Event Timelines — CMSF-00 §3.6

3.6.1. SAP Type timeline

CMSF defines a special instance of an Event Timeline track, termed the SAP Type timeline track. Its purpose is to convey information about the distribution of Stream Access Point types and their associated Earlist Presentation Times.

In the catalog, the SAP-type timeline track MUST include a 'packaging' value of 'eventtimeline" and MUST include an 'eventType' value of 'org.ietf.moq.cmsf.sap'.

In the SAP Type timeline JSON payload:

The index reference MUST be 'l' for Location
The data field is a JSON Array containing two integers. The first integer defines SAP type with an allowed value of 0,1,2 or 3. The value 0 indicates that the Object does not start with an ISOBMFF stream access point. The value equal to 1, 2, or 3 indicates that the Object begins with a stream access point of SAP type 1, 2, or 3, respectively. When the Object is the first Object in the Group, the value MUST be equal to 1 or 2. The second integer defines the earliest media presentation timestamp, rounded to the nearest millisecond, of all media samples in the Object defined by the Location of that record.

3.6.2. SAP-type timeline track example

This shows an example of 30-fps HEVC-encoded content, in which each 4s Group beings with SAP-type 2 (i.e., the first picture in the Group is an IDR picture, while there may be one or more pictures in the Group following the IDR picture in decoding order but preceding it in output order). After 2 seconds in each Group, there is a SAP-type 3, i.e., a CRA picture, which is associated with one or more Random Access Skipped Leading (RASL) pictures. A small buffer of frames (10 frames at 30 fps) is skipped/discarded (RASL pictures) when the streaming session starts from the SAP-type 3 location. In this example, the EPT is the presentation time of the first picture after the RASL pictures in decoding order; all pictures after the RASL pictures can be fully correctly decoded and are thus presentable when the streaming session starts from the SAP-type 3 location. Note that if the streaming session starts from the start of the Group, then these RASL pictures can be fully correctly decoded and are thus presentable.

[
    {
        "l": [0,0],
        "data": [2,0]
    },
    {
        "l": [0,60],
        "data": [3,2100]
    },
    {
        "l": [1,0],
        "data": [2,4000]
    },
    {
        "l": [1,60],
        "data": [3,6100]
    }
]