Technical Q&A QA1622

Accessing Image Metadata in iOS

Q: How do I get image metadata in iOS?

A: The UIImagePickerController allows a developer to prompt the user to take a picture from the camera, or to choose an existing picture from the photo library. Beginning in iOS 4.1 the NSDictionary object that the UIImagePickerControllerDelegate returns contains an NSDictionary that contains the metadata of the photo that was just captured in addition to the UImage. To access the metadata dictionary use the UIImagePickerControllerMediaMetadata key of the NSDictionary returned by the UIImagePickerControllerDelegate. To store the metadata along with the image in the Camera Roll, use the writeImageToSavedPhotosAlbum:metadata:completionBlock: method of the Assets Library framework.

Listing 1 Example of accessing and saving the image metadata with the UIImagePickerController.

// Respond to the user accepting a newly-captured picture
- (void) imagePickerController: (UIImagePickerController *) picker
 didFinishPickingMediaWithInfo: (NSDictionary *) info {

    NSString *mediaType = [info objectForKey: UIImagePickerControllerMediaType];
    UIImage *originalImage, *editedImage, *imageToSave;

    // Handle a still image capture
    if (CFStringCompare ((CFStringRef) mediaType, kUTTypeImage, 0)
        == kCFCompareEqualTo) {

        editedImage = (UIImage *) [info objectForKey:
                                   UIImagePickerControllerEditedImage];
        originalImage = (UIImage *) [info objectForKey:
                                     UIImagePickerControllerOriginalImage];

        if (editedImage) {
            imageToSave = editedImage;
        } else {
            imageToSave = originalImage;
        }

        // Get the image metadata
        UIImagePickerControllerSourceType pickerType = picker.sourceType;
        if(pickerType == UIImagePickerControllerSourceTypeCamera)
        {
            NSDictionary *imageMetadata = [info objectForKey:
                                           UIImagePickerControllerMediaMetadata];
            // Get the assets library
            ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
            ALAssetsLibraryWriteImageCompletionBlock imageWriteCompletionBlock =
            ^(NSURL *newURL, NSError *error) {
                if (error) {
                    NSLog( @"Error writing image with metadata to Photo Library: %@", error );
                } else {
                    NSLog( @"Wrote image with metadata to Photo Library");
                }
            };

            // Save the new image (original or edited) to the Camera Roll
            [library writeImageToSavedPhotosAlbum:[imageToSave CGImage] 
                                         metadata:imageMetadata 
                                  completionBlock:imageWriteCompletionBlock];
        }
    }


    [[picker parentViewController] dismissModalViewControllerAnimated: YES];
    [picker release];
}

To access the metadata of existing pictures in the photo library you need to use the Assets Library framework which can be used to access the pictures and videos managed by the Photos application including their metadata.

Listing 2 Example of accessing the image metadata with the Assets Library framework.

// Get the assets library
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];

// Enumerate just the photos and videos group by using ALAssetsGroupSavedPhotos.
[library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos
                       usingBlock:^(ALAssetsGroup *group, BOOL *stop)
 {

     // Within the group enumeration block, filter to enumerate just photos.
     [group setAssetsFilter:[ALAssetsFilter allPhotos]];

     // For this example, we're only interested in the first item.
     [group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:0]
                             options:0
                          usingBlock:^(ALAsset *alAsset, NSUInteger index, BOOL *innerStop)
      {

          // The end of the enumeration is signaled by asset == nil.
          if (alAsset) {
              ALAssetRepresentation *representation = [alAsset defaultRepresentation];
              NSDictionary *imageMetadata = [representation metadata];
              // Do something interesting with the metadata.
          }
      }];
 }
                     failureBlock: ^(NSError *error)
 {
     // Typically you should handle an error more gracefully than this.
     NSLog(@"No groups");
 }];

[library release];

Document Revision History

Date	Notes
2011-08-22	Discusses the new APIs available in iOS 4 to access the metadata of an image.
2009-08-25	New document that describes how to access image metadata using the UIImagePickerController and the Assets Library Framework.