Using Help
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154
Adobe Photoshop Help
Making Color and Tonal Adjustments
Using Help
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Contents
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Index
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154
Using the Threshold command (Photoshop)
The Threshold command converts grayscale or color images to high-contrast, black-and-
white images. You can specify a certain level as a threshold. All pixels lighter than the
threshold are converted to white; all pixels darker are converted to black. The Threshold
command is useful for determining the lightest and darkest areas of an image.
To use the Threshold command to convert images to black and white:
1
Open the Threshold dialog box. (See
“Making color adjustments” on page 132
.)
The Threshold dialog box displays a histogram of the luminance levels of the pixels in the
current selection.
2
Drag the slider below the histogram until the threshold level you want appears at the
top of the dialog box, and click OK. As you drag, the image changes to reflect the new
threshold setting.
To use the Threshold command to identify representative highlights and shadows:
1
Open the Threshold dialog box. (See
“Making color adjustments” on page 132
.)
2
Select Preview.
3
To identify a representative highlight, drag the slider to the far right until the image
becomes pure black. Drag the slider slowly toward the center until some solid white areas
appear in the image, and place a color sampler on one of the areas.
4
To identify a representative shadow, drag the slider to the far left until the image
becomes pure white. Drag the slider slowly toward the center until some solid black areas
appear in the image, and place a color sampler on one of the areas.
5
Click Cancel to close the Threshold dialog box without applying changes to the image.
You can use the Info palette readouts of the two color samplers to determine your
highlight and shadow values.
Using the Posterize command (Photoshop)
The Posterize command lets you specify the number of tonal levels (or brightness values)
for each channel in an image and then maps pixels to the closest matching level. For
example, choosing two tonal levels in an RGB image gives six colors, two for red, two for
green, and two for blue.
This command is useful for creating special effects, such as large, flat areas in a photo-
graph. Its effects are most evident when you reduce the number of gray levels in a
grayscale image. But it also produces interesting effects in color images.
If you want a specific number of colors in your image, convert the image to grayscale
and specify the number of levels you want. Then convert the image back to the
previous color mode, and replace the various gray tones with the colors you want.
To use the Posterize command:
1
Open the Posterize dialog box. (See
“Making color adjustments” on page 132
.)
2
Enter the number of tonal levels you want, and click OK.
