Apple Final Cut Pro 6 - Manual (Page 1874)

Appendix A Video Formats 391
V
In fact, 8-bit Y´C
B
C
R
video does not use all 256 codes to represent picture information.
Black is stored as code 16 and white is code 235. Codes 1–15 and 236–254 are retained
for footroom and headroom, respectively. These areas allow for quick spikes in the signal
caused by filtering in analog-to-digital conversions and, in the case of white levels, can
prevent clipping for highlights that may exceed 235 (white). Levels above 235 are
sometimes referred to as super-white levels. For more information about super-white
levels, see Volume III, Chapter 29, “Rendering and Video Processing Settings.”
Internally, Final Cut Pro can do pixel calculations using 32-bit floating-point precision,
which allows for very accurate calculations without rounding errors. This leads to much
more accurate color reproduction when applying filters and compositing layers of
video. This is especially important when you are going to show your movie on film
or broadcast-quality video monitors. In Final Cut Pro, the Video Processing tab in the
Sequence Settings window allows you to choose the rendering bit depth for a
sequence. For more information, see Volume III, Chapter 29, “Rendering and Video
Processing Settings.”
Perceptual Coding and Gamma
The limited number of brightness steps in 8-bit digital video requires efficient use
of the 256 available codes. Because perception of brightness follows a power law
function, humans are more sensitive to absolute intensity changes in dark areas. In
other words, the amount of light required to make a perceptual shift in brightness
increases exponentially. Therefore, a gamma correction is applied to video so that the
step between each code represents a perceptual shift in brightness. Without this
gamma correction, the darker areas would appear to abruptly jump from one
brightness level to the next (“banding”) and white levels would waste many codes with
imperceptible brightness shifts. This gamma correction is reversed by video monitors
so that the viewer sees the original light intensity of the image. For more information
about gamma, see Volume III, Chapter 29, “Rendering and Video Processing Settings.”
Video Compression
Once a video signal is digital, it requires a large amount of storage space and
transmission bandwidth. To reduce the amount of data, several strategies are employed
that compress the information without negatively affecting the quality of the image.
Some methods are lossless, meaning that no data is lost, but most are lossy, meaning
that information is thrown away that can’t be retrieved.