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Luma (video)
In video, luma represents the brightness in an image (the "black-and-white" or achromatic portion of the image). Luma is typically paired with chrominance. Luma represents the achromatic image, while the chroma components represent the color information. Converting R'G'B' sources (such as the output of a 3CCD camera) into luma and chroma allows for chroma subsampling: because human vision has finer spatial sensitivity to luminance ("black and white") differences than chromatic differences, video systems can store chromatic information at lower resolution, optimizing perceived detail at a particular bandwidth.
==Luma versus luminance==
Luma is the weighted sum of gamma-compressed R'G'B' components of a color video – the ''prime symbols'' (') denote ''gamma-compression''. The word was proposed to prevent confusion between luma as implemented in video engineering and luminance as used in color science (i.e. as defined by CIE). Luminance is formed as a weighted sum of ''linear'' RGB components, not gamma-compressed ones. Even so, luma is erroneously called luminance.〔Charles Poynton, "YUV and luminance considered harmful: a plea for precise terminology in video," (online )〕 SMPTE EG 28 recommends the symbol Y' to denote luma and the symbol Y to denote luminance.〔Engineering Guideline EG 28, "Annotated Glossary of Essential Terms for Electronic Production," SMPTE, 1993.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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