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50 ADVANCED IMAGE PROCESSING
The tone curve is a graphic representation of the
brightness and color levels of the image. The bottom axis
is the 256 levels of the original image (input data) from
black to white. The vertical axis is the corrected image
(output data) with the same scale from top to bottom.
The bottom left portion of the graph represents the dark
colors and shadow areas of the image. The middle section
represents the mid-tones: skin, grass, blue sky. The top
right section is the highlights: clouds, lights. Changing the
tone curve can affect the brightness, contrast, and color of
the image.
A SHORT GUIDE TO TONE
-CURVE CORRECTIONS
Input
Output
Highlights
Mid-tones
Shadows
Bring out detail in the shadows
With the RGB channel selected, place the smooth-curve
cursor on the center of the curve. Click and drag the curve
up. Look at the displayed image to judge the result. The
adjustment can be very small and still have a significant
impact on the image. Moving the tone curve down will
make the image darker.
This is a simple technique to make a subject hidden in the
shadows brighter. Unlike the brightness level control (page
33), this method of correction will not loose details in the
highlight areas of the image.
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