Here's an image from Nintendo's Video target control and sensing circuit for photosensitive gun patent on how the hardware works: More images on the entire process flow are also available in that patent link. Improve this answer.
Here's a great video explaining it as well. To your last comment, I remember reading that later games would blank the entire screen first, and make sure the photodiode reads low, then put a white box on the screen.
It delayed the process by 1 frame, but defeated the cheat of pointing at a bright light. CortAmmon Indeed, agent86's answer on Arquade linked by angussidney describes that exact technique. I actually find the second part of the OP's question, which wasn't addressed here, more interesting: Why there was a general switch from video feedback to motion tracking technology.
The quote's description of "the computer in the NES" doing things is a bit misleading, since it sounds like this is some kind of hardware feature that happens automatically. In reality it's a software thing, where the game checks if the Zapper's trigger has been pulled, and if so, temporarily sets the background to a dark color while drawing a bright shape in place of each target sprite one at a time and checking if a hit is registered for that target.
Show 1 more comment. While quality light guns work that way, the video linked to in BruceWayne's comment explains how the NES light gun works and demonstrates that the NES light gun doesn't work that way at all. In particular, the video explains why it's not possible to just aim the gun at a bright light to always score a hit -- the screen blacks out completely first before showing the squares that represent the targets.
If the black screen isn't detected first, the gun won't register a hit. I figured that the question asked about the NES in particular, it would likely be read by people wondering how any kind of vintage light gun worked.
Interestingly, the Atari was intended to use the "high-quality" approach to reading a light gun even though it would be unable to show anything on the screen while it was watching for a light-gun trigger; I think one game was actually written for the light gun Crossbow , and it shows a gray screen for a frame when the trigger is pulled. I wonder if the light gun might have been more popular if the inputs could have NES programs must poll the Zapper at least every couple of scanlines, or it will not detect the light.
It is possible to make the entire screen white, and precisely detect which scanline the Zapper is pointed at by counting the elapsed CPU cycles.
Problem is, everyone looks at Duck Hunt and assumes that's just how the Zapper works. Duck Hunt is polling the Zapper, but isn't counting scanlines. But the capability is there, I'm sure some later games do. Show 2 more comments. Light guns hit home video game consoles with Shooting Gallery on the Magnavox Odyssey in Because the included shotgun-style light gun was only usable on a Magnavox television, the game flopped.
While older light guns like the Ray-O-Lite rifle emitted beams of light, the Zapper and many other recent light guns work by receiving light through a photodiode on or in the barrel and using that light to figure out where on the TV screen you're aiming. When you point at a duck and pull the trigger, the computer in the NES blacks out the screen and the Zapper diode begins reception. Not a single one. I was convinced the zapper was broken but then I plugged the NES and zapper into an old s-era tube television out in my garage and, behold, the zapper works!
From my little test I know enough to say that the issue seems to be a CRT vs. HDTV issue, but I have no idea why. A lot of people, and certainly most of the kids that played the NES back in the day, were under the impression that the Zapper actually shot something towards the television, much like a TV remote sending a signal to a television set.
The Zapper does nothing of the sort and what, exactly, on the television would be able to receive the signal and send it to the NES? The only connection between the Zapper and the NES is the cord, and for good reason. This was true of all light-gun accessories for all video game systems of the era and preceding it. They were all just simple light sensors housed in deceptively gun-like cases. Does that mean the Zapper was actively tracking all those ducks on the screen with surgical precision?
Nintendo designers came up with a very clever way to ensure the simple sensor in the Zapper could keep up. Every time a player pulled the trigger on the Zapper, the screen would for just a fraction of a second blink black with a large white targeting box drawn over everything on the screen that was a valid target such as the ducks.
It repeated the process, all within that fraction of a second, for every available target on the screen. If the box was centered enough, it counted as a hit.
If the target box was outside of the center zone, it was a miss. It was a very clever way to deal with the limitations of the hardware and provide a fluid user experience. Unfortunately, despite being clever, it was very hardware dependent.
Much like early PC video game designers used hardware quirks to help build their games like knowing the clock speed of the platform they were working on was fixed and could be used to time in-game events , Nintendo and other early game companies relied heavily on the quirks of CRT displays and their respective display standards.
Specifically, in the case of the Zapper, the mechanism is completely dependent on the characteristics of CRT display. First, it requires extremely precise timing between the trigger pull on the Zapper and the response on the screen.
The original timing sequence was based on the very dependable response time of a CRT hooked up to the analog NES signal. Whether the old tube TV was big, small, cutting edge or 10 years old, the speed of the signal via the CRT display standard was reliable. By contrast, the latency in modern digital sets is not reliable and is not the same as the old consistent delay in the CRT system.
However, this latency completely destroys communication between the Zapper, the NES, and the events on the screen. This extremely precise timing was possible and consistent because Nintendo designers could count on the refresh rate of the CRT being consistent.
CRT displays use an electron gun to activate phosphors in screen hidden behind the display glass. This gun sweeps across the screen from the top to the bottom at a very dependable frequency. Even though it happens faster than the human eye can detect, every single frame of every single video game or television broadcast is displayed as if some hyperactive robot is drawing it line by line from the top to the bottom.
By contrast, modern digital displays make all the changes simultaneously. They are displayed all at once in their respective standards. Have a pressing tech question, big or small?
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