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Step 4—Setting the Gray Point
The point midway between the maximum and minimum input
values is known as the "gray point" (or "midpoint"). This point
represents a neutral shade corresponding to an output level of
128. The gray point is controlled by a gray-point slider at the
bottom of the curve edit display. Moving the gray-point slider to
the left increases the brightness of mid-tones without "washing
out" details in highlights. Moving the slider to the right darkens
mid-tones without obliterating detail in shadows.
Gamma
Gamma (also written " ") is a fundamental property of video systems, one that determines the intensity of the output signal
relative to the input. When calculating gamma, the maximum possible input intensity is assigned a value of one, and the minimum
possible intensity (no input) is assigned a value of zero. Output is calculated by raising input to a power that is the inverse of the
gamma value (output = input
to the left, raising mid-tone output values and brightening the image while leaving the maximum and minimum values untouched.
Lowering the gamma value has the same effect as moving the gray-point slider to the right, lowering mid-tone output values and
darkening the image. The default value for gamma is one, which produces a linear curve. Gamma can be set to any value between
0.45 and 3.00.
Reference: The Tool Chest
(1/ )
). In practical terms, raising the gamma value has the same effect as moving the gray-point slider
The Gray-Point Slider
The gray-point slider is located at the bottom of the curve edit
display.
Gray-point
slider
After clicking the slider once to activate it, move it to the left to
increase the brightness of mid-tones in the selected channel, or
move it to the right to decrease brightness. Alternatively, you
can enter a value for gamma directly in the text box under the
slider. The effects of changes to the gray point can be seen in the
active image.
Gamma
value
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