Adobe Photoshop CS4 - Manual (Page 124)

117 USING PHOTOSHOP CS4
Color fundamentals
Last updated 1/10/2010
• Matte Specifies the background color used to fill anti-aliased edges that lie adjacent to transparent areas of the
image. When Transparency is selected, the matte is applied to edge areas to help blend the edges with a web
background of the same color. When Transparency is deselected, the matte is applied to transparent areas. Choosing
None for the matte creates hard-edged transparency if Transparency is selected; otherwise, all transparent areas are
filled with 100% white. The image must have transparency for the Matte options to be available.
Dithering Unless you’re using the Exact color table option, the color table may not contain all the colors used in the
image. To simulate colors not in the color table, you can dither the colors. Dithering mixes the pixels of the available
colors to simulate the missing colors. Choose a dither option from the menu, and enter a percentage value for the
dither amount. A higher amount dithers more colors but may increase file size. You can choose from the following
dither options:
• None Does not dither colors but instead uses the color closest to the missing color. This tends to result in sharp
transitions between shades of color in the image, creating a posterized effect.
• Diffusion Uses an error-diffusion method that produces a less-structured dither than the Pattern option. To
protect colors in the image that contain entries in the color table from being dithered, select Preserve Exact Colors.
This is useful for preserving fine lines and text for web images.
• Pattern Uses a halftone-like square pattern to simulate any colors not in the color table.
• Noise Helps to reduce seam patterns along the edges of image slices. Choose this option if you plan to slice the
image for placement in an HTML table.
Customize indexed color tables
The Color Table command lets you make changes to the color table of an indexed-color image. These customization
features are particularly useful with pseudocolor images—images displaying variations in gray levels with color rather
than shades of gray, often used in scientific and medical applications. However, customizing the color table can also
produce special effects with indexed-color images that have a limited number of colors.
Note: To shift colors simply in a pseudocolor image, choose Image > Adjustments, and use the color adjustment
commands in the submenu.
Edit colors and assign transparency with a color table
You can edit colors in the color table to produce special effects, or assign transparency in the image to a single color in
the table.
1 Open the indexed-color image.
2 Choose Image > Mode > Color Table.
3 To change a single color, click the color and choose a new color in the Color Picker.
4 To change a range of colors, drag in the table to choose the range of colors you want to change. In the Color Picker,
choose the first color you want in the range and click OK. When the color picker redisplays, choose the last color
you want in the range and click OK.
The colors you selected in the Color Picker are placed in the range you selected in the Color Table dialog box.
5 To assign transparency to a color, select the Eyedropper tool in the Color Table dialog box, and click the color in
the table or in the image. The sampled color is replaced with transparency in the image.
6 Click OK in the Color Table dialog box to apply the new colors to the indexed-color image.
Choose a predefined color table
1 Open the indexed-color image.