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Luma is a video signal formed by combining nonlinear RGB component signals. It is conventionally written $Y^{\prime }$, with the prime warning that the components have already undergone a nonlinear encoding. It differs from physical luminance, which describes light with a photometric scale.

Under Recommendation ITU-R BT.709-6,The coefficients sum to one, so a neutral signal $R^{\prime }=G^{\prime }=B^{\prime }=t$ has luma $Y^{\prime }=t$. The same recommendation specifies a nonlinear conversion $f$ from a normalized linear primary signal $E$:The primed components are formed separately before combining them.^[ref]

An original calculation shows why the order matters. For a linear red primary $(R,G,B)=(1,0,0)$, the primed triple is also $(1,0,0)$ and luma is 0.2126. Combining the linear channels first gives normalized linear luminance $Y=0.2126$; applying $f$ to that scalar gives approximately 0.4485. ThusNonlinear encoding does not commute with this channel combination.

In YCbCr, luma is stored alongside colour-difference information. Chroma subsampling can preserve the luma sampling grid while reducing the other components' sampling. A luma image can therefore retain fine spatial variation without carrying every colour distinction of the original image; it is not a complete colour description.

YCbCr represents colour using a luma component and two scaled colour-difference components. It is a family of encodings: the matrix coefficients, numerical ranges and color space must be specified. Recommendation ITU-R BT.709-6 defines one important set of signal and digital-coding conventions.

For its normalized nonlinear RGB signals $R^{\prime },G^{\prime },B^{\prime }$, defineHere $P_b,P_r$ denote normalized colour differences before digital scaling. For nominal eight-bit video codes, the corresponding relationships areThese are the recommendation's signal equations and quantization scales, with nearest-integer rounding applied at the end and half-integers rounded upward.^[ref]

For an original red example, $(R^{\prime },G^{\prime },B^{\prime })=(1,0,0)$ gives $Y^{\prime }=0.2126$, $P_b\approx -0.1146$ and $P_r=0.5$. The rounded code triple is approximately $(63,102,240)$. Neutral gray $R^{\prime }=G^{\prime }=B^{\prime }=t$ instead gives $P_b=P_r=0$, hence neutral colour-difference codes 128 regardless of gray level. An RGB triple cannot simply be copied into the YCbCr component slots.

This conversion alone need not reduce the number of samples. Chroma subsampling separately reduces the spatial sampling of the colour-difference components. NV12 and NV21 additionally specify how those samples occupy computer memory. A colour conversion, a sampling choice and a storage layout therefore describe three different operations.