'===========================================================================
' Subject: ROTATING SPIRAL EFFECT            Date: 07-25-99 (11:58)       
' Author:  Marc Santa                        Code: QB, QBasic, PDS        
' Origin:  santa@tir.com                   Packet: GRAPHICS.ABC
'===========================================================================
colour = 256 ^ 2 '0=red, 1=green, 2=blue
CONST pi = 3.141592653#
CONST k2 = 31.5 / pi
DEF fnatn (z) = (ATN(z) * k2)
CLS : SCREEN 13: xc = 160: yc = 100
FOR i = 1 TO 31: PALETTE i, i * colour: NEXT
FOR i = 1 TO 32: PALETTE i + 31, (32 - i) * colour: NEXT
FOR r = 1 TO 200
  col = col + 64 / r: IF col > 63.5 THEN col = col - 63
  col1 = 32 + CINT((col - 32))' * r / 160)
  CIRCLE (160, 100), r, col1: CIRCLE (161, 100), r, col1
NEXT
'PRINT ATN(TAN(PI / 4!)), PI / 4!
FOR x = 0 TO 160: FOR y = 0 TO 100
   IF (x = 0) THEN cc = 15.75 ELSE cc = fnatn(y / x)
   c1 = POINT(xc + x, yc + y): PSET (xc + x, yc + y), (c1 + cc) MOD 63 + 1
   IF y THEN c1 = POINT(xc + x, yc - y): PSET (xc + x, yc - y), (c1 + 63 - cc) MOD 63 + 1
   IF x * y THEN c1 = POINT(xc - x, yc - y): PSET (xc - x, yc - y), (c1 + 31.5 + cc) MOD 63 + 1
   IF x THEN c1 = POINT(xc - x, yc + y): PSET (xc - x, yc + y), (c1 + 31.5 - cc) MOD 63 + 1
NEXT y: NEXT x

da = -1
DO
  'da = -da
  FOR b = 1 TO 6280
    da = SIN(b / 1000)
    fa = fa + da * 3
    IF fa <= .5 THEN fa = fa + 63
    IF fa > 63.5 THEN fa = fa - 63
    a = CINT(fa)
    FOR i = 1 TO 31
      IF i + a > 63 THEN PALETTE (i + a) - 63, i * colour ELSE PALETTE (i + a), i * colour
    NEXT
    FOR i = 1 TO 32
      IF i + a + 31 > 63 THEN PALETTE (i + a) - 32, (32 - i) * colour ELSE PALETTE (i + a + 31), (32 - i) * colour
    NEXT
  NEXT
'  col2 = col2 + 1: IF col2 = 3 THEN col2 = 0
'  rand = INT(RND * 4)
'  IF rand = 0 THEN colour = 256 ^ 2 + 256 ^ 2 + col2
'  IF rand = 1 THEN colour = 257
'  IF rand = 2 THEN colour = 258
'  IF rand = 3 THEN colour = 256 ^ 2 + 256
LOOP WHILE INKEY$ = ""