Adobe PHOTOSHOP 5.0 User Manual page 140

132 CHAPTER 6
Making Color and Tonal Adjustments
To use the Threshold command:
Open the Threshold dialog box, as described in
1
"Using color adjustment commands" on page 105.
The Threshold dialog box displays a histogram of
the luminance levels of the pixels in the current
selection.
Drag the slider below the histogram until the
2
threshold level you want appears at the top of the
dialog box. As you drag, the image changes to
reflect the new threshold setting.
Original image, and Threshold command applied: 170
Using the Posterize command
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 photograph. 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 specify the number of levels in an image:
Open the Posterize dialog box, as described in
1
"Using color adjustment commands" on page 105.
Enter the number of levels you want, and
2
click OK.
Original image, and Posterize command applied:
2 levels of color