从UIImagePickerControlleHow的结果来看,如何获得包含元数据的JPEG？

Question

在iOS 4.2上,当我使用UIImagePickerController让用户从照片库中选择一个图像时,这些是返回给我的字典键:

2011-03-02 13:15:59.518 xxx[15098:307] didFinishPickingMediaWithInfo: 
  info dictionary: {
    UIImagePickerControllerMediaType = "public.image";
    UIImagePickerControllerOriginalImage = "<UIImage: 0x3405d0>";
    UIImagePickerControllerReferenceURL = 
      "assets-library://asset/asset.JPG?id=1000000050&ext=JPG";
}

使用这些键中的一个或多个,如何获得包含图像元数据(例如曝光信息和GPS位置数据)的JPEG表示,以便我可以在某处上传并包含元数据(不会被剥离)？

我从Warren Burton看到的非常好的答案显示从iPhone中的ALAsset中检索到的URL显示图像？如何使用UIImagePickerControllerReferenceURL和ALAssetsLibrary assetForURL方法来获取ALAsset和ALAssetRepresentation.但是,我该怎样做才能获得包含所有元数据的JPEG？

或者是否有通过UIImage的机制？

这里的底线是我想获得包含在其中的元数据的JPEG ...

Answer 1

自从我问了这个问题后,我做了一些实验,并认为我现在知道了答案.所有结果都是在iOS 4.2上得到的,这是我关心的......

首先,我们使用了UIImageJPEGRepresentationala:

NSData *imageData = UIImageJPEGRepresentation(self.selectedImage, 0.9);

这似乎没有给你(大部分)图像中的元数据(EXIF,GPS等).很公平,我认为这是众所周知的.

我的测试表明,图像资产的"默认表示"中的JPEG将包含所有元数据,包括EXIF和GPS信息(假设它首先出现在那里).您可以通过从资产URL到资产到资产的默认表示(ALAssetRepresentation)然后使用getBytes方法/消息来检索JPEG图像的字节来获取该图像.该字节流具有上述元数据.

这是我用于此的一些示例代码.它需要一个资产URL,假设是一个图像,并返回带有JPEG的NSData.注意你的使用,代码中的错误处理等.

/*
 * Example invocation assuming that info is the dictionary returned by 
 * didFinishPickingMediaWithInfo (see original SO question where
 * UIImagePickerControllerReferenceURL = "assets-library://asset/asset.JPG?id=1000000050&ext=JPG").
 */
[self getJPEGFromAssetForURL:[info objectForKey:UIImagePickerControllerReferenceURL]];
// ...

/* 
 * Take Asset URL and set imageJPEG property to NSData containing the
 * associated JPEG, including the metadata we're after.
 */
-(void)getJPEGFromAssetForURL:(NSURL *)url {
    ALAssetsLibrary* assetslibrary = [[ALAssetsLibrary alloc] init];
    [assetslibrary assetForURL:url
        resultBlock: ^(ALAsset *myasset) {
            ALAssetRepresentation *rep = [myasset defaultRepresentation];
#if DEBUG
            NSLog(@"getJPEGFromAssetForURL: default asset representation for %@: uti: %@ size: %lld url: %@ orientation: %d scale: %f metadata: %@", 
            url, [rep UTI], [rep size], [rep url], [rep orientation], 
            [rep scale], [rep metadata]);
#endif

            Byte *buf = malloc([rep size]);  // will be freed automatically when associated NSData is deallocated
            NSError *err = nil;
            NSUInteger bytes = [rep getBytes:buf fromOffset:0LL 
                                length:[rep size] error:&err];
            if (err || bytes == 0) {
                // Are err and bytes == 0 redundant? Doc says 0 return means 
                // error occurred which presumably means NSError is returned.

                NSLog(@"error from getBytes: %@", err);
                self.imageJPEG = nil;
                return;
            } 
            self.imageJPEG = [NSData dataWithBytesNoCopy:buf length:[rep size] 
                                     freeWhenDone:YES];  // YES means free malloc'ed buf that backs this when deallocated
        }
        failureBlock: ^(NSError *err) {
            NSLog(@"can't get asset %@: %@", url, err);
        }];
    [assetslibrary release];
}