调整图片质量而不损失

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我必须用PHP调整图片大小。但是...质量非常非常糟糕！（看图）调整图片质量而不损失

这里我的代码：

function _copyAndResizeImage($id, $wanted_width, $isAdvert=true) { 
// The file 
if ($isAdvert) { 
    $filename = "../upload/images/advert/".$id."/1.jpg"; 
} else { 
    $filename = "../upload/images/user/".$id.".jpg"; 
} 

list($width, $height) = getimagesize($filename); 

if ($width == 0) { 
    $percent=1; 
} else { 
    $percent = $wanted_width/$width; 
} 

// Content type 
header('Content-Type: image/jpeg'); 

// Get new dimensions 
$new_width = $width * $percent; 
$new_height = $height * $percent; 

// Resample 
$image_p = imagecreatetruecolor($new_width, $new_height); 
$image = imagecreatefromjpeg($filename); 
imagecopyresampled($image_p, $image, 0, 0, 0, 0, $new_width, $new_height, $width, $height); 

// Output 
if ($isAdvert) { 
    imagejpeg($image_p, '../upload/images/advert/'.$id.'/1-'.$wanted_width.'.jpg'); 
} else { 
    imagejpeg($image_p, '../upload/images/user/'.$id.'-'.$wanted_width.'.jpg'); 
} 

}

enter image description here