Design Light gun

1 design

1.1 sequential targets
1.2 cathode ray timing
1.3 combined method
1.4 infrared emitters
1.5 image capture
1.6 multiplayer

design

the light gun named because uses light method of detecting on screen user targeting. name leads 1 believe gun emits beam of light, in fact light guns receive light through photodiode in gun barrel.

there 2 versions of technique commonly used, concept same: when trigger of gun pulled, screen blanked out black, , diode begins reception. or part of screen painted white in way allows computer judge gun pointing, based on when diode detects light. user of light gun notices little or nothing, because period in screen blank fraction of second (see persistence of vision).

sequential targets

the first detection method, used zapper, involves drawing each target sequentially in white light after screen blacks out. computer knows if diode detects light drawing square (or after screen refreshes), target @ gun pointed. essentially, diode tells computer whether or not player hit something, , n objects, sequence of drawing of targets tell computer target player hit after 1 + ceil(log2(n)) refreshes (one refresh determine if target @ hit , ceil(log2(n)) binary search object hit).

a side effect of on poorly designed games, player can point gun @ light bulb or other bright light source, pull trigger, , cause system falsely detect hit on first target every time. better games account either detecting if targets appear match or displaying black screen , verifying no targets match.

cathode ray timing

the black guncon made namco playstation.



the blue (top) , pink (middle) konami justifiers made super nintendo entertainment system , green (bottom) 1 made playstation

the second method, used super nintendo entertainment system s super scope , computer light pens, more elaborate , more accurate.

the trick method lies in nature of cathode ray tube inside video monitor (crts affordable tv monitors in late 1980s , 1990s, when method popularized). screen drawn scanning electron beam travels across screen starting @ top until hits end, , moves down update next line. done repeatedly until entire screen drawn, , appears instantaneous human eye done quickly.

when player pulls trigger, computer (often assisted display circuitry) times how long takes electron beam excite phosphor @ location @ gun pointed. light gun sends signal after sensing sudden small change in brightness of point on screen when electron gun refreshes spot. computer calculates targeted position based on monitor s horizontal refresh rate (the fixed amount of time takes beam left right side of screen). either computer provides time base horizontal refresh rate through controller s connector (as in super scope), or gun reads composite video signal through t-connector on a/v cable (as in guncon 2). once computer knows gun pointed, can tell through collision detection if coincides target or not.

many guns of type (including super scope) ignore red light, red phosphors have slower rate of decay green or blue phosphors. result, (but not all) games brighten entire screen s green/blue pixels when trigger pulled in order more reliable fix on position.

display timing not possible plasma, lcd, , dlp monitors, since not have off state between refreshes. digital signal processing electronics may introduce nontrivial lag between signal input , display output, not predictable because varies between monitor models , brands , between mode settings of single monitor. lag not significant player feedback may enough destroy accuracy of display-timing based light gun system.

combined method

some light guns designed sequential targeting not timed precisely enough (x, y) reading of video signal, can use combination of sequential targeting cathode ray timing. first, screen brightened , response time measured in cathode ray timing, computer measures scanline hit , not horizontal pixel hit. not need fast timer pure cathode ray timing uses, on order of 15 khz y vs. 5 mhz (x, y) on standard resolution display. then, using sequential targets, game cycles among targets on line.

infrared emitters

a new method developed compensate display technologies other crt. relies on 1 or several infrared light emitters placed near screen, , 1 ir sensor on muzzle of gun. when trigger pressed, gun sends intensity of ir beam detects. since intensity depends upon both distance , relative angle screen, angle sensors located in gun. way trigonometric equation system solved, , muzzle s 3d position relative screen calculated. then, projecting muzzle on screen measured angles impact point determined. example of technology (though not using ir) can seen in nes power glove accessory, used 3 ultrasonic sensors serving same function ir emitters used in lightguns.

a simpler variant commonly used in arcades, there no angle detectors 4 ir sensors. however, can prove inaccurate when shooting distances , angles, since calculation of angles , 3d position has larger margin of error.

other variants include 3 or more emitters different infrared wavelengths , same number of sensors. method , proper calibration 3 or more relative angles obtained, not needing angle detectors position gun.

sometimes, sensors placed around screen , emitter on gun, calculations similar.

the wii remote uses infrared video camera in handheld controller, rather simple sensor.

this family of methods used wii remote, guncon 3, , modern arcade light gun games.

the downside of method s not accurate traditional light gun. additional ir image processing results in lag or cursor drift , i.e. when sweeping light gun across screen crosshair seem drag behind light gun pointing.

image capture

when user pulls trigger screen replaced split-second seemingly random display of black , white pixels, or groups of pixels (blocks). light gun contains fine-resolution low pixel count digital camera narrow field of view. handful of dot image pixels captured, gun converts small image binary array allows computer locate exact position gun pointed at. method compatible screen of size. size of screen , distance shooter entered gun driver software determine dimensions of random blocks/pixels best allow rendering on light gun ccd.

multiplayer

a game uses more 1 gun reads both triggers continuously , then, when 1 player pulls gun s trigger, game reads gun until knows object hit.