What are Icons?

An icon is a graphic image, a small picture or object that represents a file, program, web page, or command. Icons help you execute commands, open programs or documents quickly. To execute a command by using an icon, click or double-click on the icon. It is also useful to recognize quickly an object in a browser list. For example, all documents using the same extension have the same icon.

The structure of an icon

An icon is a group of images of various formats (size and colors). Furthermore, each image can include transparent area. That's why you'll find a transparent color in the drawing palette for 16 or 256 color images and a variable opacity parameter using an alpha channel for the new RGB/A image format (opacity = 0 means transparent).

Icons for Windows or Other Applications

You need to include several images in an icon to permit Windows® or Macintosh® OS to choose the appropriate format when displaying it on screen. It may change based on screen number of colors and display location. For example, in the Windows® task bar icons are displayed using 16x16 image format and on the desktop, they are displayed at larger sizes (48x48, 96x96, 256x256). In Windows Vista™, some intermediate sizes are used (24x24, 32,32, 48x48 and 64x64). If a size does not exist in the icon, Windows Vista™displays it automatically by resizing "on the fly" the largest size 256x256. The Macintosh® OSX uses the large formats 128x128, 256x256 and even 512x512 in OSX 10.5. All those formats include an alpha channel to create smooth transparency and permits nice resize effects.