Except for the EBML Header and the CRC-32 Element, the EBML specification does not require any particular storage order for Elements. The Matroska specification however defines mandates and recommendations for ordering certain Elements in order to facilitate better playback, seeking, and editing efficiency. This section describes and offers rationale for ordering requirements and recommendations for Matroska.
The Info Element is the only REQUIRED Top-Level Element in a Matroska file. To be playable, Matroska MUST also contain at least one Tracks Element and Cluster Element. The first Info Element and the first Tracks Element MUST either be stored before the first Cluster Element or both SHALL be referenced by a SeekHead Element occurring before the first Cluster Element.
It is possible to edit a Matroska file after it has been created. For example, chapters, tags or attachments can be added. When new Top-Level Elements are added to a Matroska file, the SeekHead Element(s) MUST be updated so that the SeekHead Element(s) itemize the identity and position of all Top-Level Elements. Editing, removing, or adding Elements to a Matroska file often requires that some existing Elements be voided or extended; therefore, it is RECOMMENDED to use Void Elements as padding in between Top-Level Elements.
As noted by the EBML specification, if a CRC-32 Element is used then the CRC-32 Element MUST be the first ordered Element within its Parent Element. The Matroska specification recommends that CRC-32 Elements SHOULD NOT be used as an immediate Child Element of the Segment Element; however all Top-Level Elements of an EBML Document SHOULD include a CRC-32 Element as a Child Element.
If used, the first SeekHead Element SHOULD be the first non-CRC-32 Child Element of the Segment Element. If a second SeekHead Element is used, then the first SeekHead Element MUST reference the identity and position of the second SeekHead. Additionally, the second SeekHead Element MUST only reference Cluster Elements and not any other Top-Level Element already contained within the first SeekHead Element. The second SeekHead Element MAY be stored in any order relative to the other Top-Level Elements. Whether one or two SeekHead Element(s) are used, the SeekHead Element(s) MUST collectively reference the identity and position of all Top-Level Elements except for the first SeekHead Element.
It is RECOMMENDED that the first SeekHead Element be followed by a Void Element to allow for the SeekHead Element to be expanded to cover new Top-Level Elements that could be added to the Matroska file, such as Tags, Chapters and Attachments Elements.
The Cues Element is RECOMMENDED to optimize seeking access in Matroska. It is programmatically simpler to add the Cues Element after all Cluster Elements have been written because this does not require a prediction of how much space to reserve before writing the Cluster Elements. However, storing the Cues Element before the Cluster Elements can provide some seeking advantages. If the Cues Element is present, then it SHOULD either be stored before the first Cluster Element or be referenced by a SeekHead Element.
The first Info Element SHOULD occur before the first Tracks Element and first Cluster Element except when referenced by a SeekHead Element.
The Chapters Element SHOULD be placed before the Cluster Element(s). The Chapters Element can be used during playback even if the user does not need to seek. It immediately gives the user information about what section is being read and what other sections are available. In the case of Ordered Chapters it RECOMMENDED to evaluate the logical linking even before playing. The Chapters Element SHOULD be placed before the first Tracks Element and after the first Info Element.
The Attachments Element is not intended to be used by default when playing the file, but could contain information relevant to the content, such as cover art or fonts. Cover art is useful even before the file is played and fonts could be needed before playback starts for initialization of subtitles. The Attachments Element MAY be placed before the first Cluster Element; however if the Attachments Element is likely to be edited, then it SHOULD be placed after the last Cluster Element.
The Tags Element is most subject to changes after the file was originally created. For easier editing, the Tags Element SHOULD be placed at the end of the Segment Element, even after the Attachments Element. On the other hand, it is inconvenient to have to seek in the Segment for tags, especially for network streams. So it's better if the Tags Element is found early in the stream. When editing the Tags Element, the original Tags Element at the beginning can be overwritten with a Void Element and a new Tags Element written at the end of the Segment Element. The file size will only marginally change.
- SeekHead
- Info
- Tracks
- Chapters
- Attachments
- Tags
- Clusters
- Cues
- SeekHead
- Info
- Tracks
- Chapters
- Attachments
- Void
- Clusters
- Cues
- Tags
- SeekHead
- Info
- Tracks
- Chapters
- Attachments
- Tags
- Cues
- Clusters
The Timestamp Element MUST occur as in storage order before any SimpleBlock, BlockGroup, or EncryptedBlock within the Cluster Element.