Apple Final Cut Pro 5 - Manual (Page 1338)

Chapter 24 Rendering 551
III
About Color Space Conversion
Each color space has a certain range, or gamut, of colors that can be represented. Some
colors represented in the Y´C
B
C
R
color space cannot be represented in RGB, and are said
to be out of gamut. If the color space of your media files doesn’t match the color space
of your sequence, Final Cut Pro maps the media file color values to the color space of
the sequence. In some cases, colors get “clipped” to the nearest value during
conversion. This can cause very saturated colors to become less intense and is referred
to as chroma clamping.
Rendering in Native Y´C
B
C
R
(YUV) Color Space
Final Cut Pro supports native Y´C
B
C
R
color space rendering. Since most video formats
use Y´C
B
C
R
, this allows you to edit without worrying about color space conversions
to RGB.
About Maximum White Levels
Most digital camcorders record 8-bit video from 16 (black) to 235 (maximum white),
while RGB files use the entire range from 0 to 255. Normally, when converting from RGB
to Y´C
B
C
R
, the maximum RGB white value (255) is mapped to the maximum Y´C
B
C
R
value (235). However, this is not always the correct way to convert the pixel values. This
is because many digital camcorders use 235 as a broadcast-safe maximum white level,
but allow even brighter pixel values up to 254. This allows you to set your camera
exposure to keep your levels generally around 235, while allowing additional headroom
for extreme highlights such as shiny objects. In these cases, 235 in Y´C
B
C
R
corresponds
to 100 IRE, which is the broadcast safe limit for white values. Anything brighter is
considered to be super-white, meaning that although the pixel value is allowed in the
video signal, it is not considered legal for traditional NTSC and PAL broadcast.
If you work exclusively with Y´C
B
C
R
video footage, you don’t need to worry about this
issue. However, if you incorporate any RGB media files in a sequence that uses a Y´C
B
C
R
codec, you need to choose how the RGB values are mapped to the Y´C
B
C
R
color space.
In Final Cut Pro, you can choose whether RGB values are mapped to 235 (white—or 100
IRE) or super-white (above 100 IRE). If your Y´C
B
C
R
footage contains super-white values,
and you include RGB footage in your sequence, you should choose the super-white
option in the Video Processing tab. If the maximum white value of your video footage
never exceeds 100 IRE, then you should choose white.