Chroma subsampling
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.
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'.
Therefore it is possible to sample color information at a lower resolution while maintaining good image quality.
Gamma-correcting electro-optical transfer functions (EOTF) are used due to the nonlinear response of human vision.
This encoding uses more levels for darker colors than for lighter ones, accommodating human vision sensitivity.
[2] The subsampling scheme is commonly expressed as a three-part ratio J:a:b (e.g. 4:2:2) or four parts, if alpha channel is present (e.g. 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.
Initially, 4:1:1 chroma subsampling of the DV format was not considered to be broadcast quality and was only acceptable for low-end and consumer applications.
[3][4] However, DV-based formats (some of which use 4:1:1 chroma subsampling) have been used professionally in electronic news gathering and in playout servers.
However, in most equipment, especially cheap TV sets and VHS/Betamax VCRs, the chroma channels have only the 0.5 MHz bandwidth for both Cr and Cb (or equivalently for I/Q).
This fits reasonably well with the PAL color encoding system, since this has only half the vertical chrominance resolution of NTSC.
Different variants of 4:2:0 chroma configurations are found in: Cb and Cr are each subsampled at a factor of 2 both horizontally and vertically.
There are four main variants of 4:2:0 schemes, having different horizontal and vertical sampling siting relative to the 2×2 "square" of the original input size.
As well, the spatial displacement between both fields can result in the appearance of comb-like chroma artifacts.
A number of legacy schemes allow different subsampling factors in Cb and Cr, similar to how a different amount of bandwidth is allocated to the two chroma values in broadcast systems such as CCIR System M. These schemes are not expressible in J:a:b notation.
Instead, they adopt a Y:Cb:Cr notation, with each part describing the amount of resolution for the corresponding component.
Chroma subsampling suffers from two main types of artifacts, causing degradation more noticeable than intended where colors change abruptly.
Gamma-corrected signals like Y'CbCr have an issue where chroma errors "bleed" into luma.
In those signals, a low chroma actually makes a color appear less bright than one with equivalent luma.
[20] This issue persists in HDR video where gamma is generalized into a transfer function "EOTF".
2020 defines a "constant luminance" Yc'CbcCrc, which is calculated from linear RGB components and then gamma-encoded.
Other sub-sampling filters (especially the averaging "box") have a similar issue that is harder to make a simple example out of.
Similar artifacts arise in the less artificial example of gradation near a fairly sharp red/black boundary.
[26] The term Y'UV refers to an analog TV encoding scheme (ITU-R Rec.
The luma (Y') of video engineering deviates from the luminance (Y) of color science (as defined by CIE).
[27] Similarly, the chroma of video engineering differs from the chrominance of color science.
[clarification needed] This knowledge allowed RCA to develop a system in which they discarded most of the blue signal after it comes from the camera, keeping most of the green and only some of the red; this is chroma subsampling in the YIQ color space and is roughly analogous to 4:2:1 subsampling, in that it has decreasing resolution for luma, yellow/green, and red/blue.
