Line 150
is
critical, since it jumps
over a
RESET.
Without this jump, the
light
blocks
would
be
erased
when they are interrogated.
(Try deleting the line and RUNning
to
see.)
Line 160
is just
a foxy display trick. It
causes
a
blinking light to "appear" to
be scanning
all
points.
Actually, line
160
is just
turning blocks ON and OFF as the Computer
interrogates
them. Since
we
don't want to turn
off blocks that are already supposed to
be lit,
we hop over
this
line when a real live block
is
hit. In reality then,
this
"roving
eye"
never
actually
"hits"
an
"ON"
block.
Line
170
merely closes the FOR-NEXT
loops started
in line
110
Line
180
replaces the
heading
that was erased by the
POINT
scanning process.
Line
190
simply moves the
cursor
(which can
be
a pesky light in
graphics
displays)
back
to the far upper-left
corner,
to get it out of the way.
It
could have been
moved to any
point,
or
just left alone.
Line
200
is
the
locking loop used to keep
READY
and the prompt from
goofing up
the
display.
Pretty simple
when taken a
line
at
a time, isn't it?
Oh
yes,
did you notice
that
the "moving dot" turned
off the original heading? Did you also
notice
that
it took two passes
of
the dot to equal the
width of one
printed character (which
of
course fits
right
in with what
the layout sheet shows)?
The reason the
heading
was erased is that we
deliberately chose not to protect it
(like
we
protected the
blocks)
from the RESET
in
line
160
in order to make the point.
You could
write a little
protective line if you
wanted
to
(or reduce the vertical length of
the scan to
avoid it)
.
Alpha
or Omega?
There you have
it
—
a good running
start
into graphics.
Go
now to part C where you
will
find more
ready-to-run
programs. By
giving them careful study,
you
will see the four
graphics statements
in
use, plus most of the rest you
have learned about
BASIC program-
ming. The
possibilities from here
are
unlimited.
Learned in
Chapter
22
Gr
aphfes Functions
':.,€*'
POINT
(X,Y)
PRINT
AT
140
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