Scrolling Grid background v4

[DrChip]

REM Background - Scrolling grid v4
REM iPhone 6 Plus / iOS 8.3 b1
REM you can add the touch command or
REM accely to move forward and
REM backward.
REM or you can use it as a background!
REM v2 - scrolls up and down automatically
REM v3 - adjust size 60,120,180,240...
REM you can make it go left and right too!
REM v4 - adjust the scroll length
REM enjoy!

graphics
sw=Screen_Width()
sh=Screen_Height()
DIM trix(4),triy(4)

speed = 10 'pos forward 10, neg backward -10
deltaspeed = 1 ' rate of inc...
stepp = 160 ' block size
ste = stepp/2
scrollmin=80
scrollmax=80

loop:
speed=speed+deltaspeed
IF speed > scrollmin OR speed < -scrollmax THEN
deltaspeed = -deltaspeed
END IF

'Draw Background
refresh off

'clear screen
graphics clear 0,0,0

z=z+speed
IF z>=0 THEN
z=z-speed
END IF

FOR h=z TO sh STEP stepp
FOR w=0 TO sw STEP stepp
fill COLOR 15/255,15/255,60+(h/3)/255
fill RECT w,h to w+ste,h+ste
fill RECT w+ste,h+ste to w+stepp,h+stepp
NEXT w
NEXT h
refresh on
GOTO loop