More graphics

irt.ese two pages show how you can use PLOT and UNPLOT to make

NM noving pictures on the screen. Moving pictures are called animated

Varapnicsand they are useful (or games programs, or to illustrate

(programs which explain, say, the principles of gravity or ballistics and
ih ohtpaths
4

The pictures for video and arcade games
Vare controlled by a small computer. The
£omputer is programmed to play only the
"games and the programs are in the
"£omputer's own code, not in BASIC.

Pic unplot program

A general purpose microcomputer
programmed in BASIC makes slower,
simpler pictures. It cannot handle all the
instructons for the screen guickly enough
to make really fast moving graphics.

This is how the
spot moved. ln a

moment this spot
will be unplotted too.

When the spot reaches the edge ofthe
screen the program may stop with an

error message as the values for X and Y

are outside the screen range of the computer.

Th short program makes a spot oflight
Move across the screen. Remember, the
sammands [or PLOT and UNPLOT vary on

When the ball reaches the top ofthe
screen the amount to be added to Y is

1. ad ball video games use programs
lhe one above to move the ball onthe

n. There are simple programrulesto subtracted instead. In the same way, when
Pthe bali moving when it reachesthe it reaches the right edge, the amount is
Me ol the screen. subtracted from X.

Line pattern program

Thus program plots a line across the
screen and when it reaches the sides,
sends it back again in another direction. It
does not use UNPLOT so the lines leavea
pattern on the screen. The picture onthe
night shows what happens when you run
the program. The program is set by line
100 to plot 10,000 pixels. You can change
this figure to make it shorter, or BREAK the
program at a pattern you like.

10 REM: SET UP GRAPHICS MODE HERE IF NECESSARY

20 PRINT "HOW MANY PIXELS ACROSS?, Lines 20 to 50 ask for the height and width
30 INPUT H of the screen. The semi-colon puts your
40 PRINT "AND UP?" reply on the same line as (he guestion.
50 INPUT V

60 LET X<H2 |

This makes X and Y start at (he of the screen.
70 LET Y<V/2 "a GRRETA
80 LET S-] S and Tare the amounts that will be added to X and
90 LET T-<] Y to make the line move

100 FOR I<l TO 10000 IM— —<— — Theloop from lines 100 to 190 is repeated 10,000 times
110 LET S<S4(NT(RNDC(l) x 10-- 1)—5)/50 Each time, Xand Y are changed by a small amount.
120 LET X-X15S This gives a very smalj number to add to X. The

130 LET Y<Y1T number varies each time the loop is repeated.

140 IF X<5 THEN LET S<-S

180 IF X>H-8 THEN LET S<-S kato op poe raj pi pom
z- reverne when come
160 IF Y<8 THEN LET Te-T Eve er z

170 IF Y>V-S THEN LET Ts-T
180 GOSUB 300 ]————————— Sends the computer to the subroutine to plot the line
190 NEXT |

200 STOP

300 REM: PLOT LINE

310 PLOT (X,Y) pore ii Plots the pixel with the current valne (or X and Y.

Try changing the numbers in lines 80 and
90 to; say, 8 or 10 (or larger on a computer
with high resolution graphics). This
makes the computer plot the pixels al
intervals. s]