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. 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 two chromatic channels as well as a luminance channel, and in color science there are two chromatic dimensions as well as a luminance dimension. In neither the vision nor the science is there complete independence of the chromatic and the luminance. Luminance information can be gleaned from the chromatic information; e.g. the chromatic value implies a certain minimum for the luminance value. But there can be no question of color influencing luminance in the absence of a post-processing of the separate signals. 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.
Original without color subsampling. 200% zoom.
Image after color subsampling (compressed with Sony Vegas DV codec, box filtering applied.)
Read more about this topic: Chroma Subsampling
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