Dare to Dither
By Lynda Weinman
Dithering is the placement of pixels in patterns to emulate colors that the computer can't display. It occurs when 24-bit graphics (millions of colors) are displayed on 8-bit systems (256 colors), or when 24-bit photographs or graphics are reduced to 8-bit depths or lower, such as when saving in GIF or PNG file formats. If, like most Web designers, you regard dithering as an unwanted graphics villain that should be eliminated or at least reduced, this month's column might surprise you: In addition to learning how to reduce dithering where it's unwanted, I'll recommend that you learn the art of dithering to enhance certain types of Web graphics.
What Causes Dithering?
The great irony of Web design is that most artists typically create artwork on souped-up systems that don't match those of their intended audience. A graphic designer is likely to have a video card that supports 16- or 24- bit color, while a typical Web surfer might only maintain an 8-bit color system good enough for typical accounting, word processing, or database operations. And then there are those who have 16- or 24-bit video capabilities but don't know how to access the additional colors. Changing bit depth is not a common procedure for the average computer user.
GIF, the most common file format on the Web, requires that images be reduced to 8-bit or lower depths, and to reduce colors, the computer often defaults to mathematical procedures that produce dithering of one type or another.
For this and other reasons, working with 8-bit color is a fact of Web-design life, and if you ignore it, your graphics suffer.
