Operation Manual

72 Working with Graphics, Animation, and Multimedia
♦ With JPEGs, image quality goes down (along with file size) as the
compression setting increases. With high compression, some
images may still look fine, but photographs in particular will
visibly degrade. Preview your site, and let your own eye be the
judge. You can adjust the global or local settings for .JPG
compression as required.
♦ For images using variable transparency, use the .PNG format.
Low-color systems
Note: If you’re quite sure all your intended visitors can display more
than 256 colors, the following section isn’t required reading. On the
other hand, the knowledge is likely to prove useful when working with
any on-screen graphics, just as knowing basic color theory serves you
in the realm of printed sites.
Bear in mind that not all your visitors’ computers will match your own
for graphic display. Like the .GIF format, some monitors are still
limited to 256 colors. It’s generally a good idea to switch your display
to 256 colors and test the page in a browser before finally publishing it.
This will ensure you’re using colors in a way that won’t present
problems when displayed on average systems. The more you know
about color palettes and image formats, the fewer display problems
are likely to creep in.
A major constraint imposed by 256-color systems is that not all
systems use the same 256 colors. The operating system reserves a
certain number of palette slots for “system colors,” and system palettes
differ, for example, between Windows and Macintosh computers.
Applications, including Web browsers, declare their own palettes and
use dithering to approximate colors outside that palette. This means,
for example, alternating pixels of red and blue (from within the palette)
to approximate a purple color outside the palette.
If you’re not careful, dithering can ruin your best efforts. Let’s say
you’ve created a graphic with some solid color areas, but the user’s
browser can’t display one of those colors. So the browser dithers the
color, resulting in a mottled and degraded image. If the solid color
happened to include text, the text becomes less legible.