protected function reduceImage()
    {
        $imResampled = imagecreatetruecolor($this->width, $this->height);
        imagecopyresampled($imResampled, $this->im,
            0, 0, 0, 0,
            $this->width, $this->height,
            $this->width*$this->scale, $this->height*$this->scale
        );
        imagedestroy($this->im);
        $this->im = $imResampled;
    }