Apple Color 1.0 - Manual (Page 24)

24 Chapter 1 Color Correction Basics
Image Encoding Standards
The following section provides important information about the image encoding
standards supported by Color. The image data you’ll be color correcting is typically
encoded either using an RGB or Y´C
B
C
R
(sometimes referred to as YUV) format. Color is
extremely flexible and capable of working with image data of either type.
The RGB Additive Color Model
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.
While a few high-quality QuickTime codecs encode video using RGB (Animation is one
of the most commonly used), RGB-encoded images are typically stored as
uncompressed image sequences.
The Y’C
B
C
R
Color Model
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. 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.