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62 ADVANCED IMAGE PROCESSING
A short guide to tone curve corrections
Image processing is a highly specialized and difficult field that takes years of practice
to master. This basic guide to using tone curves covers a few simple procedures to
improve your pictures. For more about digital-image processing, consult your local
book dealer about self-help guides on this subject.
About the tone curve
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.
Input
Output
Highlights
Mid-tones
Shadows
Bring out detail in the shadows
This is a simple technique to make a subject
hidden in the shadows brighter. Unlike the
brightness level control (p. 40), this method of
correction will not loose details in the highlight
areas of the image.
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.