uses Printers;

Procedure BildAusSchnitt(hWND: HWND; iLeft, iTop, iWidth, iHeight: integer);
var
  c: TCanvas;
begin
  BringWindowToTop(hWND);
  c:= TCanvas.Create;
  c.Handle:= GetWindowDC(hWND);
  try
    printer.begindoc;
    printer.Canvas.CopyRect(Rect(0, 0, iWidth* 10, iHeight* 10), c, Rect(iLeft, iTop, iLeft+ iWidth, iTop+ iHeight));
    printer.enddoc;
  finally
    ReleaseDC(hWND, c.handle);
    c.Free;
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
   BildAusSchnitt(Panel1.Handle, 0, 0, Panel1.Width, Panel1.Height);
end;