手机照片不能生成缩略图问题以及解决方案

一、出现的问题
这几天做了手机上传照片并裁出缩略图的接口的测试，发现不管怎么，生成的缩略图都是一片漆黑。:-(
然后就把这个缩略图类单拿出来进行测试，发现只要是手机拍出来的照片都不能进行缩略图的处理。。。。
二、问题分析以及解决方案
经过群里的请教，发现问题可能是出现在文件的类型的判断上，因为png图片自带一个透明的图层，导致不能直接转换成jpg的文件，而手机排出的照片扩展名是jpg.
所以，得出的结论是 手机拍出的是jpg扩展名的png图片 。
由于扩展名是可以随意修改的，不是很能保证文件的信息的准确性，所以我们采用了 getimagesize 函数进行文件类型的获取。(转)

 
//获取真实的图片类型
 list($width, $height, $type, $attr) = getimagesize($this->sur_file);
  switch($type) {  
    case 1 :  
      $img_type = 'gif';  
      break;  
    case 2 :  
      $img_type = 'jpeg';  
      break;  
    case 3 :  
      $img_type = 'png';  
      break;  
    case 15 :  
      $img_type = 'wbmp';  
      break;  
    default :  
      return false;  
  }
  
//下面把修改后的生成缩略图的类贴出来，供大家指正~
/**
 * php生成缩略图类
 * 修改者 点点细雨 
 * 文章出处 ： http://blog.csdn.net/diandianxiyu_geek
 * 2014-07-23 解决了图片类型不能正常识别的问题
 */
class thumb {
  
  public $sur_file; //读取的原图片
  public $des_file; //生成目标图片
  public $tem_file; //临时图片
  public $tag;  //缩略标签  0,默认,按固定的高宽生成  1,按比列或固定最大长度生成  -1,按某个宽度或某个高度缩小
  public $resize_width;  //$tag为0时,目标文件宽
  public $resize_height;  //$tag为0时,目标文件高
  public $sca_max; //$tag为1时,<0$sca_max<1时为缩小比例;$sca_max>1时为最大长度(高或宽之中的最大值)
  public $type;  //图片类型
  public $width;  //原图片宽
  public $height;  //原图片高
  public $size;  //原图大小
  
  //构造函数
  
  public function __construct($surpic, $reswid, $reshei, $despic, $mark, $scamax) {
    $this->sur_file = $surpic;
    $this->resize_width = $reswid;
    $this->resize_height = $reshei;
    $this->tag = $mark;
    $this->sca_max = $scamax;
    list($width, $height, $type, $attr) = getimagesize($this->sur_file);
    switch ($type) {
      case 1 :
        $img_type = 'gif';
        break;
      case 2 :
        $img_type = 'jpeg';
        break;
      case 3 :
        $img_type = 'png';
        break;
      case 15 :
        $img_type = 'wbmp';
        break;
      default :
        return false;
    }
    $this->type = $img_type; //获取图片类型
    $this->init_img(); //初始化图片
    $this->des_file = $despic; //目标图片地址
    $this->width = $width;
    $this->height = $height;
    $this->size = filesize($surpic);
    $this->new_img();
    imagedestroy($this->tem_file);
  }
  
  //图片初始化函数
  private function init_img() {
    if ($this->type == 'jpeg') {
      $this->tem_file = imagecreatefromjpeg($this->sur_file);
    } elseif ($this->type == 'jpg') {
      $this->tem_file = imagecreatefromjpeg($this->sur_file);
    } elseif ($this->type == 'gif') {
      $this->tem_file = imagecreatefromgif($this->sur_file);
    } elseif ($this->type == 'png') {
      $this->tem_file = imagecreatefrompng($this->sur_file);
    } elseif ($this->type == 'bmp') {
      $this->tem_file = imagecreatefrombmp($this->sur_file); //bmp.php中包含
    }
  }
  
  //图片生成函数
  private function new_img() {
    $ratio = ($this->width) / ($this->height); //原图比例
    $resize_ratio = ($this->resize_width) / ($this->resize_height); //缩略后比例
    $newimg = imagecreatetruecolor($this->resize_width, $this->resize_height); //生成新图片
    imagealphablending($newimg, false); //这里很重要,意思是不合并颜色,直接用$img图像颜色替换,包括透明色;
    imagesavealpha($newimg, true);
    if ($this->tag == 0) { //按固定高宽截取缩略图
      $newimg = imagecreatetruecolor($this->resize_width, $this->resize_height); //生成新图片
      if ($ratio >= $resize_ratio) {//即等比例下,缩略图的高比原图长,因此高不变
        imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, $this->resize_width, $this->resize_height, (($this->height) * $resize_ratio), $this->height);
      } elseif ($ratio < $resize_ratio) {//即等比例下,缩略图的宽比原图长,因此宽不变
        imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, $this->resize_width, $this->resize_height, $this->width, (($this->width) / $resize_ratio));
      }
    } elseif ($this->tag == 1) { //按固定比例或最大长度缩小
      if ($this->sca_max < 1) { //按比例缩小
        $newimg = imagecreatetruecolor((($this->width) * ($this->sca_max)), (($this->height) * ($this->sca_max))); //生成新图片
        imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, (($this->width) * ($this->sca_max)), (($this->height) * ($this->sca_max)), $this->width, $this->height);
      } elseif ($this->sca_max > 1) { //按某个最大长度缩小
        if ($ratio >= 1) { //宽比高长
          $newimg = imagecreatetruecolor($this->sca_max, ($this->sca_max / $ratio)); //生成新图片
          imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, $this->sca_max, ($this->sca_max / $ratio), $this->width, $this->height);
        } else {
          $newimg = imagecreatetruecolor(($this->sca_max * $ratio), $this->sca_max); //生成新图片
          imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, ($this->sca_max * $ratio), $this->sca_max, $this->width, $this->height);
        }
      }
    } elseif ($this->tag == -1) { //按某个宽度或某个高度缩小
      if ($resize_ratio >= 1) {//新高小于新宽,则图片按新宽度缩小
        $newimg = imagecreatetruecolor($this->resize_width, ($this->resize_width / $ratio)); //生成新图片
        imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, $this->resize_width, ($this->resize_width / $ratio), $this->width, $this->height);
      } elseif ($resize_ratio < 1) {//新宽小于新高,则图片按新高度缩小
        $newimg = imagecreatetruecolor(($this->resize_height * $ratio), $this->resize_height); //生成新图片
        imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, ($this->resize_height * $ratio), $this->resize_height, $this->width, $this->height);
      }
    }
  
    //输出新图片
    if ($this->type == 'jpeg' || $this->type == 'jpg') {
      imagejpeg($newimg, $this->des_file);
    } elseif ($this->type == 'gif') {
      imagegif($newimg, $this->des_file);
    } elseif ($this->type == 'png') {
      imagepng($newimg, $this->des_file);
    } elseif ($this->type == 'bmp') {
      imagebmp($newimg, $this->des_file); //bmp.php中包含
    }
  }
  
#function new_img() end
}