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 encoding schemes — both analog and digital — and also in JPEG encoding.

Digital signals are often compressed to save transmission time and reduce file size. 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.

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. 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).

In human vision there are three channels for color detection, and for many color systems, three "channels" is sufficient for representing most colors. For example: red, green, blue or magenta, yellow, cyan. But there are other ways to represent the color. In many video systems, the three channels are luminance and two chroma channels. In video, the luma and chroma components are formed as a weighted sum of gamma-corrected (tristimulus) R'G'B' components instead of linear (tristimulus) RGB components. As a result, luma must be distinguished from luminance. That there is some "bleeding" of luminance and color information between the luma and chroma components in video, the error being greatest for highly saturated colors and noticeable in between the magenta and green bars of a color bars test pattern (that has chroma subsampling applied), should not be attributed to this engineering approximation being used. Indeed, similar bleeding can occur also with gamma = 1, whence the reversing of the order of operations between gamma correction and forming the weighted sum can make no difference. The chroma can influence the luma specifically at the pixels where the subsampling put no chroma. Interpolation may then put chroma values there which are incompatible with the luma value there, and further post-processing of that Y'CbCr into R'G'B' for that pixel is what ultimately produces false luminance upon display.

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