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A light gun is a pointing device for computers and a control device for
arcade and video games. The first light gun was used on the MIT
Whirlwind computer.
The light gun, and its descendant, the light pen, are now rarely used as
computer pointing devices, because of the popularity of the mouse and
changes in monitor display technology.
Light guns in video games
The video game light gun is typically modeled on a ballistic weapon (usually a
pistol or bazooka) and is used for targeting objects on a video screen. With force feedback, the light gun can also simulate the recoil of the weapon.
Light guns are very popular in arcade games, but have never caught on in the home video game console market. This may be
because people are reluctant to buy more than one extra controller for their
system, let alone a special-purpose and often expensive peripheral, or because
light guns are less satisfactory to use with the small television screens in peoples homes than on the large screens in arcade
game cabinets.
The most popular example of the light gun is Nintendo's Zapper gun for the Nintendo Entertainment System, though there are also light guns for Sony PlayStation, Sega
Dreamcast, Magnavox Odyssey and many other console and arcade
systems.
How light guns work
The "light gun" is so named because it uses light as its method of detecting where on screen you are targeting. The name leads
one to believe that the gun itself emits a beam of light, but in fact all light guns actually receive light through a
photoreceptor diode in the gun barrel. The diode uses light reception to do its targeting, in conjunction with a timed mechanism between the trigger of the gun
and some rather smart graphics programming. There are two versions of this
technique that are commonly used, but the concept is the same: when you pull the trigger of the gun, the screen is blanked out to
black, and the diode begins reception. All or part of the screen is painted white in a way that allows the computer to judge
where the gun is pointing, based on when the diode detects light. The user of the light gun notices nothing, because the period
in which the screen is blank is very short.
The first detection method, used by the Zapper, involves drawing each target sequentially in white light after the screen
blacks out. The computer knows that if the diode detects light as it is drawing a square (or after the screen refreshes), that is
the target the gun is pointed at. Essentially, the diode tells the computer whether or not you hit something, and for
n objects, the sequence of the drawing of the targets tell the computer which target you hit after 1 +
ceil(log2(n)) refreshes (one to determine if any target at all was hit and
ceil(log2(n)) to do a binary search for the object
that was hit).
An interesting side effect of this is that on poorly designed games, often a player can point the gun at a light bulb, pull
the trigger and hit the first target every time. Better games account for this by not using the first target for anything.
The second method, used by the Super Nintendo Entertainment System's Super Scope and the EGA's light pen, is more elaborate but more accurate.
The trick to this method lies in the nature of the cathode ray
tube inside the video monitor (it does not work with LCD projectors). The screen is drawn
by a scanning electron beam that travels across the screen starting at the top
until it hits the end, and then moves down to update the next line. This is done repeatedly until the entire screen is drawn, and
appears instantaneous to the human eye as it is done very quickly.
When the player pulls the trigger, the game brightens the entire screen for a split second, and the computer (often assisted
by the display circuitry) times how long it takes the electron beam to excite the phosphor at the location the gun is pointed at. It then calculates the targeted position based on the monitor's
horizontal refresh rate (the fixed amount of time it takes the beam to get from the left to right side of the screen).
Once the computer knows where the gun is pointed at, it can tell if it coincides with the target or not. However, many guns of
this type (including the Super Scope) ignore red light, as red phosphors have a much slower rate of decay than green or blue
phosphors.
A game that uses more than one gun reads both triggers continuously and then, when one player pulls a gun's trigger, the game
poll that gun's diode until it knows which object was hit.
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