SOURCE CODE ()

CONST numr  =  1000 : REM num of random position in pool 
CONST nums  =    20 : REM number of stars in starfield
CONST speed =     8 : REM speed of stars (smaller = faster)
CONST clock =    50 : REM Number of TI per second
CONST bg    = BLACK : REM background color
CONST fg    = WHITE : REM forteground color

DIM xrnd AS POSITION (numr+1)
DIM yrnd AS POSITION (numr+1)

DIM xtab AS POSITION (nums+1)
DIM ytab AS POSITION (nums+1)

BITMAP ENABLE (320,200,2)
REM BITMAP ENABLE(320,200,4)
REM BITMAP ENABLE(160,200,16)
CLS bg
COLOR BORDER bg 

REM init pool of random position (faster than calling RND over and over again)
FOR i=0 TO numr
	xrnd(i) = RND(SCREEN WIDTH)
	yrnd(i) = RND(SCREEN HEIGHT)
NEXT
irnd = 0

REM initial stars
FOR i=0 TO nums
	' REPEAT : c = RND(SCREEN COLORS) : UNTIL c<>bg
	xtab(i) = RND(SCREEN WIDTH)
	ytab(i) = RND(SCREEN HEIGHT) 
	
	PLOT xtab(i),ytab(i),fg
NEXT

VAR x0 AS SIGNED WORD = (SCREEN WIDTH / 2)
VAR y0 AS SIGNED WORD = (SCREEN HEIGHT / 2)

VAR x AS SIGNED WORD = 0
VAR y AS SIGNED WORD = 0
VAR z AS SIGNED WORD = 0
VAR t AS SIGNED WORD = 0
VAR dx AS SIGNED WORD = 0
VAR dy AS SIGNED WORD = 0

TI=0: frames = 0
DO
	INC frames
	PLOT x0,y0,WHITE
	IF TI>=clock THEN 
		LOCATE 0,0 : PRINT frames;"    ";
		frames = 0 
		TI = 0
	ENDIF
	FOR i=0 TO nums
		z = xtab(i)
		x = z + (z - x0)/speed

		t = ytab(i)	
		y = t + (t - y0)/speed
		
		IF (((x OR y)<0) OR (y>=SCREEN HEIGHT)) OR (x>=SCREEN WIDTH) THEN
			REPEAT
				x = xrnd(irnd)
				y = yrnd(irnd)
				ADD irnd,1,0 TO numr
				REM avoid fixed stars, these are the ones where
				REM ((x-x0)/speed)==0 or ((y-y0)/speed)==0
				REM which result in the folowing bitwise AND expression
			UNTIL ((x-x0)/speed) AND ((y-y0)/speed)
		ENDIF
		
		xtab(i) = x
		ytab(i) = y
		
		PLOT z,t,bg
		PLOT x,y,fg
		'PLOT x,y,bg
	NEXT
    WAIT VBL
	joy = JOY(0) : dx = 0 : dy = 0
	IF joy HAS BIT LEFT  THEN : DEC dx : ENDIF
	IF joy HAS BIT RIGHT THEN : INC dx : ENDIF
	IF joy HAS BIT UP    THEN : DEC dy : ENDIF
	IF joy HAS BIT DOWN  THEN : INC dy : ENDIF
	IF (dx OR dy)<>0 THEN
		IF joy HAS BIT FIRE THEN : MUL dx,4 : MUL dy,4 : ENDIF
		PLOT x0,y0,bg
		ADD x0,dx,0 TO SCREEN WIDTH
		ADD y0,dy,0 TO SCREEN HEIGHT
	ENDIF
LOOP

Commodore 128 (CPU 8502)

In order to compile the example, type this command on the command line:

Linux

ugbc.c128 -O prg -o contrib_starfield.prg contrib_starfield.bas

Windows

ugbc.c128.exe -O prg -o contrib_starfield.prg contrib_starfield.bas

For Microsoft Windows users we suggest using UGBASIC-IDE, which allows you to download and compile this example with just one click.