Chroma subsampling

Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance.

It is used in many video and still image encoding schemes – both analog and digital – including in JPEG encoding.

Digital signals are often compressed to reduce file size and save transmission time. Since the human visual system is much more sensitive to variations in brightness than color, a video system can be optimized by devoting more bandwidth to the luma component (usually denoted Y'), than to the color difference components Cb and Cr. In compressed images, for example, the 4:2:2 Y'CbCr scheme requires two-thirds the bandwidth of non-subsampled "4:4:4" R'G'B'. This reduction results in almost no visual difference as perceived by the viewer.

The human vision system processes color information (hue and colorfulness) at about a third of the resolution of luminance (lightness/darkness information in an image). Therefore it is possible to sample color information at a lower resolution while maintaining good image quality.

This is achieved by encoding RGB image data into a composite black and white image, with separated color difference data (chroma). For example with ${\displaystyle Y'C_{b}C_{r}}$, gamma encoded ${\displaystyle R'G'B'}$ components are weighted and then summed together to create the luma ${\displaystyle Y'}$ component. The color difference components are created by subtracting two of the weighted ${\displaystyle R'G'B'}$ components from the third. A variety of filtering methods can be used to limit the resolution.

Gamma encoded luma ${\displaystyle Y'}$ should not be confused with linear luminance ${\displaystyle Y}$. The presence of gamma encoding is denoted with the prime symbol ${\displaystyle '}$.

Gamma-correcting electro-optical transfer functions (EOTF) are used due to the nonlinear response of human vision. The use of gamma improves perceived signal-to-noise in analogue systems, and allows for more efficient data encoding in digital systems. This encoding uses more levels for darker colors than for lighter ones, accommodating human vision sensitivity.

The subsampling scheme is commonly expressed as a three-part ratio J:a:b (e.g. 4:2:2) or four parts, if alpha channel is present (e.g. 4:2:2:4), that describe 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):