Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual page 245

AFTER EFFECTS CS3 239
User Guide
About color spaces
A color model is a way of describing color using numbers so that computers can operate on them. The color model
used within After Effects is the RGB color model, in which each color is described in terms of amounts of red, green,
and blue light added together to make the color. Other color models include CMYK, HSB, Y'CbCr, and XYZ.
A color space is a variant of a color model. A color space is distinguished by a gamut (range of colors), a set of primary
colors (primaries), a white point, and a tone response. For example, within the RGB color model are a number of color
spaces, including—in decreasing order of gamut size—ProPhoto RGB, Adobe RGB, sRGB IEC61966-2.1, and Apple
RGB. Although each of these color spaces defines color using the same three axes (R, G, and B), their gamuts and
tone response curves are different.
For information on color spaces and color management in After Effects, go to the Adobe website at
www.adobe.com/go/learn_ae_colormanagementpaper.
Though many devices use red, green, and blue components to record or express color, the components have different
characteristics—for example, one camera's blue is not exactly the same as another camera's blue. Each device that
records or expresses color has its own color space. When an image moves from one device to another, image colors
may look different because each device interprets the RGB values in its own color space.
Color management uses color profiles to convert colors from one color space to another, so colors look the same
from one device to another.
See also
"Color management" on page 240
About gamma and tone response
A color space's tone response is the relationship of light intensity to the signal that creates or records (perceives) the
light.
The human visual system does not respond linearly to light. In other words, our perception of how bright a light is
does not double when twice as many photons hit our eyes. Similarly, a CRT monitor's display elements do not emit
light that is twice as bright when a voltage twice as great is applied. The relationship of light intensity to signal
intensity for a display device is expressed by a power function. The exponent of this power function is called gamma.
The relationship of light intensity to signal intensity for an input device is the inverse of the relationship for an output
device, though the gamma values may differ for input and output devices to accommodate the difference between
scene lighting and lighting of the viewing environment.
Raising any number to the power of 1 gives the original number as a result. A gamma of 1.0 is used to express the
behavior of light in the natural world, outside the context of our nonlinear perceptual systems. A system with gamma
of 1.0 is sometimes said to operate in linear light, whereas a system encoded with a gamma other than 1.0 to match
the human visual system is said to be perceptual.
Note: Moving the midtone slider (such as the Levels effect's Gamma control) in a color-correction histogram has the same
result as modifying gamma, changing the tone response curve without moving the white point. Modifying the curve in
the Curves effect also modifies tone response, but not necessarily with a gamma curve.
If you have enabled color management (by specifying a working color space), you can perform all color operations
in linear light by linearizing the working color space. A linearized color space uses the same primaries and white
point as the nonlinear version; the tone response curve is just made straight.
If you have not enabled color management, you can still perform blending operations using a gamma of 1.0.