A list of attributes, that should be used as descriptions of tracks or
differentiation criteria for tracks in the same alternate or switch
group. Each differentiating attribute is associated with a pointer to the
field or information that distinguishes the track.
The following attributes are descriptive:
| Name | Attribute | Description |
|---|---|---|
| Temporal scalability | 'tesc' | The track can be
temporally scaled. |
| Fine-grain SNR scalability | 'fgsc' | The track can
be fine-grain scaled. |
| Coarse-grain SNR scalability | 'cgsc' | The track can
be coarse-grain scaled. |
| Spatial scalability | 'spsc' | The track can be
spatially scaled. |
| Region-of-interest scalability | 'resc' | The track
can be region-of-interest scaled. |
The following attributes are differentiating:
| Name | Attribute | Pointer |
|---|---|---|
| Codec | 'cdec' | Sample Entry (in Sample Description
box of media track) |
| Screen size | 'scsz' | Width and height fields of
Visual Sample Entries. |
| Max packet size | 'mpsz' | Maxpacketsize field in RTP
Hint Sample Entry |
| Media type | 'mtyp' | Handlertype in Handler box (of
media track) |
| Media language | 'mela' | Language field in Media
Header box |
| Bitrate | 'bitr' | Total size of the samples in the
track divided by the duration in the track header box |
| Frame rate | 'frar' | Number of samples in the track
divided by duration in the track header box |
Descriptive attributes characterize the tracks they modify, whereas
differentiating attributes differentiate between tracks that belong to
the same alternate or switch groups. The pointer of a differentiating
attribute indicates the location of the information that differentiates
the track from other tracks with the same attribute.
media type: one for video, one for audio, etc. Such a file may include
several video tracks, although, at any point in time, only one of them should
be played or streamed. This is achieved by assigning all video tracks to the
same alternate group. (See subclause 8.3.2 for the definition of alternate
groups.)
All tracks in an alternate group are candidates for media selection, but it
may not make sense to switch between some of those tracks during a session.
One may for instance allow switching between video tracks at different
bitrates and keep frame size but not allow switching between tracks of
different frame size. In the same manner it may be desirable to enable
selection – but not switching – between tracks of different video codecs or
different audio languages.
The distinction between tracks for selection and switching is addressed by
assigning tracks to switch groups in addition to alternate groups. One
alternate group may contain one or more switch groups. All tracks in an
alternate group are candidates for media selection, while tracks in a switch
group are also available for switching during a session. Different switch
groups represent different operation points, such as different frame size,
high/low quality, etc.
For the case of non-scalable bitstreams, several tracks may be included in a
switch group. The same also applies to non-layered scalable bitstreams, such
as traditional AVC streams.
By labelling tracks with attributes it is possible to characterize them. Each
track can be labelled with a list of attributes which can be used to describe
tracks in a particular switch group or differentiate tracks that belong to
different switch groups.
author Alexander Simm