Detailed overview of screen savers, or desktop wallpaper, including their historical and technical background and main purposes.

1. ScreenSavers Demystifying The Big Picture

2. Screensavers A screensaver is a type of computer program initially designed to prevent phosphor burn-in on CRT and plasma computer monitors by blanking the screen or filling it with moving images or patterns when the computer is not in use. Contemporary screensavers are used primarily for entertainment or security. Decades before the first computers utilizing this technology were invented, Robert A. Heinlein postulated that they would be needed in Stranger In A Strange Land.

3. Purpose Before the advent of LCD screens, most computer screens were based on cathode ray tubes (CRTs). When the same image is displayed on a CRT screen for long periods of time, the properties of the exposed areas of phosphor coating on the inside of the screen gradually and permanently change, eventually leading to a darkened shadow or "ghost" image on the screen. Cathode raytelevisions, oscilloscopes and other devices that use CRTs are all susceptible to phosphor burn-in, as are plasma displays to some extent. Screen-saver programs were designed to help avoid these effects by automatically changing the images on the screen during periods of user inactivity.

4. Purpose For CRTs used in public, such as ATMs and railway ticketing machines, the risk of burn-in is especially high because a stand-by display is shown whenever the machine is not in use. Older machines designed without burn-in problems taken into consideration often display evidence of screen damage, with images or text such as "Please insert your card" (in the case of ATMs) visible even when the display changes while the machine is in use. Blanking the screen is out of the question as the machine would appear to be out of service. In these applications, burn-in is prevented by shifting the position of the display contents every few seconds, or by having a number of different images that are changed regularly.

5. Purpose Modern CRTs are much less susceptible to burn-in than older models due to improvements in phosphor coatings, and because modern computer images are generally lower contrast than the stark green- or white-on-black text and graphics of earlier machines. LCD computer monitors, including the display panels used in laptop computers, are not susceptible to burn-in because the image is not directly produced by phosphors (although they can suffer from a less extreme and usually non-permanent form of image persistence). For these reasons, screensavers today are primarily decorative or for entertainment, and usually feature moving images or patterns and sometimes sound effects.

6. Purpose One increasingly popular application is for screensavers to activate a useful background task, such as a virus scan (for example, Avast comes with this feature, and it shows a screensaver [from your computer, you can choose it] with an overlaying blue window showing the progress. If a virus is detected, the window turns red and the scan is stopped, and the alert is shown once you exit the screensaver) or a distributed computing application (such as the SETI@home project). This is convenient because these applications use resources only when the computer would be otherwise idle.

7. Entertainment After Dark was an early screensaver for the Macintosh platform, and later PC/Windows, which prominently featured whimsical designs such as flying toasters. Perhaps in response to the workplace environment in which they are often viewed, many screensavers continue this legacy of whimsy by populating the idle monitor with animals or fish, games, and visual expressions of mathematics equations (through the use of fractals) as in the Electric Sheep screensaver.

8. Entertainment The ability of screensavers to divert and entertain is used for promotion, especially to build buzz for "event-based" products such as feature films. The screensaver is also an outlet for the work of creative computer programmers. The Unix-based screensaver XScreenSaver collects the display effects of other Unix screensavers, which are termed "display hacks" in the jargon file tradition of US computer science academics. It also collects forms of computer graphics effects called demo effects, originally included in demos created by the demo scene.

9. Security Screensaver software can also be used as a rudimentary security measure. Many screensavers can be configured to ask users for a password before permitting the user to resume work. However, a user might be able to circumvent the password by restarting the computer if the computer's owner has set their account to automatically log in upon startup.

10. History The first screensaver was allegedly written for the original IBM PC by John Socha, best known for creating the Norton Commander; he also coined the term screen saver. The screensaver, named scrnsave, was published in December 1983 issue of the Softalk magazine. It simply blanked the screen after three minutes of inactivity (an interval which could be changed only by recompiling the program). The first screensaver that allowed users to change the activating time was released on Apple's Lisa, in 1983.

11. History The Atari 400 and 800's screens would also go through random screensaver-like color changes if they were left inactive for too long. The user had no control over this. These computers, released in 1979, are technically earlier "screen savers." And prior to these computers, the 1977 Atari VCS/2600 gaming console included color cycling in games like Combat or Breakout, in order to prevent burn-in of game images to 1970's-era televisions. These are examples of screensavers in ROM or the firmware of a computer. Today with the help of modern graphics technologies there is a wide variety of different screensavers. Because of 3D computer graphics, which provide realistic environments, 3D screensavers are available.

12. Underlying architecture Screensavers are usually designed and coded using a variety of programming languages as well as graphics interfaces. Typically the authors of screensavers use the C or C++ programming languages, along with Graphics Device Interface (GDI), DirectX, or OpenGL, to craft their final products. Several Mac OS X screensavers are created and designed using the Quartz Extreme graphics layer. The screensaver interfaces indirectly with the operating system to cause the physical display screen to be overlaid with one or more graphic 'scenes'. The screensaver typically terminates after receiving a message from the operating system that the mouse has been moved or a key has been pressed.