Metadata key atom
An atom that contains a key that corresponds to the timed metadata track containing it.
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Overview
A metadata key atom is identified by a local_key_id corresponding to a 32-bit integer (or FourCC) type code local to the timed metadata track containing it. This local_key_id will correspond to atom types within the metadata sample data.
For example, if the metadata key atom has the atom type of 'stuf', any atoms of type 'stuf' in timed metadata samples sharing this sample description hold the value for this key. Any value fitting in a 32-bit big endian integer can be used (such as 'stuf' or the integer 72). If a FourCC is used, it is recommended that the value be mnemonic if possible. For example, a metadata key atom type of ‘actr’ might contain information about actors in a movie. See Timed metadata sample data format.
There are two reserved atom types for metadata key atoms: 0 and 0xFFFFFFFF.
A local_key_id of 0 indicates the metadata key atom is unused and should not be interpreted. This indication allows the key to be marked as unused in the timed metadata sample description without requiring the sample description and parent atoms to be rewritten or resized.
A local_key_id of 0xFFFFFFFF must not occur in a metadata key atom. It is reserved for future use and can occur as an atom type in timed metadata samples.
All other type codes are available for use as a local_key_id.
Each metadata key atom contains a variable number of atoms that define the key structure, optionally the data type for values, and optionally locale information for values. Atoms may be introduced in the future.
A metadata key atom must contain a metadata declaration atom.
The layout of a metadata key atom is as follows:
Data field | Bytes |
|---|---|
4 | |
Type = | 4 |
variable |