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Chapter 16
Red/Cyan, Green/Magenta and
Blue/Yellow (RGB) Gamma Curves
The Red/Cyan, Green/Magenta and Blue/Yellow gamma curves can be viewed
individually by clicking on their respective buttons in the Gamma Curve Editor dialog.
The appropriate gamma curve is then displayed in the curve window. It remains active
until another gamma curve button is selected. A typical curve is shown in Figure 16.12,
in this case the Red/Cyan gamma curve.
Control Point
Gamma Curve
Figure 16.12 The Red/Cyan gamma curve shown graphically in the curve window.
This two-dimensional graph represents the input/output transfer function. The
horizontal axis represents the input, or original gamma levels. The vertical axis repre-
sents the output, or new gamma levels. The bottom of the gamma curve represents the
shadow values in an image. The middle area represents the midtone values, and the top
of the curve represents the highlight values in an image.
The input and output values on 8-bit scanner models range from 0 to 255. The
input values on 12-bit scanner models range from 0 to 4095, while the output values are
still limited to the range of 0 to 255.
A diagonal line connecting the lower-left and upper-right corners would represent
a linear transfer function. For example, an input value of 100 would produce an output
value of 100. Similarly, a 200 input would produce a 200 output, and so on. A hori-
zontal line running along the bottom border would map all inputs into a zero output,
consequently creating a black image.
Monochrome Gamma Curve
The Monochrome gamma curve is viewed by clicking on the Monochrome but-
ton in the Gamma Curve Editor dialog, shown in Figure 16.13. The Monochrome
curve is identical to the color curves, but is active only when a Monochrome film type is
selected. The resultant Monochrome gamma curve sent to the scanner is the combina-
Software Reference for Scanners
The Gamma Curve Editor Dialog
Page 145
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