上传图像不失质量

我使用这个类来上传图像并调整图像大小。问题是图像质量差！我需要上传图像而不会丢失质量！上传图像不失质量

class SimpleImage { 

    var $image; 
    var $image_type; 

    function load($filename) { 
     $image_info = getimagesize($filename); 
     $this->image_type = $image_info[2]; 
     if($this->image_type == IMAGETYPE_JPEG) { 
     $this->image = imagecreatefromjpeg($filename); 
     } elseif($this->image_type == IMAGETYPE_GIF) { 
     $this->image = imagecreatefromgif($filename); 
     } elseif($this->image_type == IMAGETYPE_PNG) { 
     $this->image = imagecreatefrompng($filename); 
     }  
    } 

    function save($filename, $image_type=IMAGETYPE_JPEG, $compression=75, $permissions=null) { 
     if($image_type == IMAGETYPE_JPEG) { 
     imagejpeg($this->image,$filename,$compression); 
     } elseif($image_type == IMAGETYPE_GIF) { 
     imagegif($this->image,$filename);   
     } elseif($image_type == IMAGETYPE_PNG) { 
     imagepng($this->image,$filename); 
     } 
     if($permissions != null) { 
     chmod($filename,$permissions); 
     } 
    } 
    function output($image_type=IMAGETYPE_JPEG) { 
     if($image_type == IMAGETYPE_JPEG) { 
     imagejpeg($this->image); 
     } elseif($image_type == IMAGETYPE_GIF) { 
     imagegif($this->image);   
     } elseif($image_type == IMAGETYPE_PNG) { 
     imagepng($this->image); 
     } 
    } 

    function getWidth() { 
     return imagesx($this->image); 
    } 

    function getHeight() { 
     return imagesy($this->image); 
    } 

    function resizeToHeight($height) { 
     $ratio = $height/$this->getHeight(); 
     $width = $this->getWidth() * $ratio; 
     $this->resize($width,$height); 
    } 

    function resizeToWidth($width) { 
     $ratio = $width/$this->getWidth(); 
     $height = $this->getheight() * $ratio; 
     $this->resize($width,$height); 
    } 

    function scale($scale) { 
     $width = $this->getWidth() * $scale/100; 
     $height = $this->getheight() * $scale/100; 
     $this->resize($width,$height); 
    } 

    function resize($width,$height) { 
     $new_image = imagecreatetruecolor($width, $height); 
     imagecopyresampled($new_image, $this->image, 0, 0, 0, 0, $width, $height, $this->getWidth(), $this->getHeight()); 
     $this->image = $new_image; 
    }  
}

任何人都可以请帮忙吗？

来源

2010-10-27 King Julien

质量问题已得到解决，但还有一个问题。当我上传透明PNG图像时，透明部分变黑。这怎么能修复？ – 2010-10-27 14:43:52

参见例如这里：http://stackoverflow.com/questions/313070/png-transparency-with-php – mercator 2010-10-27 14:57:51