Screensaver

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. Today, screensavers are also used for entertainment or security.

Purpose

Before the advent of LCD screens, most computer screens depended 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. Televisions, oscilloscopes and other devices that use CRTs are all susceptible to phosphor burn-in, as are plasma displays to some extent.

Screensaver programs were designed to help avoid these effects by automatically changing the images on the screen during periods of user inactivity.

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.

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.

One increasingly popular application is for screensavers to activate a useful background task, such as a virus scan or a distributed computing application (such as the SETI@home project). This is convenient because these applications only use resources when the computer would be otherwise idle.

Considerations

Monitors running screensavers consume the same amount of power as when running normally, which can be anywhere from a few watts for small LCD monitors to several hundred for large plasma displays. Most modern computers can be set to switch the monitor into a lower power mode, blanking the screen altogether. A power saving mode for monitors is usually part of the power management options supported in most modern operating systems, though it must also be supported by the computer hardware and monitor itself.

Additionally, using a screensaver with a flat panel or LCD screen instead of powering down the screen can actually reduce the lifetime of the display, since the fluorescent backlight remains lit and ages faster than it would if the screen was turned off completely. As fluorescent tubes age they grow progressively dimmer, and they can be expensive or difficult to replace. A typical LCD screen loses about 50% of its brightness during a normal product lifetime, if left on continuously. (In most cases, the tube is an integral part of the LCD and the entire assembly needs to be replaced.)

Thus the term "screen saver" is somewhat a misnomer—the best way to save the screen (and also save electricity) would simply be to have the computer turn off the monitor.

Notice also that screensavers may utilize a significant amount of CPU time. On a busy server this might be a problem.

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 such as fractals.

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.

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.

Screensavers are not to be confused with power management features, which place the computer in a low power state after it has been idle for a specified amount of time. In fact, screensavers can actually waste power, because they can prevent the computer from entering the lower power (or sleep) state, and they often cause the CPU and GPU to perform more calculations, and keep the hard disk running for longer than if the computer were idle.

Microsoft Windows

On older versions of Microsoft Windows the native screensaver format had the potential to install a virus when run (as a screen saver was just an ordinary application with a different extension). When any file with the file suffix ".scr" was opened, for example from an e-mail attachment, Windows would execute the .scr (screensaver) file automatically — this had the potential to allow a virus or malware to install itself. Modern versions of Windows can read tags left by applications such as Internet Explorer and verify the publisher of the file, presenting a confirmation to the user.

In addition, on August 5 2006, the BBC reported that "screensavers" and "free screensavers" respectively were the most and third most likely search terms to return links to malware. [ [http://news.bbc.co.uk/1/hi/technology/4765199.stm BBC News: Warning on search engine safety] ]

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 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.

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.

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