Quicktime File Format (2012-08-14).pdf

MetadataMetadata StructurePredefinedA 32-bit integer that is set to 0Handler typeA 32-bit integer that indicates the structure used in the metadata atom, set to ‘mdta’ReservedAn array of 3 const unsigned 32-bit integers set to 0NameThe name is a NULL-terminated string in UTF-8 characters which gives a human-readable name for ametadata type, for debugging and inspection purposes. The string may be empty or a single bytecontaining 0.Note A reader parsing a metadata atom should confirm the handler type in the metadata handleratom is ‘mdta’ before interpreting any other structures in the metadata atom according to thespecification presented here. If the handler type is not ‘mdta’, the interpretation is defined byanother specification.Metadata Header AtomThe metadata format optionally assigns unique identifiers to metadata items for such purposes as definingstable identifiers for external references into the set of metadata items. This is accomplished by including anitem information atom in added metadata item atoms contained by the metadata item list atom. Such uniqueidentifiers must be guaranteed to be unique.To make the assignment of unique item identifiers more efficient, the metadata atom may contain a metadataheader atom holding the integer value for the next unique item identifier to assign stored in the nextItemIDfield. In general it holds a value one greater than the largest identifier used so far.Important The metadata header atom must exist if there are metadata item atoms containing an iteminformation atom indicating the item’s unique ID.Upon assigning the identifier to a metadata item, if the value of the nextItemID field is less than 0xFFFFFFFF,it should be incremented to the next unused value. If the value of nextItemID is equal to 0xFFFFFFFF, it shouldnot be changed: in that case, a search for an unused item identifier value in the range from 0 to 0xFFFFFFFFis needed for all additions.The metadata header atom is a full atom with an atom type of ‘mhdr’. It contains the following fields:SizeA 32-bit unsigned integer that indicates the size in bytes of the atom structure2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.131