0

我已经通过 iTunes 将图像同步到 2 个 Ipad（比如说 IPAD1 和 IPAD2）。然后，当我使用 ALAssetLibrary 块检索图像时，2 个 Ipad 中的文件大小不同。

（IPAD1 文件大小：0.024059，IPAD2 文件大小：0.024325）。

我可以知道，为什么相同的文件在 IPAD1 和 IPAD2 中的大小不同？

但是，我通过 safari 浏览器将图像保存到 IPAD1 和 IPAD2，通过点击同一网页中的图像，当我通过 ALAssetLibarary 检索时，Ipad1 和 Ipad2 中的图像文件大小相同。

请告诉我您的宝贵建议......

用法

I am doing the image comparison in Ipad PhotoLibrary.
Whenever an Image transfer request is coming from another device, i have to test the image file exist in PhotoLibrary.
So mentor image request will have CRC code of the requested image which will uneque for the same image file and i am generating the CRC code for all my photoLibrary images and comparing it with the requested image CRC code.
So whenever these 2 CRC's are equal , i can easily identify the files are same.

我用于检索 PhotoLibrary Image 的代码是：

ALAssetsLibraryAssetForURLResultBlock resultblock = ^(ALAsset *myasset)
  {
   ALAssetRepresentation *rep = [myasset defaultRepresentation];

   NSLog(@"GOT ASSET, File size: %f", [rep size] / (1024.0f*1024.0f));

   uint8_t* buffer = malloc([rep size]);

   NSError* error = NULL;

   NSUInteger bytes = [rep getBytes:buffer fromOffset:0 length:[rep size] error:&error];

       if (bytes == [rep size])
       {   

            defaultRepresentationData = [[NSData dataWithBytes:buffer length:bytes] retain];

            CGImageRef iref = [rep fullResolutionImage];
            UIImage *photLibraryImage  = [UIImage imageWithCGImage:iref];


             NSData *imageData = UIImagePNGRepresentation(photLibraryImage); //convert image into .png format.
             const int imageCRC = [self CRCForImage:imageData]]; //getting CRC value for image data

             NSLog([NSString stringWithFormat:@"@@@@@@@@ image CRC is:%u",imageCRC ]);       

       }
       else
       {
            NSLog(@"Error '%@' reading bytes from asset: '%@'", [error localizedDescription]); //assetURL);
       }

   free(buffer);

   // notifies the lock that "all tasks are finished"

  };

     //
      ALAssetsLibraryAccessFailureBlock failureblock  = ^(NSError *myerror)
      {
           NSLog(@"NOT GOT ASSET");  
      };

  NSURL *asseturl = [NSURL URLWithString:fileName]; 

   ALAssetsLibrary* assetslibrary = [[[ALAssetsLibrary alloc] init] autorelease];
   [assetslibrary assetForURL:asseturl  resultBlock:resultblock  failureBlock:failureblock];    

 }

NSLog(@"GOT ASSET, File size: %f", [rep size] / (1024.0f*1024.0f)); 在不同设备中为同一文件打印不同的值。

控制台日志详细信息

Ipad1

GOT 资产，文件大小：0.024059 图像 CRC 为：2659650838

iPad2

GOT 资产，文件大小：0.024325 @@@@@@@@image CRC 为：331786167

IPAD 版本详情

IPAD1：版本：4.2.1(8C148) 型号：MB292LL

IPAD2：版本：4.3.5(8L1) 型号：MB292LL

谢谢。

4

1 回答 1

1

It's an implementation detail. There's no guarantee they'll be identical. Metadata (such as hardware info, serial number, time of day, etc.) may differ between the devices, for example.

If you tell us the broader goal you're trying to accomplish, we may be able to help you find an alternate solution to comparing file sizes.

于 2011-11-23T16:16:02.250 回答