The subsampling scheme is commonly expressed as a three-part ratio J: a: b (e.g. This encoding uses more levels for darker colors than for lighter ones, accommodating human vision sensitivity. The use of gamma improves perceived signal-to-noise in analogue systems, and allows for more efficient data encoding in digital systems. While CRTs are no longer widely used, gamma or electro-optical transfer curves (EOTF), are still very useful due to the nonlinear response of human vision. In very early video systems, gamma-correction was necessary due to the nonlinear response of a cathode-ray tube (CRT). This is achieved by encoding RGB image data into a composite black and white image, with separated color difference data ( chroma). Therefore it is possible to sample color information at a lower resolution while maintaining good image quality. The human vision system (HVS) processes color information, meaning hue and colorfulness, at about a third of the resolution of luminance, meaning the lightness/darkness information in an image. This reduction results in almost no visual difference as perceived by the viewer. 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'. 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. The lower row shows the resolution of the color information.ĭigital signals are often compressed to reduce file size and save transmission time.
Note how similar the color images appear. In full size, this image shows the difference between four subsampling schemes.
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