Light gun: Encyclopedia II - Light gun - How light guns work

Light gun - 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 little or nothing, because the period in which the screen is blank is very short.

Light gun - Method one

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 refresh 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 computer light pens is more elaborate but more accurate.

Light gun - Method two

The trick to this method lies in the nature of the cathode ray tube inside the video monitor (it does not work with LCD screens, projectors, etc.). 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).

Light gun - Combining the methods

Some light guns designed for method one are not timed precisely enough for method two to work exactly as described, but they can use a combination of the two methods. First the screen is brightened and the response time is measured as in method two, but the computer measures only which scanline was hit and not which horizontal pixel was hit. This does not need nearly as fast a timer that pure method 2 uses. Then using method one, the game cycles among those targets on the line.

Light gun - Detection

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 polls that gun's diode until it knows which object was hit.

Adapted from the Wikipedia article "How light guns work", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki