DECLARE SUB Box(x1!,y1!,x2!,y2!,dir%)
DECLARE SUB PreBox(x1!,y1!,x2!,y2!)
DECLARE FUNCTION Iter%(px!,py!)
DIM SHARED MaxIter AS INTEGER
MaxIter=100
SCREEN 1
PreBox 0,0,319,199
SUB Box(x1,y1,x2,y2,dir%)
IF INKEY$="" THEN SYSTEM
IF y2-y1<1 THEN EXIT SUB
IF dir% THEN
xm=.5*(x1+x2)
FOR y=y1 TO y2
k%=Iter%(xm,y)
IF k%<=MaxIter THEN
PSET(xm,y),k% MOD 4
ELSE
PSET(xm,y),1
END IF
NEXT
Box x1,y1,xm,y2,0
Box xm,y1,x2,y2,0
ELSE
tv%=-1
ym=.5*(y1+y2)
FOR x=x1 TO x2
k%=Iter%(x,ym)
IF tv%=-1 THEN tv%=k% MOD 4 ELSE IF k% MOD 4<>tv% THEN tv%=-2
IF k%<=MaxIter THEN
PSET(x,ym),k% MOD 4
ELSE
PSET(x,ym),1
END IF
NEXT
IF tv%<>POINT(x1,y1)OR tv%<>POINT(x1,y2)OR tv%<>POINT(x2,y1)OR tv%<>POINT(x2,y2)THEN
Box x1,y1,x2,ym,1
Box x1,ym,x2,y2,1
ELSE
LINE(x1,y1)-(x2,y2),tv%,BF
END IF
END IF
END SUB
FUNCTION Iter%(px,py)
zx=px*.0125-2
zy=py*.015-1.5
cx=zx:cy=zy
FOR i%=1 TO MaxIter
zx2=zx*zx-zy*zy+cx
zy=2*zx*zy+cy
zx=zx2
IF zx*zx+zy*zy>4 THEN EXIT FOR
NEXT
Iter%=i%
END FUNCTION
SUB PreBox(x1,y1,x2,y2)
FOR x=x1 TO x2
k%=Iter%(x,y1)
IF k%>MaxIter THEN
PSET(x,y1),1
ELSE
PSET(x,y1),k% MOD 4
END IF
k%=Iter%(x,y2)
IF k%>MaxIter THEN
PSET(x,y2),1
ELSE
PSET(x,y2),k% MOD 4
END IF
NEXT
FOR y=y1 TO y2
k%=Iter%(x1,y)
IF k%>MaxIter THEN
PSET(x1,y),1
ELSE
PSET(x1,y),k% MOD 4
END IF
k%=Iter%(x2,y)
IF k%>MaxIter THEN
PSET(x2,y),1
ELSE
PSET(x2,y),k% MOD 4
END IF
NEXT
Box x1,y1,x2,y2,1
END SUB