Apple Color 1.5 - Manual (Page 24)

The RGB Additive Color Model Explained
In the RGB color model, three color channels are used to store red, green, and blue values
in varying amounts to represent each available color that can be reproduced. Adjusting
the relative balance of values in these color channels adjusts the color being represented.
When all three values are equal, the result is a neutral tone, from black through gray to
white.
More typically, you’ll see these ratios expressed as digital percentages in the Color Parade
scope or Histogram. For example, if all three color channels are 0%, the pixel is black. If
all three color channels are 50%, the pixel is a neutral gray. If all three color channels are
100% (the maximum value), the pixel is white.
Animation (an older, 8-bit codec) and Apple ProRes 4444 (a newer 10-bit codec) are the
two most commonly used RGB QuickTime codecs. In digital intermediate workflows,
RGB-encoded images are typically stored as uncompressed DPX or Cineon image
sequences.
The Y
′
C
B
C
R
Color Model Explained
Video is typically recorded using the Y
′
C
B
C
R
color model. Y
′
C
B
C
R
color coding also employs
three channels, or components. A shot’s image is divided into one luma component (luma
is image luminance modified by gamma for broadcast) and two color difference
components which encode the chroma (chrominance). Together, these three components
make up the picture that you see when you play back your video.
• The Y
′
component represents the black-and-white portion of an image’s tonal range.
Because the eye has different sensitivities to the red, green, and blue portions of the
spectrum, the image “lightness” that the Y
′
component reproduces is derived from a
weighted ratio of the (gamma-corrected) R, G, and B color channels. (Incidentally, the
Y
′
component is mostly green.) Viewed on its own, the Y
′
component is the
monochrome image.
• The two color difference components, C
B
and C
R
, are used to encode the color
information in such a way as to fit three color channels of image data into two. A bit
of math is used to take advantage of the fact that the Y
′
component also stores green
information for the image. The actual math used to derive each color component is C
B
= B
′
- Y
′
, while C
R
= R
′
- Y
′
.
Note: This scheme was originally created so that older black-and-white televisions would
be compatible with the newer color television transmissions.
Chroma Subsampling Explained
In Y
′
C
B
C
R
encoded video, the color channels are typically sampled at a lower ratio than
the luma channel. Because the human eye is more sensitive to differences in brightness
than in color, this has been used as a way of reducing the video bandwidth (or data rate)
requirements without perceptible loss to the image.
24 Chapter 1 Color Correction Basics