Introduction to luma and chroma

This chapter introduces the concepts of luma and chroma. Video systems convey image data in the form of one component that represents lightness, and two components that represent color, disregarding lightness. The lightness component is conveyed in a perceptually uniform manner that minimizes the amount of noise (or quantization error) introduced in processing, recording, and transmission. The term luminance and the symbol Y were established by the Commission Internationale de l’Eclairage (CIE), the standards body for color science. The term luminance has come to mean the video signal representative of luminance even though the components of the video signal have been subjected to a nonlinear transfer function. Both the term luminance and the symbol Y conflict with their CIE definitions, making them ambiguous. The chapter uses the term “luma” to refer to the video signal and provides attention to designate nonlinear quantities with a prime. However, many video engineers, computer graphics practitioners, and image-processing specialists use these terms carelessly. The chapter also highlights color difference coding that is also called “chroma”.