Light Pens ...

From: Douglas Quebbeman <dhquebbeman_at_theestopinalgroup.com>
Date: Tue Jul 17 11:49:55 2001

> >By putting up some sort of target which does provide the light-pen
> >with a light signal. The target is designed in such a way that you
> >cannot move the pen without hitting some part of it, and the software
> >sees a light-pen hit and *re-centers* the target on that point,
> >effectively moving it to where the pen is now pointing.
>
> I think the technical term for that is a "cursor". :-)

There was an article in an early issue of Byte that described
how to build a lightpen and rudimentary code to drive it. I
believe it was for the Processor technology VDM-1 (and so you
could do it with a SOL too).

The full-matrix block cursor was just one of many characters
stored in the character generator chip. You'd have to calibrate
the pen interface so that only a cursor block would put out enough
light to trigger the interface to have a hit available. So you'd
write out a row of cursors, starting at the top row, looking for
a signal from the pen. Once you find the row, you'd repeat the
sequence colmun-by-column until you again got a hit. Now you
know the X,Y coordinates of then pen, and you can do whatever
you want at that point. Select the object under the pen, repeat
the depositing of some character sequence, etc.

To draw with this particular display, of course, you'd have to
draw using some pre-defined screen character as the "nib". I
never got that far- I build the pen, still have it, and I put
quite a bit of effort into re-writing the code, all on paper..

then I moved out, forgot it, and the girlfriend threw my code
away. but I digress...

-dq
Received on Tue Jul 17 2001 - 11:49:55 BST