Glossary

Color sampling, or Chroma sub-sampling, is the practice of encoding images by reducing the input resolution for the chroma information. It is used in many video encoding schemes — both analog and digital.

Since the human visual system is much more sensitive to variations in brightness (knows as Luma component) than color (known as Chroma compomemts), a video system can be optimized by devoting more to the luma component (usually denoted Y), than to the color difference components Cb and Cr. For example, The 4:2:2 scheme requires two-thirds the bandwidth, as for each 4 Y samples, only 2 Cb and Cr samples are used. At normal viewing distances, there is no perceptible loss incurred by color sampling.

As in the example above, the sub-sampling scheme is commonly expressed as a three part ratio J:a:b. This ratio describes the number of luminance and chrominance samples in a conceptual region that is: J pixels wide, and 2 pixels high. The parts are (in their respective order):

• J pixels wide. Usually, 4.
• Contains a chrominance samples (Cr, Cb) in the first row.
• Contains b chrominance samples (Cr, Cb) in the second row.