Randomize ' initiate random number generator

Tail = 4
ScreenX = 640
ScreenY = 200

' Declare some array that will conain drawed lone info 
' (to be able to undraw lines)
Dim x1(Tail), y1(Tail), x2(Tail), y2(Tail) as integer
Dim a1(Tail), b1(Tail), a2(Tail), b2(Tail) as integer

' Initiate first line points
x1[0] = 10+Rnd(ScreenX-10)
x2[0] = 10+Rnd(ScreenX-10)
y1[0] = 10+Rnd(ScreenY-10)
y2[0] = 10+Rnd(ScreenY-10)

a1[0] = 10+Rnd(ScreenX-10)
a2[0] = 10+Rnd(ScreenX-10)
b1[0] = 10+Rnd(ScreenY-10)
b2[0] = 10+Rnd(ScreenY-10)

c = Rnd(Rgb(255,255,255)) ' setup random colour
c2 = Rnd(Rgb(255,255,255)) ' setup random colour

xd1 = 5 : xd2 = 4 : yd1 = 3 : yd2 = 4
ad1 = 4 : ad2 = 5 : bd1 = 4 : bd2 = 3

Change = 0
Change2 = 0

SetBrushColor 0
DrawRect 0,0,ScreenX,ScreenY

while true ' works forever, till Exit while won't be called
  KeepBacklightActive
  SetPenColor c
  DrawLine x1(0),y1(0),x2(0),y2(0) ' draw new line
  SetPenColor c2
  DrawLine a1(0),b1(0),a2(0),b2(0) ' draw new line
  SetPenColor 0
  DrawLine x1(Tail),y1(Tail),x2(Tail),y2(Tail) ' undraw last line
  DrawLine a1(Tail),b1(Tail),a2(Tail),b2(Tail) ' undraw last line

  ' Scroll array
  i = Tail
  while i>0
    x1(i) = x1(i-1)
    y1(i) = y1(i-1)
    x2(i) = x2(i-1)
    y2(i) = y2(i-1)

    a1(i) = a1(i-1)
    b1(i) = b1(i-1)
    a2(i) = a2(i-1)
    b2(i) = b2(i-1)

    i = i - 1
  wend

 ' Initite new pos of next line
  x1(0) = x1(0) + xd1
  x2(0) = x2(0) + xd2
  y1(0) = y1(0) + yd1
  y2(0) = y2(0) + yd2

  a1(0) = a1(0) + ad1
  a2(0) = a2(0) + ad2
  b1(0) = b1(0) + bd1
  b2(0) = b2(0) + bd2

  ' direction corection
  if x1[0] < 1 or x1[0]>=ScreenX-1 then
    xd1 = -xd1
    Change = Change+1
  endif

  if (x2[0] < 1 or x2[0]>=ScreenX-1) then
    xd2 = -xd2
    Change = Change+1
  endif

  if (y1[0] < 1 or y1[0]>=ScreenY-1) then
    yd1 = -yd1
    Change = Change+1
  endif

  if (y2[0] < 1 or y2[0]>=ScreenY-1) then
    yd2 = -yd2
    Change = Change+1
  endif

  if (Change >=4) then
    c = Rnd(Rgb(255,255,255)) ' setup random colour
    Change = 0
  endif

  ' direction corection 2
  if a1[0] < 1 or a1[0]>=ScreenX-1 then
    ad1 = -ad1
    Change2 = Change2+1
  endif

  if (a2[0] < 1 or a2[0]>=ScreenX-1) then
    ad2 = -ad2
    Change2 = Change2+1
  endif

  if (b1[0] < 1 or b1[0]>=ScreenY-1) then
    bd1 = -bd1
    Change2 = Change2+1
  endif

  if (b2[0] < 1 or b2[0]>=ScreenY-1) then
    bd2 = -bd2
    Change2 = Change2+1
  endif

  if (Change2 >=4) then
    c2 = Rnd(Rgb(255,255,255)) ' setup random colour
    Change2 = 0
  endif

  delay 0.001
 
  if readkey then ' exit on any key press
      exit while
  endif 
wend