Survey of Web Graphics Tools 311Web PaletteSurvey of Web Graphics ToolsWeb Design in a Nutshell, eMatter EditionRGB values 51, 51, 153, if you convert it using the Adaptive Palette option, these
values may shift to 49, 49, 156—a color thatwill dither on 8-bit displays.
Some other drawing programs, notably Corel Draw, will also shift colors when
exporting to GIF format. Do some tests by opening the exported images in an
image-editing program and check your RGB values to determine whether there is
a problem.
Preventing color shift.Using Photoshop 4.0 alone, the only way to ensure that
your colors will stay web-safe is to choose Web from the Palette options when
converting to Indexed Color. Although this is a perfect solution for graphical
images made up of flat colors, the quality of photographic images may suffer as a
result of a simple Web Palette conversion. You can reduce the number of colors in
the palette as outlined above.
If you are using Photoshop 5.0, checking the box next to Preserve Exact Colors
will prevent the colors from shifting from their web-safe settings when converting
to Indexed Color with an Adaptive Palette.
Fixing Color Shift.If your image is very simple, made up of just a few dominant
flat colors, you can easily make them web-safe again by manually editing their
RGB values in the Color Table:
- Open the GIF file. You can check for color shift quickly by placing your
cursor over the color and checking the RGB values in the Info Palette. Combi-
nations of 0, 51, 102, 153, 204, and 255 are web safe. - If the values have shifted, view the Color Table (Open ImageÝModeÝ
Color Table). - Find the color in the grid (it may be tricky to find the right color if the Color
Table is full). Double-click on the color, and type in the correct RGB values.
As you might guess, this process can become quite tedious if you have more than
one or two colors to adjust. For complex images, consider using the Web Scrubber
filter or a tool like ImageReady to correct the colors for you.
Adobe ImageReady 1.0
ImageReady has many fine web-specific tools and features, however, palette
control (with a special regard for the Web Palette) is one of its specialties. Image-
Ready offers unprecedented control over palette colors and makes it easy to select
nondithering colors.
Converting to Indexed Color
Before saving an image as an 8-bit GIF or PNG, you can first manipulate it in the
Optimize Palette. Here you select the palette to which you would like the image to
be reduced. The default is Perceptual (the best 256 colors based on human visual
perception of color), but other choices include Adaptive and Web.
The number of colors can be reduced easily by selecting one of the common bit-
depths from the Colors pop-up menu. Selecting “Auto” strips out unused colors
and reduces the palette to just the necessary colors.