A Short Guide To Tone-Curve Corrections - Minolta Dimage Scan Software Manual

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.
The tone curve is a graphic representation of the
About the tone curve
brightness and color levels of the image. The bottom
axis is the 256 levels of the original image (input data)
Highlights
from black to white. The vertical axis is the corrected
image (output data) with the same scale from top to
bottom.
Mid-tones
The bottom left portion of the graph represents the dark
colors and shadow areas of the image. The middle
Shadows
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,
Input
contrast, and color of the image.
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. 26), 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.
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DVANCED IMAGE PROCESSING
Increasing image contrast
The contrast of an image can be changed. The light
blue 45° line on the tone-curve graph represents the
original contrast of the image. Making the angle of the
tone curve greater than 45° will increase the contrast.
Making the angle less than 45° will reduce the
contrast.
Correcting color
By selecting individual color channels on the tone curve, adjustments to the overall
color of an image can be made. This can be used to eliminate unnatural color casts or
add warmth to a picture.
With the RGB channel
selected, click on the tone
curve near the top and bottom
to add two nodes. Slightly
move the top node up and the
bottom node down. This will
increase the angle of the
central portion of the tone
curve and increase the
contrast of the image without
making an overall change in
image brightness.
If the image is too red, green, or
blue, simply drag the
corresponding color-channel curve
down until the color appears
natural. If the color cast is
predominantly one of the
secondary colors, cyan, magenta,
or yellow, move the curve of the
complementary color up. For
example, if the image is too yellow,
move the blue curve up, see the
color example on page 2. For
more on complementary colors,
see page 27.
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