Matroska supports storage of related files and data in the Attachments Element (a Top-Level Element). Attachment Elements can be used to store related cover art, font files, transcripts, reports, error recovery files, picture, or text-based annotations, copies of specifications, or other ancillary files related to the Segment.

Matroska Readers MUST NOT execute files stored as Attachment Elements.

Cover Art

This section defines a set of guidelines for the storage of cover art in Matroska files. A Matroska Reader MAY use embedded cover art to display a representational still-image depiction of the multimedia contents of the Matroska file.

Only JPEG and PNG image formats SHOULD be used for cover art pictures.

There can be two different covers for a movie/album: a portrait style (e.g., a DVD case) and a landscape style (e.g., a wide banner ad).

There can be two versions of the same cover, the normal cover and the small cover. The dimension of the normal cover SHOULD be 600 pixels on the smallest side – for example, 960x600 for landscape, 600x800 for portrait, or 600x600 for square. The dimension of the small cover SHOULD be 120 pixels on the smallest side – for example, 192x120 or 120x160.

Versions of cover art can be differentiated by the filename, which is stored in the FileName Element. The default filename of the normal cover in square or portrait mode is cover.(jpg|png). When stored, the normal cover SHOULD be the first Attachment in storage order. The small cover SHOULD be prefixed with “small_”, such as small_cover.(jpg|png). The landscape variant SHOULD be suffixed with “_land”, such as cover_land.(jpg|png). The filenames are case sensitive.

The following table provides examples of file names for cover art in Attachments.

FileName Image Orientation Pixel Length of Smallest Side
cover.jpg Portrait or square 600
small_cover.png Portrait or square 120
cover_land.png Landscape 600
small_cover_land.jpg Landscape 120

Table: Cover Art Filenames{#coverartFilenames}

Font files

Font files MAY be added to a Matroska file as Attachments so that the font file may be used to display an associated subtitle track. This allows the presentation of a Matroska file to be consistent in various environments where the needed fonts might not be available on the local system.

Depending on the font format in question, each font file can contain multiple font variants. Each font variant has a name which will be referred to as Font Name from now on. This Font Name can be different than the Attachment’s FileName, even when disregarding the extension. In order to select a font for display, a Matroska player SHOULD consider both the Font Name and the base name of the Attachment’s FileName, preferring the former when there are multiple matches.

Subtitle codecs, such as SubStation Alpha (SSA/ASS), usually refer to a font by its Font Name, not by its filename. If none of the Attachments are a match for the Font Name, the Matroska player SHOULD attempt to find a system font whose Font Name matches the one used in the subtitle track.

Since loading fonts temporarily can take a while, a Matroska player usually loads or installs all the fonts found in attachments so they are ready to be used during playback. Failure to use the font attachment might result in incorrect rendering of the subtitles.

If a selected subtitle track has some AttachmentLink elements, the player MAY use only these fonts.

A Matroska player SHOULD handle the official font media types from [@!RFC8081] when the system can handle the type:

  • font/sfnt: Generic SFNT Font Type,
  • font/ttf: TTF Font Type,
  • font/otf: OpenType Layout (OTF) Font Type,
  • font/collection: Collection Font Type,
  • font/woff: WOFF 1.0,
  • font/woff2: WOFF 2.0.

Fonts in Matroska existed long before [@!RFC8081]. A few unofficial media types for fonts were used in existing files. Therefore it is RECOMMENDED for a Matroska player to support the following legacy media types for font attachments:

  • application/x-truetype-font: Truetype fonts, equivalent to font/ttf and sometimes font/otf,
  • application/x-font-ttf: TTF fonts, equivalent to font/ttf,
  • application/ OpenType Layout fonts, equivalent to font/otf
  • application/font-sfnt: Generic SFNT Font Type, equivalent to font/sfnt
  • application/font-woff: WOFF 1.0, equivalent to font/woff

There may also be some font attachments with the application/octet-stream media type. In that case the Matroska player MAY try to guess the font type by checking the file extension of the AttachedFile\FileName string. Common file extensions for fonts are:

  • .ttf for Truetype fonts, equivalent to font/ttf,
  • .otf for OpenType Layout fonts, equivalent to font/otf,
  • .ttc for Collection fonts, equivalent to font/collection The file extension check MUST be case insensitive.

Matroska writers SHOULD use a valid font media type from [@!RFC8081] in the AttachedFile\FileMediaType of the font attachment. They MAY use the media types found in older files when compatibility with older players is necessary.