' Qbasic Mandelbrot set program
CLS : SCREEN 13

' This makes a nice rainbow palette

FOR t% = 0 TO 192
LINE (t%, 195)-(t%, 199), t%
NEXT t%
PALETTE 193, 1973790

FOR p& = 0 TO 63 STEP 2
a% = a% + 1
PALETTE a%, 63 + 256 * p&
PALETTE a% + 32, 63 - p& + 63 * 256
PALETTE a% + 64, 256 * 63 + p& * 65536
PALETTE a% + 96, (63 - p&) * 256 + 63 * 65536
PALETTE a% + 128, p& + 65536 * 63
PALETTE a% + 160, (63 - p&) * 65536 + 63
NEXT p&

' screen size
sx% = 320
sy% = 190
xmax = 2: ymax = 2: xmin = -3: ymin = -2
FOR y% = 0 TO sy%
FOR x% = 0 TO sx%
p = xmin + x% * (xmax - xmin) / sx%
q = ymin + y% * (ymax - ymin) / sy%
it% = 0
cx = 0: cy = 0
here:
xn = cx * cx - cy * cy + p
yn = 2 * cx * cy + q
r = xn * xn + yn * yn
IF r > 4 OR it% > 192 THEN GOTO plt
it% = it% + 1
cx = xn: cy = yn
GOTO here
plt:
PSET (x%, y%), 1 + it%
NEXT x%: NEXT y%
SLEEP 0