314 Chapter 17 – Designing Graphics with the Web PaletteSurvey of Web Graphics Tools
Web Design in a Nutshell, eMatter EditionAmong the palette choices are Web 216 and WebSnap Adaptive. Web 216 is the
standard Web Palette. Clicking the “Optimize” checkbox just below the format
menu will strip any unused colors from the color table, keeping the file size as
small as possible. You can set the color depth in the Number of Colors pop-up
menu. The number below the entry field is the actual number of colors used in
the image. Severely limiting the amount of dithering while in the Web 216 Palette
pushes colors to their nearest web-safe values.
The WebSnap Adaptive Palette shifts colors that are near in value to browser-safe
colors to their closest browser-safe color. WebSnap Palettes will snap a color to
web safe if its RGB values are within 7 bits of a colorcube value (e.g., 57-57-57
will snap to 51-51-51, while 60-57-57 will not). There is no way to adjust this toler-
ance as there is in ImageReady and Web Scrubber.Web Scrubber Plug-in Filter (Furbo Filters)
If you haven’t yet invested in a web-graphics tool such as ImageReady or Fire-
works, you can lay out a smaller chunk of change for special web utilities that
work as plug-ins for Photoshop, PaintShop Pro, or any program that accepts
Photoshop-compatible plug-ins.
Web Scrubber from Furbo Filters is particularly good at optimizing images that
contain a combination of photographic imagery and areas of flat-web-safe color. It
provides “web-snap” abilities similar to ImageReady and allows you to reduce the
number of colors in a paletted image (something Photoshop can’t do on its own).
As of this writing, Web Scrubber is available only for the Mac as part of Furbo
Filters’ Webmaster series of plug-ins. These plug-ins are available athttp://www.
furbo-filters.com/. They are working on a similar set for Windows, but in the
meantime, Windows users may download the WebScrub filter for free from David
Siegel’s site athttp://www.verso.com/agitprop/dithering/.
Web Scrubber uses an algorithm (developed by Todd Fahrner and Philip Gwyn)
that works like a snap-to device for colors. As stated in the documentation, “colors
that are nearly web safe are shifted so that they use one of the 216 colors. Colors
that aren’t close to being web safe are left alone to dither when displayed in the
browser.” You can control the amount of shift you would like to occur.
The Web Scrubber control panel allows you to select the number of colors in the
image with the Colors pop-up menu. Use the Red, Green, and Blue sliders to
control how much color shift you want to allow. The higher the number, the more
colors will be forced to shift to the nearest web-safe color. The lower the number,
the more the browser may dither the image. The effects of your changes appear
immediately in the preview window. (See Figure 17-4.)
If you’ve used Photoshop 4.0’s Adaptive Palette to convert your image, you can
use Web Scrubber to correct all the shifted RGB values. Generally, Photoshop
doesn’t shift values more than five units, so a slider setting of 5 in Web Scrubber
should clean colors up sufficiently.
Settings higher than 10 tend to affect the quality of the image, particularly at lower
bit depths. You can experiment with different values and see how they affect file