![]() ![]() CCIR 601 / Serial Digital Interface / D-1. ![]() ![]() Many high-end digital video formats and interfaces use this scheme: 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. The two chroma components are sampled at half the horizontal sample rate of luma: the horizontal chroma resolution is halved. Formats such as HDCAM SR can record 4:4:4 R'G'B' over dual-link HD-SDI. Note that "4:4:4" may instead be wrongly referring to R'G'B' color space, which implicitly also does not have any chroma subsampling (except in JPEG R'G'B' can be subsampled). This scheme is sometimes used in high-end film scanners and cinematic post-production. Types of sampling and subsampling 4:4:4 Įach of the three Y'CbCr components has the same sample rate, thus there is no chroma subsampling. To calculate required bandwidth factor relative to 4:4:4 (or 4:4:4:4), one needs to sum all the factors and divide the result by 12 (or 16, if alpha is present). Also note that the diagram does not indicate any chroma filtering, which should be applied to avoid aliasing. The mapping examples given are only theoretical and for illustration. 4:1:0 (where the height of the region is not 2 pixels, but 4 pixels, so if 8 bits per component are used, the media would be 9 bits per pixel) and 4:2:1. This notation is not valid for all combinations and has exceptions, e.g. May be omitted if alpha component is not present, and is equal to J when present. Alpha: horizontal factor (relative to first digit).Note that b has to be either zero or equal to a (except in rare irregular cases like 4:4:1 and 4:2:1, which do not follow this convention). b: number of changes of chrominance samples ( Cr, Cb) between first and second row of J pixels.a: number of chrominance samples ( Cr, Cb) in the first row of J pixels.J: horizontal sampling reference (width of the conceptual region).The parts are (in their respective order): 4:2:2:4), that describe the number of luminance and chrominance samples in a conceptual region that is J pixels wide and 2 pixels high. 4:2:2) or four parts, if alpha channel is present (e.g. 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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