Reference: The Settings Window 69
Gradation Curve tool (LCH mode only)
The Gradation Curve tool lets you control the gamma, which measures the
intensity affecting the mid-level grays (midtones) of an image.
Gamma is commonly used to describe the relationship between output density
to the original density across the mid-tones. Adjusting the gamma lets you
change the values of the middle range of gray tones without dramatically
altering the shadows and highlights.
In many ways, the Gradation Curve tool gives you the most control for adjusting
an image's values, but beginners may take some time to master its intricacies.
The Gradation Curve tool applies to grayscale and color images and is not
available for use with line-art or halftone scan modes.
How to read the curve
The curve shows the relationship of the brightness changes across the middle
pixels between the resulting image and the original.
When you open the Curves dialog box, the line on the graph is diagonal
because the Input and Output values are the same.
The x axis of the graph represents the original brightness values of the pixels,
from 0 to 255, for 8-bit scanners (see the table on next page); the y axis
represents the new brightness values. Clicking on the diagonal line then plots a
point that can be adjusted.
Modified curve with
points moved up
Original curve: Straight
diagonal line
