Light pen breakthrough!
Pixel resolution is possible
By MIKE COOK
EVERYONE knows that light pens cannot draw lines as fine as
one pixel. Fortunately no-one has told the BBC Micro that!
This month we present a program which used in conjunction with
The Micro User light pen finally achieves pixel resolution.
The pen itself was described in the October 1983 issue of The
Micro User and a painting program was given the following month.
This simulated the action of a paint brush and produced broad
and flowing strokes.
Some artistic endeavours however, are more suited to a clean-cut
graphical presentation and so the Draughtsman program has been
produced.
The basic light pen is only capable of character resolution
— that is, the computer can only sense the position of the pen
to the nearest printing character. So in order to achieve pixel
resolution we have to resort to some software trickery.
Those of you who have used the painting program will have noticed
a small offset between the pen's position and the paint on the
screen.
This is naturally compensated for by the brain, and leads to
no difficulty in use. However, to coax pixel resolution out of
the pen we can use this ability of the brain to good effect.
Consider Figure I. This shows one character position being pointed
at by a light pen. If the pixel point we are interested in is
in this square then it is not changed.
Figure I: Movement of the target pixel
However if it is in one of the adjacent squares (shown shaded)
it is moved one pixel position towards the pen.
If the point of interest is outside the immediate area it is
moved much more rapidly towards the pen. In practice this means
that the point is teased into the correct position, and due to
the cleverness of the human brain is quite naturally achieved.
As we now have a basic method of defining a pixel's position
we are able to build the software around this.
Obviously a freehand sketch using the pen would be difficult
to control. But as a draughtsman is concerned with straight lines
we can use rubber banding techniques to position our line exactly.
When it is in position the press of a key will fix it permanently
on the screen.
As well as rubber-banding single lines, we can also use triangles
and rectangles and there is a routine for coping with circles.
When running the program the screen will go white to allow the
pen to pick up light Nothing will happen until one of the modes
is entered.
To do this type the letter for the appropriate mode. For example
let's draw a straight line by pressing the L key.
The top line of the display should now say LINE. Take up the
light pen and you should see a single lit pixel following the
pen.
When it is in the correct position for the start of the line
press any key. When the pen is moved a line will be drawn from
this point to the pen.
This line will move following the pen and behaving like a rubber
band on the screen.
When the end position of the line is at the correct place press
the Return key and you will be back in the command mode, ready
to draw another shape.
If you want to carry on drawing lines press the space bar instead
of the Return key and you will stay in the line mode.
If you want the start of the new line to be the same point as
the end of the old line then instead of the space bar press J
for join.
The triangle mode works in a similar manner with first a line
being rubber-banded and then the full triangle. In this mode the
J key will join up the new triangle to the last side of the old
one.
When drawing a rectangle the first point will fix one corner
and the second point will fix the opposite corner. The join key
will join the next rectangle to the last corner of the previous
one.
When selecting the polygon mode you will be asked how many sides
the polygon is to have. If you require a circle then a large number
like 40 should be used.
In this mode the radius is defined by rubber banding but this
radius line disappears when the polygon is drawn.
If the join option is used the new polygon will be drawn with
the same centre point as the last one.
You can also change the colour of the lines by pressing the
C key. This will cycle through the three colours available in
Mode 1.
These are set up in line 90 and can be changed to suit your
own preferences.
When you have finished your masterpiece the screen can be saved
as a file by typing S. You will be asked to provide a file name
and the memory locations that make up the screen will be saved.
For those of you with discs this file will be placed in the
directory S for screen dump.
This will reduce the confusion between a screen dump file and
a program. If you have tape then the characters S. will be added
to the beginning of the file name.
The program also allows you to load a previously dumped file
back to the screen to be worked on further. This is done by typing
L.
Hints on typing in the program: Line 80 defines a text window
of one line at the top of the screen. This line is best left out
until all the typing errors are corrected as any error messages
will scroll off the top before you can read them.
There are some variables that start with the LETTER O such as
OX% (Old X). Do not confuse these with the number 0.
In line 300 the space between the quote marks and the number
is vital. You will get an error message if it is left out.
Unfortunately the error message is not all that helpful. In
various lines (such as 150) note there is no space between the
quote marks.
