program julia_bw; {Almost exactly the same as julia_col. There is only one
               main difference: this creates the Julia sets in
               black and white as opposed to colour.}

uses crt, graph;

var     xcorner, y2corner, x2corner, ycorner: real;
        xsize, ysize, k, i, xgap, ygap, max: integer;
        ca, cb, za2, za1, za, zb: real;
        gd, gm: integer;
        count: word;



procedure iterate;
begin
count := 0;
za2 := za;
while 1=1 do
begin
za1 := za2;
za2 := (za1 * za1 - zb * zb) + ca;
zb := za1 * zb * 2 + cb;
IF za2 * za2 + zb * zb > 4 THEN EXIT;
count := count + 1;
IF count > max THEN EXIT
end;
END;



procedure main;
begin
gd := Detect;
InitGraph(gd, gm, 'd:\tp\bgi');
xcorner := -1.5;
y2corner := 1.5;
x2corner := 1.5;
ycorner := -1.5;
max := 50;
xsize := 400;
ysize := 400;
xgap := 10;
ygap := 10;
setcolor(white);
{CLS}
FOR k := ysize downTO 1 do
FOR i := 1 TO xsize do
begin
za := (i / xsize) * (x2corner - xcorner) + xcorner;
zb := (k / ysize) * (y2corner - ycorner) + ycorner;
ca := 0.360284;
cb := 0.100376;
iterate;            {The line below is the only different part.}
if count > max then LINE (xgap + i, ygap +ysize- k, xgap+i, ygap +ysize- k)
end;
while not keypressed do
end;



begin
main;
end.