Overview of timed metadata
Synchronize metadata references to media tracks for particular media time periods with timed metadata.
Overview
A timed metadata track synchronizes metadata references to media tracks for particular media time periods through the track reference, edit, and edit list structures. It is a specialization of the track structure which uses the base media information atom of type ‘minf’, and a track handler type value set to ‘meta’. The base media information atom contains the generic media header atom of type ‘gmhd’.
Because a metadata track is neither visual nor aural, the following track properties should have these values:
Track width and track height each set to
0.The track volume set to
0.The track matrix set to the identity matrix.
A QuickTime movie can contain none, one, or several timed metadata tracks. Timed metadata tracks can refer to multiple tracks. Metadata tracks are linked to the tracks they describe using a track-reference of type ‘cdsc’. The metadata track holds the ‘cdsc’ track reference. If a metadata track describes characteristics of the entire movie, there should be no track reference of type 'cdsc' between it and another track. These metadata tracks can be considered to hold global metadata for the movie.
Using a timed metadata track, any form of descriptive metadata that changes over time can be linked to a range of media times for which it is valid. Examples of timed metadata can include:
Faces detected in the scene
Location-based information (such as GPS)
Camera aperture and other changing camera-related information
Copyright and other information for individual clips edited together
Information such as scene changes and actor names added to the movie in production
As with other tracks, each metadata sample is associated with a single timed metadata sample description. This sample description signals information needed to interpret the data in the metadata sample in a way that is analogous to how a video track’s video sample atom indicates that video samples contain H.264 compressed sample data of particular dimensions.
Zero, one, or many metadata values can be associated with a range of media time in the track. The accommodation for no metadata values for a time allows runs of time with metadata interspersed with runs of time with no metadata. Because the timed metadata is organized as a track, it is also possible to use track edits to indicate the absence of metadata. For some situations, however, it would be better to include metadata samples that themselves carry no metadata values.