Stack Light Rifle

The following article was taken from Sinclair User - May 1984

In the dim dark past when video games consisted of black and white tennis with that distinctive 'boing' , there
was then a new idea of having a seperate gun to fire at the TV screen. The Stack Light Rifle takes the idea a
stage further with a four-part sniper's rifle and high resolution colour.
The rifle is supplied with three games on tape, High Noon, Shooting Gallery and Grouse Shoot for the 48k
Spectrum.
The main pistol is attached to 12ft of cable which ends in a dead-ended ZX81-size connector whcih plugs into
the Spectrums user port. To the pistol you can attach a barrel, stock and telescopic sight.
Of the three games, High Noon requires the greatest skill. In it a cartoon-style gun fighter will walk across the
screen and you six shots with which to kill him. Of the other games, Grouse Shoot entails shooting at rising
birds while in Shooting Gallery you have to shoot a bouncing ball.
The rifle is well-made and suprisingly accurate. It is perhaps regrettable that the present trend towards death
and destruction games should result in the appearance of such a device. If, however, that is what you want,
then it is, arguably, the best of its type.

The following review was taken from Home Computer Advanced Course 1984

The Stack Light Rifle (SLR) is designed to bring added realism to 'shoot-em-up' games on home computers. Combining the appearance of a gun with a camera-style optical system, the SLR is hardly a precision instrument, but its use of light pen technology allows the user to dispense with joystick or keyboard control.

The main component of the Stack Light Rifle System is the electronic target pistol that is connected to the
computer by a generous length of lead. At the computer end, depending on the version, there is a connector for
the appropriate socket or edge connector. On the ZX Spectrum version the connector contains two chips and a
couple of simple components to interface the main electronics inside the gun to the computer. To make the pistol more accurate and to turn it into a rifle - it is supplied with a shoulder stock that clips and secures to the rear of the pistol, a barrel and a make-believe telescopic sight.
The electronics inside the pistol consist of a light detector or photo-diode and a small amplifier and buffer. Light
coming down the barrel is focused by a small plastic lens onto the photo-diode, and the device is sensitive
enough to detect the changes in intensity of the picture. Once boosted by the amplifier, the signal is clipped to
provide a digital pulse rather than an analogue waveform and is then fed to the computer via the switch. The
screen position that is being scanned at that moment is the position the rifle is pointing at. As the computer
receives the pulse from the Light Rifle it compares the value of its scan registers with the screen position of the
target and, if a match is found, the played has scored a direct hit.

Variants of the Light Rifle are currently available for the ZX Spectrum, Commodore Vic20 and Comrnodore 64 and all perform the same function. Stack provides three games on cassette with the Light rifle but that's about the limit of the support provided. Although various independent software houses produce games that would appear to be eminently suited to this type of user control, very few have actually produced or converted programs to work with it; Micrornania is an exception. Possibly even more damaging to potential sales of the Light rifle is the fact that Stack doesn't provide any driver routines to allow users to write their own programs. This omission, together with the lack of any technical details on how it works, means that the Light Rifle is not a good alternative to a joystick.

The Light Rifle is based on the same principle of operation as a light pen, but is much bigger and is designed to
be held up to about three metres (10 feet) from the television set rather than in contact with the screen. To help
filter out any ambient light, the Light Rifle is provided with both a long dark tube (the barrel) and a lens. These
combine to provide a reasonable - if not perfect - degree of accuracy, and allow the user to shoot-em-up from the comfort of an armchair. The games that are supplied are rather poor examples of what should be possible; both the use of graphics and the 'playability' are hardly outstanding.

One of the major problems in programming light pens, or even giant versions such as the Light Rifle, is that the
program needs to be very efficiently written. In all the examples supplied by Stack, the games come to a halt
when the trigger is pulled. This is because the requirement of continuously scanning the screen, as is usually
done for a light pen, would slow the games down too much. So when the trigger is pulled on the Light Rifle, the
software must freeze the action and establish whether the target on the screen is aligned with the position of
the gun. Once the software has determined whether or not the player has hit the target, the game can continue.
In theory when the trigger has been pulled, the amount of code necessary to establish the screen position of
the next scan detected by the gun should be very small indeed, but observing the software in action indicates
that this isn't always the case.

On a computer such as the BBC, for which there is as yet no version of the Light Rifle, the provision of a light
pen facility within the video chip would make the task of the software much simpler. The Commodore 64 offers
such a system, but the ZX Spectrum, on which the Light Rifle was tested, lacks the facility and the deficiency
shows up in the time taken to calculate the position of the rifle when the trigger is pulled.