Chroma subsampling has goal data compression
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.Rationale
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.4:2:2
The two chroma components are sampled at half the horizontal sample rate of luma: the horizontal chroma resolution is halved. This reduces the bandwidth of an uncompressed video signal by one-third, which means for 8 bit per component without alpha (24 bit per pixel) only 16 bits are enough, as in NV16.
Many high-end digital video formats and interfaces use this scheme: