Editing Chroma subsampling (section)

==Rationale==
[[File:Colorcomp.jpg|thumb|In [{{filepath:Colorcomp.jpg}} full size], this image shows the difference between four subsampling schemes. Note how similar the color images appear. The lower row shows the resolution of the color information.]]

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 (video)|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']].{{efn|The prime signs indicates gamma-correction or any non-linear EOTF.}} This reduction results in almost no visual difference as perceived by the viewer.

===How subsampling works===
The [[Visual perception|human vision system]] processes color information ([[hue]] and [[colorfulness]]) at about a third of the resolution of [[Relative luminance|luminance]] (lightness/darkness information in an image). Therefore it is possible to [[Sampling (signal processing)|sample]] color information at a lower resolution while maintaining good image quality.

This is achieved by encoding [[RGB]] image data into a composite [[black and white]] image, with separated color difference data ([[Chrominance|chroma]]). For example with <math>Y'C_bC_r</math>, [[gamma correction|gamma encoded]] <math>R'G'B'</math> components are weighted and then summed together to create the [[Luma (video)|luma]] <math>Y'</math> component. The color difference components are created by subtracting two of the weighted <math>R'G'B'</math> components from the third. A variety of [[Image scaling|filtering]] methods can be used to limit the resolution.

====Regarding gamma and transfer functions====
Gamma encoded luma <math>Y'</math> should not be confused with linear [[Relative luminance|luminance]] <math>Y</math>. The presence of gamma encoding is denoted with the [[prime symbol]] <math>'</math>.

Gamma-correcting electro-optical transfer functions (EOTF) are used due to the nonlinear response of human vision. The use of gamma improves perceived signal-to-noise in analogue systems, and allows for more efficient data encoding in digital systems. This encoding uses more levels for darker colors than for lighter ones, accommodating human vision sensitivity.<ref name=plea>Poynton, Charles. [http://www.poynton.com/PDFs/YUV_and_luminance_harmful.pdf "YUV and ''luminance'' considered harmful: A plea for precise terminology in video"].</ref>