| **Definition:** | | Video originates with linear-light (tristimulus) RGB primary components, conventionally contained in the range 0 (black) to +1 (white). From the RGB triple, three gamma-corrected primary signals are computed; each is essentially the 0.45-power of the corresponding tristimulus value, similar to a square-root function.
In a practical system such as a television camera, however, in order to minimize noise in the dark regions of the picture it is necessary to limit the slope (gain) of the curve near black. It is now standard to limit gain to 4.5 below a tristimulus value of +0.018, and to stretch the remainder of the curve to place the Y-intercept at -0.099 in order to maintain function and tangent continuity at the breakpoint:
Rgamma = (1.099 * pow(R,0.45)) - 0.099
Ggamma = (1.099 * pow(G,0.45)) - 0.099
Bgamma = (1.099 * pow(B,0.45)) - 0.099
Luma is then computed as a weighted sum of the gamma-corrected primaries:
Y = 0.299*Rgamma + 0.587*Ggamma + 0.114*Bgamma
The three coefficients in this equation correspond to the sensitivity of human vision to each of the RGB primaries standardized for video. For example, the low value of the blue coefficient is a consequence of saturated blue colours being perceived as having low brightness.
The luma coefficients are also a function of the white point (or chromaticity of reference whitex). Computer users commonly have a white point with a colour temperature in the range of 9300 K, which contains twice as much blue as the daylight reference CIE D65 used in television. This is reflected in pictures and monitors that look too blue.
Although television primaries have changed over the years since the adoption of the ntsc standard in 1953, the coefficients of the luma equation for 525 and 625 line video have remained unchanged. For hdtv, the primaries are different and the luma coefficients have been standardized with somewhat different values. |