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.