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#72: How can I change a line type for drawing?

In some applications you must draw the line where each pixel is some shape. The MS Windows allows to draw line of such type - you must use the LineDDA funtion. This function needs a callback function that is called for each pixel drawing. Of course, in this callback function you can realize the any drawing routines.

For example, the next code allows to draw the ellipse every 10 pixels:

var
  i: Integer;

procedure DrawEllipse(X, Y: Integer; lpData: LParam); stdcall;

implementation

procedure DrawEllipse(X, Y: Integer; lpData: LParam);
begin
  with TObject(lpData) as TForm do
  begin
    if (i mod 10 = 0) then 
      Canvas.Ellipse(x-5, y-5, x+5, y+5);
    Inc(i);
  end;
end;

procedure TForm1.FormPaint(Sender: TObject);
begin
  i := 0;
  LineDDA(0, 0, Width, Height, @DrawEllipse, Integer(yourForm));
end;

Published: April 3, 2000

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