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 encoding schemes – both analog and digital – and also 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 (4:4:4) R'G'B'. This reduction results in almost no visual difference as perceived by the viewer.
How subsampling works
Because the human visual system is less sensitive to the position and motion of color than luminance, bandwidth can be optimized by storing more luminance detail than color detail. At normal viewing distances, there is no perceptible loss incurred by sampling the color detail at a lower rate, i.e. with a lower resolution. In video systems, this is achieved through the use of color difference components. The signal is divided into a luma (Y') component and two color difference components (chroma). A variety of filtering methods can be used to arrive at the resolution-reduced chroma values.
Luma (Y') is differentiated from luminance (Y) by the presence of gamma correction in its calculation, hence the prime symbol added here. A gamma-corrected signal has the advantage of emulating the logarithmic sensitivity of human vision, with more levels dedicated to the darker levels than the lighter ones. As a result, it is ubiquitously used in the source tristimulus signal, the a R'G'B' input. Examples of such color spaces include sRGB, the TV Rec. 601, Rec. 709, and Rec. 2020; the concept is also generalized to optical transfer functions in Rec. 2020.
Sampling systems and ratios
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):
This notation is not valid for all combinations and has exceptions, e.g. 4:1:0 (where the height of the region is not 2 pixels but 4 pixels, so if 8 bits/component are used the media would be 9 bits/pixel) and 4:2:1.