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.
A valid Matroska file requires only one Top-Level Element, the Info Element; however, to be playable Matroska MUST also contain at least one Tracks and Cluster Element. The first Info Element and the first Tracks Element MUST either be stored before the first Cluster Element or both be referenced by a SeekHead Element which occurs before the first Cluster Element.
After a Matroska file has been created it could still be edited. 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 MUST reference the identity and position of the second SeekHead, the second SeekHead MUST only reference Cluster Elements and not any other Top-Level Element already contained within the first SeekHead, and the second SeekHead 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 itself.
It is RECOMMENDED that the first SeekHead Element be followed by some padding (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 of the Cluster Elements are written because this does not require a prediction of how much space to reserve before writing the Cluster Elements. On the other hand, 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 and first Cluster Element.
The Chapters Element SHOULD be placed before the Cluster Element(s). The Chapters Element can be used during playback even if the user doesn't need to seek. It immediately gives the user information of 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 starting playing anything. The Chapters Element SHOULD be placed before the first Tracks Element and after the first Info Element.
The Attachments Element is not meant to use by default when playing the file, but could contain the cover art and/or fonts. Cover art is useful even before the file is played and fonts could be needed before playback starts for initialization of subtitles that could use them. 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 the one that is most subject to changes after the file was originally created. So 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(s) are found early in the stream. When editing the Tags Element(s), the original Tags Element at the beginning can be voided and a new one written right 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
As each BlockGroup and SimpleBlock of a Cluster Element needs the Cluster Timecode, the Timecode Element MUST occur as the first Child Element within the Cluster Element.