AVMediaCharacteristic

Each track of .mov files and .m4v files (that is, files of type mov and m4v) can optionally carry one or more tagged media characteristics, each of which declares a purpose, a trait, or some other distinguishing property of the track's media.

For example, tag a track containing audio that mixes original program content with additional narrative descriptions of visual action with the media characteristic public.accessibility.describes-video in order to distinguish it from other audio tracks stored in the same file that do not contain additional narrative.

Each tagged media characteristic in .mov and .m4v files is stored in track user data as a user data item of type 'tagc' (represented as a FourCharCode) that consists of a standard atom header (size and type) followed by an array of US-ASCII characters (8-bit, high bit clear) comprising the value of the tag. The character array is not a C string; there is no terminating zero. The user data item atom size is sum of the standard atom header size (8) and the size of the US-ASCII character array.

You can inspect the tagged media characteristics of a track as follows:

NSArray *trackUserDataItems = [myAVAssetTrack metadataForFormat:AVMetadataFormatQuickTimeUserData];
NSArray *trackTaggedMediaCharacteristics = [AVMetadataItem metadataItemsFromArray:trackUserDataItems
        withKey:AVMetadataQuickTimeUserDataKeyTaggedCharacteristic
        keySpace:AVMetadataKeySpaceQuickTimeUserData];
for (AVMetadataItem *metadataItem in trackTaggedMediaCharacteristics) {
     NSString *thisTrackMediaCharacteristic = [metadataItem stringValue];
}

You can use hasMediaCharacteristic: to determine whether a track has a particular media characteristic, whether to infer the characteristic from its media type or format descriptions (such as AVMediaCharacteristicAudible or AVMediaCharacteristicContainsOnlyForcedSubtitles) or requires explicit tagging (such as AVMediaCharacteristicTranscribesSpokenDialogForAccessibility or AVMediaCharacteristicEasyToRead). You can't use explicit tagging to override inferences from tracks' media types or format descriptions; for example, the following returns false for any audio track, even when tagging the track with the visual characteristic.

[anAVAssetTrack hasMediaCharacteristic:AVMediaCharacteristicVisual]

You can write tagged media characteristics to the QuickTime user data of an output track associated with an AVAssetWriterInput object as follows, provided that the output file type of the asset writer is either mov or m4v:

AVMutableMetadataItem *myTaggedMediaCharacteristic = [[AVMutableMetadataItem alloc] init];
[myTaggedMediaCharacteristic setKey:AVMetadataQuickTimeUserDataKeyTaggedCharacteristic];
[myTaggedMediaCharacteristic setKeySpace:AVMetadataKeySpaceQuickTimeUserData];
[myTaggedMediaCharacteristic setValue:aMeaningfulCharacteristicAsNSString];
[myMutableArrayOfMetadata addObject:myTaggedMediaCharacteristic];
[myAssetWriterInput setMetadata:myMutableArrayOfMetadata];