DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

G/PAL See PAL-B/G/H, on page 640.
(or with [Y’, P B , P R ], respectively). G’ associates with Y’ because
green dominates luma. Properly written with primes, but sometimes
sloppily written GBR.
Gamma, decoding (γ D ) The numerical value (greater than unity) of the exponent to which
a video component R’, G’, or B’ is raised, by a power function of
the form (V’) γ D, to obtain a linear-light (luminance or tristimulus)
value. A 5⁄ 2 -power function is inherent in the electron gun of
a CRT display, so decoding gamma typically has the numerical
value 2.5; this value characterizes a display system. The value is
important throughout a video system because the approximate
inverse power function is applied at the camera; R’, G’, and B’
signals, and the derivative luma signal Y’, are conveyed in gammacorrected
form throughout the system.
Gamma, encoding (γ E ) The numerical value (smaller than unity) of the exponent to which
a linear-light (luminance or tristimulus) signal is raised, by a power
function of the form L γ E, to obtain a video signal R’, G’, or B’.
(Subsequently, luma, Y’, is computed.) Encoding gamma today
typically has a value close to 0.5, though the 1953 NTSC and FCC
standards mention 1 ⁄2.2, and 576i standards mention 1 ⁄2.8. For
television, encoding gamma is typically about 1.25 times the
reciprocal of the display gamma; see Rendering intent, on
page 644. Encoding gamma properly has a value less than unity,
though sometimes its reciprocal is quoted.
Gamma, system The product of all of the power function exponents to which
image data is subjected as it traverses a set of subsystems, starting
from linear-light components captured from a scene by a camera
(or from linear-light components captured from a previously
reproduced image by a scanner), and ending with linear-light
components reproduced at an image display. The term is best
avoided owing to the difficulty of identifying exactly what constitutes
the “system,” and because it is used so widely without any
consideration of rendering intent.
Gamma See Gamma, decoding (γ D ), and Gamma, encoding (γ E ), above.
Gamma-corrected A signal to which gamma correction has been applied – that is,
a linear-light signal, such as a tristimulus value, raised to a power
in the range 0.4 to 0.5; see Gamma, encoding (γ E ), above.
Because gamma correction produces a video signal that mimics
the lightness sensitivity of human vision, a gamma-corrected
signal exhibits good perceptual uniformity: Noise or quantization
error introduced into the signal is approximately equally perceptible
across the tone range of the system from black to white.
(Gamma correction also compensates for the nonlinear voltageto-luminance
transfer function inherent in a CRT display.)
Gamma correction The process by which a quantity proportional to intensity, such as
CIE luminance or some other tristimulus signal, is transformed
into a signal by a power function with an exponent in the range
0.4 to 0.5. See Gamma, encoding (γ E ), above. In video, gamma
630 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES