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 â both analog and digital â and also in jpeg encoding. ==rationale==  because of storage and transmission limitations, there is frequently a desire to reduce (or compress) the signal. 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 for photographs, although images produced digitally containing harsh lines and saturated colors will have significant artifacts. ==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 the
Film_and_video_technology Image_compression