How does the NES light gun work?

Hi there,

Also known as the Zapper Gun, the Nintendo Light Gun works the same as
any other light gun or light pen. Contrary to the expectations of
children, it doesn't actually fire anything! Here is the three most
concise explanations I found:

The light gun works by closing measuring the timing of when the pixel
that it points at is drawn. The television draws the screen line by
line, using an electron gun that sweeps from left to right for each
line. The light gun is very sensitive to changes in brightness, so
when the trigger is pulled, it waits until it detects that the
electron gun has lit the pixel it is pointing at, and based on when
that took place, figures out where the light gun was in the drawing
process, and then what pixel it was pointing at.
http://hcs.harvard.edu/~osserman/devel/specials/techfaq/faq.html#GUN1

Here's second:

Light Gun
How does it know what you are aiming at?  Since the games console is
responsible for drawing the screen it always knows where the raster
line is that draws the screen, it starts at the top left and draws the
screen from left to right and top to bottom (it takes 1/60th a second
to draw the screen).  The light gun points at the screen and waits for
the raster line to cross the path.  As soon as the light gun detects
the change from a dark, unrefreshed pixel on the screen to a light,
newly refreshed pixel.  The gun informs the console which in turn can
then work out the coordinates of where the raster line would have been
at that point.  If there is a target at those coordinates then a hit
is registered.
http://www.anangelcame.co.uk/j-web/glossary/glossary.htm

And a third - follow the link for a good technical explanation:

When the photocell senses the "beam" scanning across the face of the
screen, a signal is sent to the CRT controller (chip) which causes it
to record the current X,Y position of the video signal that *it* is
generating. Depending on sophistication desired, the software can
capture the limits of this "region" (i.e. which scan lines register a
"hit" and which positions on each scan line) and then determine the
"center" of that region.
http://www.epanorama.net/documents/pc/lightpen.html

I also stumbled across some pictures of inside the gun:
http://www.watershedonline.ca/community/personal/Joel/NES-Article-Light-Gun.shtml


Search strategy: "light gun work"


Best wishes,
robertskelton-ga

Very quick and complete response, although I'm still trying to get my
head round what it actually means!

Check out HTTP://www.wikipedia.com and do a search for "light gun".
there is an excellent article there that describes pretty much every
facet of how light guns work, including alogorithms, techniques, etc.