see questions below, plz
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, July 17, 2001 3:35 PM
Subject: Re: Light Pens ...
> >
> > I've only handled one system that had a light pen on a raster display. My
> > experience with a lightpen is actually limited to the vector displays on CDC
> > systems back in the early-mid '60's. They were quite precise in their
placement
> > on the rather large screeen, and didn't make first-approximations and let
you
> > correct as you went as one might do on a raster display. Since putting a
pixel
> > under the light pen was tantamount to making the entire screen white and
then
> > erasing/redrawing it, which took quite a while, and that's not what they
did,
> > what do you suppose they did on those storage tubes?
>
> Were they definitely storage CRTs? Not all vector displays were (both DEC
> and HP made storage displays using normal CRTs in the early 70's...). I
> seem to rememebr, though, that the Tektronix storage terminals could
> display a flashing cursor. I am not sure how it worked, but I guess there
> was some way to display an object without having it stored. Maybe the
> light pen cursor on the CDC system was displayed that way.
>
> > > Typically, when you put the pen on the screen, the machine moves a block
> > > or crosshairs about until it finds the light pen (sometimes it just lights
> > > up the whole screen). Once it knows where the lightpen is, it displays a
> > > 'cursor' at that location. The important thing about the cursor is that
> > > it's larger than the resolution of the light pen (say a 3*3 array of
> > > lightpen 'pixels). Then, when the user moved the lightpen, the machine
> > > detects that the lightpen is no longer over the centre block of the
> > > cursor, and moves the cursor so that it is. The cursor thus follows the
> > > lightpen around the screen, that always providing something for the
> > > lightpen to detect.
> > >
> > What do you suppose they did to keep the cursor centered? Did the integrate
the
> > waveform from the light pen? How would they handle the vertical centering?
Do
>
> No, it was genrally a lot simpler than that.
>
> The 'cursor' might have been a 3*3 array of blocks, which I will number :
>
> 1 2 3
> 4 5 6
> 7 8 9
>
Viewed from the 6845, is the block 1 scan line high? The minimal width is
probably 8 bits, or whatever else the character width is. How do you suppose
they got the resolution down to the individual pixels? IIRC, the MOT raster
system I mentioned previously had the horizontal lines modulated with the dot
clock, so horizontal lines were dotted and not any brighter than the vertical
ones. That impressed me more than the function of the light pen, but it was
interesting to see the precision with which the light pen worked.
>
> OK, the cursor is displayed so that the light pen detects block 5. If the
> pen is moved to the left, the light pen pulse will occur as block 4 is
> being displayed. Software on the host computer realises this and moves
> the entire cursor left, so that the block being detected is number 5
> again. Similarly, if you move the light pen down, the software realises
> that the position detected by the light pen corresponds to block 8, and
> moves the cursor down to compensate.
>
> You could do this in hardware, I guess (the necessary electronics is not
> exactly complicatated), but software was more usual.
>
> > The Motorola system on which I saw the demo of a "full" implementation of
the
> > light pen together with a 6845 was able to do the job all right, but I'd be
> > surprised to learn they didn't have a dedicated processor just for diddling
with
>
> Quite posslby. A single-chip microcontroller could handle this easily
> (the fast stuff -- determining the coordinates of the lightpen -- is
> still done in hardware, say by a 6845, all the software has to do is work
> out what block in the cursor currently corresponds to that location, and
> move the cursor if necessary).
>
Getting the horizontal location right, as you'd have to in order to do precise
drawing, might be a bit more complicated, don't you think? an 8-pixel extent of
uncertainty is quite a bit on a 640-pixel display.
>
>
>
Received on Tue Jul 17 2001 - 17:54:00 BST