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Cues

Introduction

The Cues Element provides an index of certain Cluster Elements to allow for optimized seeking to absolute timestamps within the Segment. The Cues Element contains one or many CuePoint Elements which each MUST reference an absolute timestamp (via the CueTime Element), a Track (via the CueTrack Element), and a Segment Position (via the CueClusterPosition Element). Additional non-mandated Elements are part of the CuePoint Element such as CueDuration, CueRelativePosition, CueCodecState and others which provide any potential Matroska reader with additional information to use in the optimization of seeking performance.

Recommendations

The following recommendations are provided to optimize Matroska performance.

  • Unless Matroska is used as a live stream, it SHOULD contain a Cues Element.
  • For each video track, each keyframe SHOULD be referenced by a CuePoint Element.
  • It is RECOMMENDED to not reference non-keyframes of video tracks in Cues unless it references a Cluster Element which contains a CodecState Element but no keyframes.
  • For each subtitle track present, each subtitle frame SHOULD be referenced by a CuePoint Element with a CueDuration Element.
  • Audio tracks SHOULD only be referenced in CuePoint Elements if no video track is present. In this case CuePoint Elements SHOULD reference audio keyframes at most once every 500 milliseconds.
  • If the referenced frame is not stored within the first SimpleBlock or first BlockGroup within its Cluster Element, then the CueRelativePosition Element SHOULD be written to reference where in the Cluster the reference frame is stored.
  • If a CuePoint Element references Cluster Element that includes a CodecState Element, then that CuePoint Element MUST use a CueCodecState Element.
  • CuePoint Elements SHOULD be numerically sorted in storage order by the value of the CueTime Element.