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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 shad-
ows brighter. Unlike the brightness level control (page 33), this
method of correction will not loose details in the highlight areas of
the image.