Apple Final Cut Pro 6 - Manual (Page 1453)

668 Part IV Real Time and Rendering
Adjusting Gamma
Final Cut Pro provides gamma control adjustments for RGB video and still-image formats.
About Gamma
Gamma is an implicit or explicit transfer function that maps input intensity to output
intensity, usually in a nonlinear way. The most common example is a CRT monitor,
where the brightness onscreen is less than expected based on the input voltage. If the
input voltage ranges between 0 (black) and 1 (white), one would expect a voltage of
0.5 to generate a brightness value of about 50 percent gray on the screen, but in fact
only 18 percent gray is generated. This is simply the nature of a CRT’s electron gun
response to input voltage.
The CRT applies an implicit power function in which the input signal is raised to the
power of 2.5 (V
2.5 ). Gamma is usually defined by the value of the exponent in the power
function—in this case, 2.5.
Nonlinear Coding
Human perception of brightness is also a power function that’s nearly opposite of CRT
gamma. This means that exponential increases in physical light intensity are required to
create perceptible shifts in brightness. To take advantage of this fact when coding luma
values in a limited bit depth (such as 8 bits, or 256 codes), incoming video signals are
modified with a gamma curve so that there are more codes concentrated in the dark
part of the signal and larger code jumps as luma increases. This corresponds to the
perception of brightness and therefore uses the available bits in the most perceptually
efficient way. If luma were coded equally from black to white, you would likely see
“banding”—perceptible jumps in brightness—in the black areas, and many subtle
changes in white levels would be imperceptible.
About Gamma Correction
Gamma correction compensates for the fact that video display devices (such as
televisions) inherently convert image signals to light intensity in a nonlinear fashion,
usually with an assumed gamma value of 2.5. The ITU-R BT.709 specification
recommends an in-camera gamma correction of 1/2.2 (or 0.45). When the camera
gamma correction is multiplied by the CRT gamma (2.5/2.2), the result is an overall
display gamma of 1.14, which has slightly more contrast than a display gamma of 1.0.
A display gamma of 1.1–1.2 is desired for television viewing in dim viewing conditions.
In summary, a gamma of 2.5 is inherent in CRT displays. Gamma correction is
automatically applied in a video camera so that the video is displayed on a CRT with a
bit of contrast, and this value is usually 1/2.2.