ZX Computings - OpenLibra

Moving graphics
Getting into
the Movies
What do you do when you want
aliens and asteriods to burn
around your TV screen? The
PRINT AT on the ZX81 gives you
one solution.
You make an object move — or
appear to move — on the TV
screen by printing it in one position,
holding the display for a
moment, then unprinting the
old position just as you reprint
in a new position.
The PRINT AT function on
the ZX81, although it is slow,
makes it easy to position an object
just where you want it. For
our first simple program, enter
PROGRAM ONE and press RUN
to see it in action. What you'll
see, all being well, is a black
square move down the screen
diagonally. Note that you can
'chain' PRINT AT commands as
in line 30.
This program shows the
simplest kind of moving
graphics programming, and
although far from satisfactory
in terms of the result displayed
on the screen, at least it should
give you a clear idea of how to
produce one kind of effect. Its
main disadvantage is that the
screen is blank while the computer
is working out the new
PRINT AT position for the black
blob. As you can see from running
this program, the RND function
— when running the computer
in SLOW — is particularly
slow. As you begin to elaborate
the program you're working on,
and you want the computer to
do more and more, you'll find
this blank between subsequent
PRINT ATs of the blob becomes
intolerable. Try, for example,
adding 35 LET Z = X' "Y and
see how slow the program
becomes. This can be overcome
to some extent by assigning
two other variables to the
PRINT AT position which hold it
for unprinting while the computer
is working out new positions.
Change the program so it
reads as the listing for program
two. Run this, and you'll see
how much more satisfactory it
is. If you want it to run forever,
add 57 IF X greater than 19
THEN RUN.
Now moving a blob down
the screen in an irregular slide is
not of much use to anyone.
Enter and run program number
three, which makes a ball
bounce around the screen.
Although this does not use a second
set of variables to unprint,
it is fairly satisfactory. You'll
see that lines 70 and 80 check
each time through the program
to ensure that the ball has not
'hit the sides', and if it has done
so, ensures that the variables
which determine the next position
of the ball (C and D) are
changed.
Program four does use new
variables (E and F) to hold the
5 REM *MOUING GRAPHICS 5 REM *HOMING GRAPHIC
PROGRAM ONE* PROGRAM TWO*
1® LET X = © 1 0 LET X=0
L.ET Y=0 20 LET Y=0
3 0 PRINT AT X..Y; ' AT X , Y ;
*• *
25 LET A =X
LET XsXfRND 27 L ET B =Y
5 0 LET Y=YfRND 3 0 PRINT AT X/Y;"B"
50 GOTO 30 4 0 LET X-X +RHD
5 0 LET Y=Y+RND
5 5 PRINT AT A,, 8; " "
60 GOTO 25
• Program one • Program two
20 LET A=UFiL "10*RND + 5
30 L E T B='vhL " A ^ R N D + S "
10 REM *BOUNCING BRLL -• • 1 * 40 LET' - A
LEI A = 10*RNL>+5 45 LPT G-C +C
3 0 LET 6-P*RND+5 50 LFT D =C
4.0 L t£T C = 1 55 L^T E=R
5 3 LET D=1 56 L. fiT F s=B
5 0 PRINT f*T P . B; 0
60 PRINT RT R,B,"0'
7 0 Ir A 19 THEN LET C = - 70 IF R 19 THEN
3 0 IF 829 THEN LET D=- 60 IF B < G OR B>29 THEN
3 5 P R I N T RT A.. B.: " "
90 LET A = A 4-C
9 0 LET R=R*C 100 LET B=B+D
1 0 0 LET S = B + D 105 PRINT AT E F; ". "
1 10 GOTO 50 110 GOTO UAL "SS"
Program three Program four
o
old position while the new one
is being worked out, and these
are used not as an 'unprint' but
rather to print a trailing series of
full stops, as you can see in
diagram one. Run these programs
a few times, and examine
the listings, and you're
sure to be able to work out
ways to make them more effective.
A second way of approaching
the moving graphics problem
is to use SCROLL to move
the screen up a line, before
reprinting what you need on the
line it has just left. Enter and run
program five, HAILSTORM,
and you'll see how it works in
practice. In this program, you
are the black block, PRINT ATted
in line 30. You control the
position of the blob by using IN-
KEY$, pressing on keys "8" (to
move the blob right) and "5"
(to move it left). The whole of
line 80 ensures that the blob
will not move off the screen,
and saves valuable memory by
placing the interpretation of
both INKEY $ and checking the
screen limits on either side,
within one line. Line 990 prints
LET
LET D^-
ZX COMPUTING SUMMER 1382 109