使用Imagine bundle调整图像大小会模糊图像

我有以下代码：

        $imagine = new 'Imagine'Gd'Imagine();
        $image = $imagine->open($src);
        $image_size = $image->getSize();
        $image_height = $image_size->getHeight();
        $image_width = $image_size->getWidth();
        $ratio = 1;

        $resized_image = $image;
        $ratio = $image_width / $desired_width;
        $this->resizedHeight = $image_height / $ratio;
        $this->resizedWidth = $image_width / $ratio;
            // thumbnailDir doesn t exist
        if(!is_dir(dirname($des)))
                mkdir(dirname($des), 0777, true);
        $resized_image = $image->resize(new Box($this->resizedWidth, $this->resizedHeight));

        $options = array(
            'resolution-units' => ImageInterface::RESOLUTION_PIXELSPERINCH,
            'resolution-x' => 500,
            'resolution-y' => 500,
            'flatten' => false
        );
        $resized_image->save($des, $options);

由于某种原因，调整大小的图像总是有点模糊，你可以在这里看到调整大小的图片，在这里看到原始图片，这是调整大小的照片。我不知道为什么会这样。知道为什么吗？