Drawing with different line types

Windows allows to draw lines where each pixel is another shape or drawing with LineDDa function. It needs a callback function that is called when each pixel must be drawn. There you can put the drawing routines. This routine draws a rectangle every 4 pixels:



TForm1 = class(TForm)

  procedure FormCreate(Sender: TObject);

  procedure FormPaint(Sender: TObject);

public

  DrawNow : Integer;

end;



var

  Form1: TForm1;



procedure DrawPoint(x,y : Integer;lpData : LParam); stdcall;



implementation



{$R *.DFM}



procedure DrawPoint(x,y : Integer;lpData : LParam);

begin

  with TObject(lpData) as TForm1 do begin

    if DrawNow mod 4 = 0 then

      Canvas.Rectangle(x-2,y-2,x+3,y+3);

    Inc(DrawNow);

  end;

end;



procedure TForm1.FormCreate(Sender: TObject);

begin

  DrawNow := 0;

end;



procedure TForm1.FormPaint(Sender: TObject);

begin

  LineDDA(0,0,Width,Height,@DrawPoint,Integer(Self));

end;