Adobe After Effects CS4 - Manual (Page 288)

282 USING AFTER EFFECTS CS4
Color
Last updated 12/21/2009
Harry Frank provides a video tutorial on his graymachine website that shows how and why to use color conversion
expressions to convert colors from the RGB color model to the HSL color model when randomly varying colors. The
specific example that he shows uses the Radio Waves effect.
Gamma and tone response
The tone response for a color space 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 in a given time. Similarly, the display elements of a CRT
monitor 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. In general, 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.
Note: Moving the midtone slider (such as the Gamma control for the Levels effect) 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.
Charles Poynton provides an excellent set of resources on his website regarding gamma and other color technology.
Linear tone response: when gamma equals 1
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.
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 into a straight line.
Many compositing operations, such as combining colors with blending modes, benefit from being performed in a
linear color space. For the most natural and realistic blending of colors, work in a linear color space. If you have not
enabled color management, you can still perform blending operations using a gamma of 1.0. (See “ Linearize working
space and enable linear blending ” on page 289.)
System gamma, device gamma, and the difference between scene and viewing environment
The gamma value for an entire system—from capture, through production, to display in the viewing environment—
is the product of the gamma values used for each of the phases in the system. This product is not always 1.0, as it would
be if the operations performed for encoding exactly matched (inverted) the operations performed for decoding. One
reason for a system gamma other than 1.0 is that a difference often exists between the lighting conditions in which a
scene is captured and the lighting conditions in which it is viewed. (Consider that you usually watch a movie in a dim
environment, but the scenes aren’t normally shot in a dim environment.)
For example, the device gamma for an HD camera is approximately 1/1.9, and the device gamma for an HD display is
approximately 2.2. Multiplying these values gives a system gamma of approximately 1.15, which is appropriate for the
somewhat dim television viewing conditions of a typical living room. The system gamma for motion picture
production is much higher (approximately 1.5–2.5) to accommodate the darker viewing environment of a movie
theater. The gamma for the film negative is approximately 1/1.7, and the gamma for the projection film is
approximately 3–4.