READERS' FEEDBACK

The Editors and Readers of COMPUTE!

What Does A Light Pen Do?

I own a VIC-20. In COMPUTE! I see advertisements for a new light pen for the VIC. I am not sure what a light pen does exactly. What does it do? Do you recommend buying one?

Rich Cope

The display on a video screen is not nearly as static as it appears. It is actually "re-drawn" many times per second by an electron beam. Moreover, it is not a solid picture, but rather a stack of closely spaced horizontal lines like a jigsaw puzzle made up entirely of long, thin rectangular pieces. An important characteristic is that the beam always "draws" the entire screen, and at a constant speed. Thus the drawing always takes the same amount of time, whether the display is blank or filled with an intricate pattern.

The light pen is a light detection device. It "sees" the electron beam as it draws the lines across the screen. By checking to see how much time passes between when the beam starts drawing the picture and when the pen detects the beam, the computer can determine how far the beam has drawn, and thus where on the screen the light pen is positioned.

A light pen is useful for pointing to things on the screen. One of the most common uses for the pen is to select items from a list simply by pointing at the desired item. Another demonstration we have seen involves "playing" a piano by pointing with the light pen to the desired "keys" on a keyboard display. Light pens also provide you with an easy way to "sketch" on the screen.