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function GroesseAendern(Bild:String; Breite:Integer; Hoehe:Integer):String;
begin
 try
  Bmp := TBitmap.Create;
  Bmp.LoadFromFile(Bild+'.bmp');
  Bmp1 := TBitmap.Create;
  Bmp1.Width := Breite;
  Bmp1.Height := Hoehe;
  SetStretchBltMode(Bmp1.Canvas.Handle, Halftone);
  StretchBlt(Bmp1.Canvas.Handle,0,0,Bmp1.width,Bmp1.height,
  Bmp.Canvas.Handle,0,0,Bmp.Width,Bmp.Height,SRCCOPY);
  Bmp1.SaveToFile(Bild+'.bmp');
 finally
  Bmp.Free;
  Bmp1.Free;
 end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  GroesseAendern('test',200,200);
end;