php调整图片尺寸

创建图像缩略图需要许多时间，此代码将有助于了解缩略图的逻辑。

    /**********************
    *@filename - path to the image
    *@tmpname - temporary path to thumbnail
    *@xmax - max width
    *@ymax - max height
    */
    function resize_image($filename, $tmpname, $xmax, $ymax)
    {
        $ext = explode(".", $filename);
        $ext = $ext[count($ext)-1];  
     
        if($ext == "jpg" || $ext == "jpeg")
            $im = imagecreatefromjpeg($tmpname);
        elseif($ext == "png")
            $im = imagecreatefrompng($tmpname);
        elseif($ext == "gif")
            $im = imagecreatefromgif($tmpname);  
        $x = imagesx($im);
        $y = imagesy($im);  
        if($x <= $xmax && $y <= $ymax)
            return $im;  
        if($x >= $y) {
            $newx = $xmax;
            $newy = $newx * $y / $x;
        }
        else {
            $newy = $ymax;
            $newx = $x / $y * $newy;
        }  
        $im2 = imagecreatetruecolor($newx, $newy);
        imagecopyresized($im2, $im, 0, 0, 0, 0, floor($newx), floor($newy), $x, $y);
        return $im2;
    }