On Jul 16, 18:39, Tony Duell wrote:
> > So here's a trick question .. what happens when a person holds
> > a lightpen to a dark section of the screen. Is the lightpen
>
> Nothing happens (literally). The light pen never sees any light, so it
> can't send a signal to the video card, so the CRT controller can never
> report the position.
>
> > completely blind unless come pixel is lit up?
>
> Yes.
That's not necessarily *quite* true. I built a lightpen for my Beeb years
and years ago, based on an article in a magazine. It used one of the RS
"Sweet Spot" devices and at first I had some trouble making it work. The
following month the magazine published a followup indicating that not all
Sweet Spot devices worked, and suggesting alternatives, but by then I'd
figured out that there was a speed-of-response problem and also a
sensitivity issue and had played with other detectors. I found that if the
brightness was set so that black was *just* not visible, a suitably
adjusted lightpen on a short-persistence monitor could detect dark pixels.
It was, however, *much* easier with lit pixels.
--
Pete Peter Turnbull
Network Manager
University of York
Received on Mon Jul 16 2001 - 19:17:01 BST