Editing Grayscale (section)

=== Luma coding in video systems ===
{{Main|luma (video)}}

For images in color spaces such as [[Y'UV]] and its relatives, which are used in standard color TV and video systems such as [[PAL]], [[SECAM]], and [[NTSC]], a nonlinear [[luma (video)|luma]] component {{math|(''Y{{prime}}'')}} is calculated directly from gamma-compressed primary intensities as a weighted sum, which, although not a perfect representation of the colorimetric luminance, can be calculated more quickly without the gamma expansion and compression used in photometric/colorimetric calculations. In the [[Y'UV]] and [[YIQ|Y'IQ]] models used by PAL and NTSC, the [[Rec. 601|rec601]] luma {{math|(''Y{{prime}}'')}} component is computed as
<math display="block">Y' =  0.299 R' + 0.587 G' + 0.114 B'</math>
where we use the prime to distinguish these nonlinear values from the sRGB nonlinear values (discussed above) which use a somewhat different gamma compression formula, and from the linear RGB components.  The [[Rec. 709|ITU-R BT.709]] standard used for [[High-definition television|HDTV]] developed by the [[Advanced Television Systems Committee standards|ATSC]] uses different color coefficients, computing the luma component as
<math display="block">Y' =  0.2126 R' + 0.7152 G' + 0.0722 B'.</math>
Although these are numerically the same coefficients used in sRGB above, the effect is different  because here they are being applied directly to gamma-compressed values rather than to the linearized values.  The [[Rec. 2100|ITU-R BT.2100]] standard for [[High Dynamic Range|HDR]] television uses yet different coefficients, computing the luma component as
<math display="block">Y' =  0.2627 R' + 0.6780 G' + 0.0593 B'.</math>

Normally these colorspaces are transformed back to nonlinear R'G'B' before rendering for viewing.  To the extent that enough precision remains, they can then be rendered accurately.

But if the luma component Y' itself is instead used directly as a grayscale representation of the color image, luminance is not preserved: two colors can have the same luma {{math|''Y{{prime}}''}} but different CIE linear luminance {{math|''Y''}} (and thus different nonlinear {{math|''Y''<sub>srgb</sub>}} as defined above) and therefore appear darker or lighter to a typical human than the original color.  Similarly, two colors having the same luminance {{math|''Y''}} (and thus the same {{math|''Y''<sub>srgb</sub>}}) will in general have different luma by either of the {{math|''Y{{prime}}''}} luma definitions above.<ref>{{cite web |last= Poynton |first= Charles A. |date= 1997-07-15 |title= The Magnitude of Nonconstant Luminance Errors |url= http://poynton.ca/PDFs/Mag_of_nonconst_luminance.pdf}}</ref>