制作缩略图时的图像质量

我有这个制作比例缩略图的代码：

function CroppedThumbnail($imgSrc,$thumbnail_width,$thumbnail_height) { //$imgSrc is a FILE - Returns an image resource.
//getting the image dimensions 
list($width_orig, $height_orig) = getimagesize($imgSrc);  
if (strtolower(substr($imgSrc, -3)) == "jpg") {
    $myImage = imagecreatefromjpeg($imgSrc);
} else if (strtolower(substr($imgSrc, -3)) == "jpeg") {
    $myImage = imagecreatefromjpeg($imgSrc);
} else if (strtolower(substr($imgSrc, -3)) == "png") {
    $myImage = imagecreatefrompng($imgSrc);
} else if (strtolower(substr($imgSrc, -3)) == "gif") {
    $myImage = imagecreatefromgif($imgSrc);
}

$ratio_orig = $width_orig/$height_orig;
if ($thumbnail_width/$thumbnail_height > $ratio_orig) {
   $new_height = $thumbnail_width/$ratio_orig;
   $new_width = $thumbnail_width;
} else {
   $new_width = $thumbnail_height*$ratio_orig;
   $new_height = $thumbnail_height;
}
$x_mid = $new_width/2;  //horizontal middle
$y_mid = $new_height/2; //vertical middle
$process = imagecreatetruecolor(round($new_width), round($new_height));
imagecopyresampled($process, $myImage, 0, 0, 0, 0, $new_width, $new_height, $width_orig, $height_orig);
$thumb = imagecreatetruecolor($thumbnail_width, $thumbnail_height);
imagecopyresampled($thumb, $process, 0, 0, ($x_mid-($thumbnail_width/2)), ($y_mid-($thumbnail_height/2)), $thumbnail_width, $thumbnail_height, $thumbnail_width, $thumbnail_height);
imagedestroy($process);
imagedestroy($myImage);
return $thumb;

}

效果良好

但是图像的质量有点低

我应该做些什么来提高创建的图片质量