such a camera, it is necessary to insert
a 3×3 matrix that transforms tristimulus signals from
the image capture primaries to the interchange primaries.
(This is the “linear matrix” built into the camera.)
Similarly, a decoder may be required to drive a display
whose primaries are different from the interchange
primaries; at the output of the decoder, in may be
CHAPTER 24 LUMA AND COLOR DIFFERENCES 293