The highlight level, shadow level, and gamma are set with the sliders under the histogram. These levels are displayed numerically in the text boxes to the right of the sliders and can be changed with the keyboard.

Gamma defines the mid-tones of the image. The input highlight slider sets the white level. All pixels to the right of the slider are set to 255 and any image detail they may contain is lost. The input shadow slider sets the black level. All pixels to the left of the slider are set to 0 and any image detail they may contain is lost.

The output levels can be adjusted. By moving the output highlight and shadow sliders, the contrast of the image can be reduced.

Click the color-histogram button to view the red, green, and blue histograms. Click the button again to hide the display.

Tone-curve processing guide

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, con- trast, and color of the image.

Input shadow slider

Input gamma slider Input highlight slider

Output highlight slider Output shadow slider

Highlights

Output

Mid-tones

 

Shadows

Input

Changing brightness

This is a simple technique to make an image brighter.

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 makes the image darker.

Increasing contrast

The contrast of an image can be changed. The 45° line on the tone-curve graph represents the original contrast of the image. Making the angle of the tone curve greater than 45° increases the contrast. Making the angle less than 45° reduces contrast.

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 increases 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.

Correcting color

By selecting individual color channels on the tone curve, adjustments to the overall color of an image can be made. 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 cyan, move the red curve up.

For more on color, see page 39.

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Advanced image processing

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