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you need to change are the values in line 40. The array MASK%
contains the values of the dots. (In order to make this program
run on the most computers, we’re using only six pins for graphics.
With many computers, you can use all eight available pins.) In
lines 90 and 100 we’ve defined some other variables you’ll be
interested in: LX, LXFAC, LY, and LYFAC are used as scaling
factors. By changing these values, you can change the size of your
printed image or even distort it (you can, for example, make our
circle print as an ellipse). Experiment a little bit!
The main calculations for plotting the image are done in the
subroutine starting at program line 600. This is where you put
the formulas that you want to plot. By changing just the lines
after 600 (with some creative mathematics!) you can plot any
function-limited only by your imagination. Some examples are
shown at the end of this section.
What the program section starting at line 600 actually does is
to calculate starting and ending points for a line (in our circle
the “lines” are very short-sometimes the starting and ending
points are the same). The coordinates of the starting point of
the line are assigned to variables Xl and Y 1. The line ends at
point X2, Y2. When these coordinates have been calculated, a
subroutine call is made to line 270. This subroutine calculates
the coordinates of individual points along that line.
After these coordinates have been determined, the subroutine
at line 400 is called. This routine turns “on” an individual dot
in our array called BIT%. (Keep in mind that no printing has
been done yet; the computer is still drawing the image on its
“graph paper” in memory.) The way an individual dot is turned
on is using the logical OR function in line 470.
When all the points have been plotted in memory, printing
begins at line 150. We first set the line spacing to 6/72 inch using
the < ESC > “A” command. This is so that there are no gaps
between rows of dots. Then the loop from line 170 to line 240
prints the dot graphics image one line (which is six dots high)
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