Radio Shack TRS-80 User Manual page 117

10
INPUT
"VERTICAL ADDRESS
C0
TO 47)
"jY
20
INPUT
"STEP SIZE"
;S
30
CLS
40
FOR X =
TO 127
STEP
S
50
SET (X,Y)
60
NEXT
X
99
GOTO 99
RUN
this
program with various values of
S.
Note
that as
you
increase
S,
the
line
is
drawn
much
faster (since the
Computer
has
less
work
to do). In fact, for
S=10
or
more, you can
hardly see the
line
being drawn. This
is
how
a
TV
picture
is
created
—
since
it
too
is
drawn
one unit
at a
time (but so
fast
you don't notice the "drawing time").
Change the program
as
follows
:
55
RESET
(X.Y-1
)
70 Y =
Y+l
80
GOTO 40
If
S
is
small,
you
can see the
lines
being
formed and
cleared.
But
if
S
is
fairly
large (try
10),
the
line
seems
to
move
in
somewhat
"old-time
movie"
fashion. This
is
the
way
the
illusion
of motion
is
created
on
a
TV
set
and
in
some of the popular video games.
Now
try this
next program
(clear
out the old one).
It
paints a
dot on
the screen
and moves
it
up
and
down.
10
INPUT
"HORIZONTAL STARTING POINT
(0
TO
127)";X
20
INPUT
"VERTICAL STARTING POINT
(0
TO 47)
";Y
30 CLS
40
RESET (X.Y-1
1
50
SET (X,Y)
H
115