June 30, 2004
Types of Images for the Web
Computer image files can= be very large. For example, a three-inch square image suitable for printing in a bo= ok can be up to three megabytes in size. Fortunately, there are ways to compre= ss image files. This is important because most people access Web pages over a modem line, and modems are relatively slow when downloading large image fil= es. For example, a three-megabyte file would take about 14 minutes to download = over a 28.8K modem connection. If the same file were compressed, it might take j= ust one or two seconds to download.
Two Popular Compressed File Formats
There are two file forma= ts that are most popular for displaying images on the Web:
![3D"*"](3D"terry_riley_web_page_files/image001.gif")
GIF (Graphic
Exchange Format)¾The
GIF format compresses a file by examining the image for consistent patterns=
and
then rewriting the digital image to a shorthand based on those repeated
patterns, reducing the number of colors used to display the picture. The GIF
format can compress images by about 4:1.
JPEG (Joint
Photographic Experts Group)=
¾JPEG
format compresses a file by tossing out subtle color differences in the ima=
ge
that human eyes are less likely to recognize. Depending on the level of ima=
ge
quality you require, JPEG can accomplish compression ratios between 10:1 and
100:1