Technical data

Color and Input/Output Options 97
To avoid that kind of calamity when displaying Web pages, both
Netscape and Microsoft browsers use the same Web-safe palette of 216
colors to display images. Since you’ve gotten this far, you may be
interested to know that the Web-safe palette is based on RGB values
that are either 0, or divisible by 51. Permissible values are in the series
0, 51, 102, 153, 204, 255. So, for example, the RGB definition
“0,102,51” would be a safe Web color, while “0,102,52” would not.
If you’re exporting at 256 colors or less, and Web display is not an
issue, there’s no question you should choose the Optimized setting—as
a quick side-by-side comparison in the preview window will always
confirm. The program will always do a better job when it’s allowed to
select a range of color values that best match those in the 24-bit version,
rather than having to apply the same 216 colors every time.
Note: When you open an image that already has an associated palette,
PhotoPlus doesn’t attempt to hold on to the palette—it always re-
optimizes, even if you use the Save Original command. Usually this
yields the best possible results; but if keeping exactly the same image
palette is essential to your particular application, our advice would be to
export from PhotoPlus in 24-bit mode and use a third-party program to
apply the palette.
Dithering
Dithering (not to be confused with “showing flustered excitement or
fear”) comes into play with images being reduced to 256 colors or less.
It’s a method of approximating colors outside the actual image palette—
for example, by alternating pixels of red and blue from within the palette
to produce the visual impression of a purple color that’s not in the
palette. Applications (including Web browsers) use dithering in 256-
color mode if the images being displayed include colors outside the
application palette. This can degrade solid-color areas and is one of the
main reasons to export Web-bound images using the Web-safe palette.
When you’re exporting to 256 colors or less, PhotoPlus lets you choose
whether or not to use dithering. If you have an image with few colors,
and preserving areas of solid color is essential, you should opt for no
dithering—and the export filter will pick “nearest-match” color values
from the palette being applied. You may see some color shifting, but the
solid color areas will be preserved. For photographic images, on the
other hand, dithering is clearly the best choice. With the “optimized
palette” option, you can choose either ordered or error diffusion
dithering. The former produces a discernibly patterned effect, while the
latter tends to average away the patterns for a more natural result.