Editing Chroma subsampling (section)

==Terminology==
The term [[Y'UV]] refers to an analog TV encoding scheme (ITU-R Rec. BT.470) while Y'CbCr refers to a digital encoding scheme.<ref name=plea/> One difference between the two is that the scale factors on the chroma components (U, V, Cb, and Cr) are different. However, the term YUV is often used erroneously to refer to Y'CbCr encoding. Hence, expressions like "4:2:2 YUV" always refer to 4:2:2 Y'CbCr, since there simply is no such thing as 4:x:x in analog encoding (such as YUV). Pixel formats used in Y'CbCr can be referred to as YUV too, for example yuv420p, yuvj420p and many others.

In a similar vein, the term luminance and the symbol Y are often used erroneously to refer to luma, which is denoted with the symbol Y'. The ''luma'' (Y') of video engineering deviates from the ''luminance'' (Y) of color science (as defined by [[International Commission on Illumination|CIE]]). Luma is formed as the weighted sum of ''gamma-corrected'' (tristimulus) RGB components. Luminance is formed as a weighed sum of ''linear'' (tristimulus) RGB components. In practice, the [[International Commission on Illumination|CIE]] symbol Y is often incorrectly used to denote luma. In 1993, [[SMPTE]] adopted Engineering Guideline EG&nbsp;28, clarifying the two terms. The prime symbol ' is used to indicate gamma correction.<ref>{{cite book |title=Annotated Glossary of Essential Terms for Electronic Production |url=https://ieeexplore.ieee.org/document/7291332 |archive-url=https://web.archive.org/web/20171130161325/http://ieeexplore.ieee.org/document/7291332 |url-status=dead |archive-date=November 30, 2017 |doi=10.5594/SMPTE.EG28.1993 |quote=luma: To avoid the interdisciplinary confusion resulting from the two distinct definitions of luminance, it has been proposed that the video documents use luma for luminance, television (i.e., the luminance signal), and chroma for chrominance television (i.e., the chrominance signal) |isbn=978-1-61482-022-2 }}</ref>

Similarly, the chroma of video engineering differs from the chrominance of color science. The chroma of video engineering is formed from weighted tristimulus components (gamma corrected, OETF), not linear components. In video engineering practice, the terms ''chroma'', ''chrominance'', and ''saturation'' are often used interchangeably to refer to chroma, but it is not a good practice, as ITU-T Rec H.273 says.<ref name="H273">{{cite web |date=2016 |title=H.273 : Coding-independent code points for video signal type identification |url=https://www.itu.int/rec/T-REC-H.273-201612-S/en |website=www.itu.int |quote=NOTE – The term chroma is used rather than the term chrominance in order to avoid the implication of the use of linear light transfer characteristics that is often associated with the term chrominance. [...] NOTE – The term luma is used rather than the term luminance in order to avoid the implication of the use of linear light transfer characteristics that is often associated with the term luminance. The symbol L is sometimes used instead of the symbol Y to avoid confusion with the symbol y as used for vertical location.}}</ref>