Apple Final Cut Pro 5 - Manual (Page 324)

Chapter 23 Using Color Bars for Video Calibration 323
IV
How Digital Video Levels Are Measured in Final Cut Pro
Using a hardware waveform monitor and vectorscope to calibrate analog input or
output is largely the same as using the software-based scopes in Final Cut Pro. When
you use the waveform monitor, black is always represented on the digital scale as
0 percent, and white is represented as 100 percent. Percentages are used because the
units vary depending on the bit depth of the digital video being measured. 8-bit video,
such as DV, has 256 (0 to 255) possible brightness (or luma) values per pixel. 10-bit
video, such as Digital Betacam and many third-party video interface codecs, uses 1024
(0 to 1023) possible luma values per pixel. Both 8- and 10-bit video are displayed as
percentages on the waveform monitor in Final Cut Pro.
Digital video black and white are not set at the extreme values. Instead, there is a small
safety margin, known as headroom, to handle exceptionally bright highlights during
shooting. For example, 8-bit digital video uses the value 16 for black (0 percent on the
waveform monitor in Final Cut Pro) and 235 for white (100 percent), but there is still
some headroom higher than 235 to capture extremely bright highlights.
Many camcorders record super-white video levels, which are video levels that exceed a
digital value of 100 percent (as high as109 percent). Since 8-bit video extends beyond
235 (almost to 255, though 255 cannot be used), it is possible to record these super-
white values even though they are not broadcast-legal. Final Cut Pro can capture these
super-white levels and maintain them throughout editing, as long as the video codec
used supports Y´C
B
C
R
(YUV) processing. For example, the Apple DV codec supports
Y´C
B
C
R
processing and allows you to capture and process DV media with super-white
video levels. If you have a third-party video interface, check its documentation to verify
that the codecs it supports use Y´C
B
C
R
processing.
Important: If your video capture codec does not support Y´C
B
C
R
processing, your video is
processed in RGB color space. 8-bit RGB video spans 0 to 255 like Y´C
B
C
R
, but black is set
to 0 and white is set to 255. In other words, RGB has no headroom, or extra bits, available
to record super-white values. A Y´C
B
C
R
level of 235, which is considered white, is mapped
to 255 when converted to RGB. However, what happens to the super-white levels such as
236, 240, 245, and so on? When converted to RGB, all these super-white values are
mapped to RGB 255, which has the effect of flattening all the subtle brightness
differences to a single white value. The result is sometimes referred to as luma clamping,
or clipping. Visually, clamping appears as a loss of details in the highlights of the image,
and a subtle darkening of the brightest values. When Final Cut Pro renders Y´C
B
C
R
media
in RGB color space, images with super-white luma levels (101 to 109 percent) are clamped
to 100 percent. This effect is most noticeable when using a transition, such as a dissolve,
between two clips. The super-white areas of the clip that’s making the transition will
suddenly appear to darken for the duration of the rendered transition. This is because the
super-white media outside the transition is displayed in Y´C
B
C
R
color space, but the
rendered portion is processed in RGB color space.