procedure TForm1.DrawLine(c : TCanvas; x1,y1,x2,y2 : Integer);
var
    dx, dy, s, sx, sy, err, e2: integer;
    ds, es, i, j : integer;
    procedure _swap(out a, b : integer);
    var
        t : integer;
    begin
        t := a;
        a := b;
        b := t;
    end;
begin

    ds := 5; // dash size
    es := 2; // empty size

    if x1 > x2 then
    begin
        _swap(x1, x2);
        _swap(y1, y2);
    end;

    dx := abs(x2 - x1);
    dy := -abs(y2 - y1);

    if x1 < x2 then sx := 1 else sx := -1;
    if y1 < y2 then sy := 1 else sy := -1;
    err := dx+dy;

    i := 0;
    j := 0;
    while x1 < x2 do begin
        if i <= ds then begin
            c.Pixels[x1, y1] := clBlack;
            inc(i);
        end
        else begin
            if j = es then begin
                i := 0;
                j := 0
            end
            else
                inc(j);
        end;

        e2 := 2*err;

        if (e2 >= dy)  then begin
            err := err + dy;
            x1 := x1 + sx;
        end; // e_xy+e_x > 0

        if (e2 <= dx) then begin
            err := err + dx;
            y1 := y1 + sy;
        end; // e_xy+e_y < 0
    end;
end;