请问:如何生成一个带有文字的透明PNG图片?

我在用PNGIMAGE,反复试验,生成的文字总是有背景色,如何才能去掉背景色?
          png := tpngobject.createblank(COLOR_PALETTE, 4, 1500, 1200);
          png.Canvas.Brush.Color := clwhite;
          png.Canvas.FillRect(Rect(0,0,png.width,png.height));                  png.Canvas.Pen.Color:=clred;
          png.canvas.pen.width := 100;
          png.canvas.brush.Style := bsClear;
          png.canvas.Ellipse(20,20,png.width-20,png.width - 20);          png.Canvas.TextOut(100,100,swords_text);          png.TransparentColor := clwhite;
          png.savetofile(swords_fullname);
          png.free;